Descriptive Glossary Table of Contents Home
    If human beings were intended to fly, The Intelligent Designer would have made their poop white.
         -- Paul Niquette

    Diagram obtained under Fair-Use from StartFlyingRC.

    AGL (Altitude) Above Ground Level, where all aircraft must fly. Always a positive number, by the way. Distinguished from MSL, (above) Mean Sea Level, which is what an altimeter measures, and can indeed be negative. Cloud levels are given as AGL.

    When in doubt, hold your altitude.  No one has ever collided with the sky.
         -- Old Aviation Saying
    In 1980, a friend of mine set one of aviation's lesser known world records: Low-Altitude Endurance, flying at -200 feet MSL (+50 feet AGL) for over five hours, in Death Valley (now her husband plans to set the high-altitude submarine record with a one-man submersible in Peru's Lake Titicaca).

    To determine altitude AGL -- the most vital measurement aloft -- the pilot must know: (a) the plane's altitude MSL, (b) the plane's geographical position, and (c) the elevation (MSL) of the ground (the G in AGL) at that location.

    aileron Movable surface on the outboard trailing edge of each wing. Operated by rotating the control wheel (or tilting the stick from side to side), the ailerons control roll (animation).

      Aileron is one of the few pure aviation terms and was derived from the diminutive form of the French aile (wing).  Other French contributions: empennage, fuselage, and longeron.

    airspeed The speed at which the airplane moves through the air expressed in knots (kts nautical miles per hour).

    Airspeed is distinguished from groundspeed and comes in two forms:  'Indicated airspeed' (IAS), corrected for altitude and temperature, becomes 'true airspeed' (TAS).  Thus "truing out" at 150 KTAS with a 15-knot tailwind "makes good" 165 knots "over the ground."
     

    In early days of aviation, airspeeds were expressed in mph (statute miles per hour) same as automobiles and trains.  Marketing psychology doubtless played a role: To say your sedan goes "100 mph" is more exciting than "87 kts" -- much as $100 is preferred for a prize, while $99.95 works better as a price.

    Back in 1937, for one flight of 2,228 nm  (2,566 sm), confusion about kts and mph for winds aloft may have participated in tragedy (see Which Way, Amelia?).

    Airspeed, altitude and brains: any two out of the three are needed to complete the flight successfully.
      -- Old Aviation Saying
    alphabet, phonetic Alpha, Bravo, Charlie (shar-lee), Delta, Echo, Foxtrot (often shortened to Fox), Golf, Hotel, India, Juliet, Kilo, Lima (pronounced as the city not the bean), Mike, November, Oscar (oss-kah), Papa, Quebec (kay-beck), Romeo, Sierra, Tango, Uniform (oo-nee-form), Victor (vik-tah), Whiskey, X-ray, Yankee, Zulu.
      Use these in casual conversation anywhere in the world and pilots will reveal themselves by asking, "What are you flying?"
         
        Never drop the airplane to fly the microphone.
          -- Old Aviation Saying

    altimeter An instrument that measures air pressure, like a barometer.  See MSL.
    The face is calibrated in 'feet MSL' and an adjustment ("Kollsman Window") that enables correcting for local atmospheric pressure, which is in turn given as 'inches of mercury' (29.92 being the so-call "Standard Atmosphere).  Clear? 

    Radar altimeters measure altitude above ground level AGL directly but are extremely rare in light aircraft but will become more common with the growth of the VLJ fleet.

    altitude Three entries: Above Ground Level , Above Means Sea Level, Density Altitude.  
    angle-of-attack The angle between the chord of the wing and the relative wind (also called angle-of-incidence).
     
    The chord is simply the straight line that connects the leading edge of the wing with the trailing edge (longest dimension front-to-back). The relative wind, not so simply, is the direction from which the air appears to be coming. In level flight, the relative wind strikes the plane horizontally from straight ahead. During ascent (or descent), the relative wind comes from above (or below) the plane.

    AOPA Aircraft Owners and Pilots Association, non-profit political organization serving the interests of its members to promote the economy, safety, utility, and popularity of flight in general aviation aircraft.

    Area Rule An ironic aerodynamic property for minimizing drag in high-speed aircraft. 
     

    Discovered back to the fifties, the rule mandates a constant crossectional area as measured at stations along the centerline of the aircraft.  Thus the fuselage on some aircraft accommodates the wings by virture of "coke-bottle" design. 

    As the VLJ segment of General Aviation fleet becomes prominent, private pilots can expect increased pertinence for jet-age lingo, like "thrust lever," "engine pressure ratio," "turbine stall," "flame-out," and "after-burner" ("reheat" in UK parlance).

    Artificial Horizon or Attitude Indicator, an instrument symbolizing the aircraft in the center, and the background controlled by a gyro.

    Depicted on the right is an artificial horizon in the panel of an aircraft that is momentarily pitching up 5 degrees and rolling right by 15 degrees.
    ATC Air Traffic Control, a service of the FAA (Federal Aviation Administration) in which air traffic controllers are responsible to guide and protect airplanes (see traffic).
     
    Most people are familiar with control towers. The 'tower' operators (also called 'local' controllers) are responsible for planes in the process of landing and taking off. 'Ground control' directs planes on the ground. Movie-makers take note: you never call ground control while in the air. At the radar scopes you have 'departure' controllers, 'enroute' controllers, and 'approach' controllers.

    The vast majority of airports do not have control towers. Most use Unicom or CTAF (Common Traffic Advisory Frequency) to exchange information among the pilots flying in the pattern. Works fine, by the way. ("Caldwell traffic, Cardinal Niner One Four, wing-up, turning right base for Runway Three Zero, behind the Cherokee.")

    ATIS Automatic Terminal Information Service (pronounced ATE-is), a transcribed radio message that gives up-to-date advisories about conditions and procedures at a particular airport. The pilot listens to the message on the ground before taxiing and in the air before approaching the airport.
     

    "Torrance Airport, Information Juliet, zero-two-zero-zero Zulu. Weather: sky partially obscured, two thousand scattered, two-five thousand broken. Visibility: five, haze. Wind: two-seven-zero at one-five. ILS Two-Niner Right approach in use, landing and departure: Runways Two-Niner Right and Two-Niner Left. Caution equipment in use on taxiway kilo. Advise on initial contact, you have received Information Juliet."

    attitudes, unusual In aviation, the expression 'unusual attitudes' has a special meaning. Just so you know, it has nothing to do with extraordinary mental states or peculiar dispositions. 'Attitude' is the general term used to describe (ahem) the instantaneous angular position of the airplane with respect to the horizon.
     

    Attitude is a vital thing for the pilot to know -- which way is up? -- and to control. The task is made easy when you can see the horizon out the window, difficult -- hard! -- when you cannot. Straight and level flight is one of the most common -- 'usual' -- attitudes, as are climbing, descending, and coordinated turns.

    So then, what is an 'unusual' attitude? It is any attitude not required for the normal conduct of flight. Unintentional attitude is more to the point.

    Airplanes tend to 'overbank' constantly. Here's why. Say a slight disturbance lifts the right wing. Suppose that, through momentary pilot inattention, it is not immediately corrected. A left turn ensues. That might not be especially inconvenient -- if you happen to desire a left turn. Inadvertent, though, and you have an unusual attitude.

    Overbanking results from the turn itself. Intended or not, while flying in a curving path the outside wing has farther to go than the inside wing. For the case under consideration, that means the right wing is travelling faster than the left wing, and develops more lift -- rolling your plane more steeply to the left. The motion can be exceedingly subtle. You are not likely to feel it.

    Most likely, though, the pilot is the problem. Carelessness in cross-checking the instruments and errors of interpretation, confusion and disorientation, lack of training or being out of practice -- these are among the foibles that beset the person at the controls.

    Unusual attitudes are not really so unusual. Safety dictates that you must learn how to recover from them, which often requires dealing with vertigo.

        Mankind has a perfect record in aviation; We never left one up there yet.
             -- Old Aviation Saying

    autopilot Airborne electronic equipment that automatically actuate the controls of an airplane.
      The simplest is called a 'wing leveler,' which is capable merely of 'roll-hold.' You might add 'heading-hold,' 'pitch-hold,' and 'altitude-hold.' Finally, there is the 'coupler' that connects the autopilot to your navigation system. Then you might ask yourself why you took up flying.

    avigation an aviation neologism (see navigation).
    The sky, to an even greater extent than the sea, is terribly unforgiving of errors.
          -- Old Aviation Saying
    AvWeb Web-based, independent aviation news resource.

    azimuth Angle measured between some reference line, generally north (either magnetic or true) and a fix or a target (in radar).  If the reference line is the longitudinal axis of the aircraft, the preferred term is bearing.

    bank Same as roll. 'Angle-of-bank' is that which the wings momentarily make with the horizon.

      In 'coordinated turns' (neither slipping nor skidding), best to think of your fanny as 'down' and what you see through the plexiglass as a crooked picture on the wall.
    balance See weight-and-balance.

    base leg See pattern.

    beacon Coded signal, either a rotating light (atop an obstacle or an airport tower) or a navigational radio (most often called an NDB for "non-directional beacon"). 

    At a civilian airport the light signal alternates between green and white; for military aerodromes, the beacon appears to flash green-white-white.

    Radio beacons are suitable for direct inbound navigation (homing) or for taking cross-bearings; they are subject to errors in outbound navigation (see ADF).

    bearing An angle measured clockwise relative to some reference, often the longitudinal axis of the airplane (see numbers) or from North (see omni).

    Best Angle of Climb Indicated airspeed (abbreviated "V X" in V-speeds) that enables the aircraft to ascend in altitude at the steepest angle (minimum distance), most relevant for clearing obstacles, distinguished from -- and slower than -- Best Rate of Climb speed.

    Best Rate of Climb Indicated airspeed (abbreviated "V Y" in V-speeds) that enables the aircraft to ascend in altitude at the fastest rate (minimum time), most relevant for minimizing time enroute, distinguished from -- and faster than -- Best Angle of Climb speed.

    blip The spot on the radar screen corresponding to the echo of an airplane, also called 'target' (see transponder). Maybe blip sounds like what a spot looks like.

    CAP Civil Air Patrol, a civilian auxiliary of the United States Airforce.

    CAVU Ceiling And Visibility Unlimited, also 'severe clear.' The extreme opposite of WOXOF, "indefinite ceiling zero, sky obscured, visibility zero with fog" (decoded from old fashioned weather teletypes).

    ceiling A broken or overcast cloud layer at some measured elevation AGL.

      For landing an aircraft, the pilot must have some prescribed forward visibility, often a mile, under a ceiling of at least a few hundred feet. These are the so-called 'landing minimums', and they vary from airport to airport.








    celestial navigation Nautical procedure appropriated by commercial and military aviation in bygone days for finding position based on angular measurements of sun, moon, and stars (more at sextant).

    checklist An ordered set of references to procedures vital to the safe conduct of flight.  Checklists are customarily printed on laminated pages bound into a notebook immediately accessible to the pilot in command.
     
     

    A given checklist pertains to a specific phase of flight: Pre-Flight  Inspection, Before-Starting-Engine, Pre-Taxi, Pre-Take-Off, Approach-to-Landing,.. Emergencies (by type). 

    Every item on a checklist has two parts, the first in the form of a noun phrase called the “challenge,” for each of which a “response” is mandated:  "Flaps -- Set"; “Landing Gear – Down and Locked, Three-Greens Showing”;  "Mixture -- Full Rich." 

    A checklist does not tell the pilot how to perform the procedure.  Technical manuals and flight training are supposed to do that.

    Aviation mnemonics are commonly used as informal checklists (CIGAR, GUMP, CCCC, TTTTT), with varying degrees of effectiveness
     

    A checklist is not a do-list.
          – Old Aviation Saying
    chronometer A precise time piece used in navigation.

    Illustrated above is an especially fancy one, perched for the night atop an E6B computer.  You see plenty of bezels and buttons plus an auxiliary movement for GMT.  Given the indicated settings, the pilot must be slumbering in which time zone?
    Time waits for no one.  Whoever first sang those words might have been thinking of aviation, where passing the middle marker on the ILS requires reading the chronometer immediately in order to declare a 'missed approach' when the time comes. 

    Nota bene, celestial navigation experiences a full mile of shift in longitude at the equator with every 3.5 seconds of error in time measurement.

    CIGAR An abbreviated pre-take-off checklist: Controls (free and correct), Instruments (heading, altimeter, horizon), Gas (fuel selector on fullest tank), Attitude (trim), Run-up (testing of the engine prior to take-off).
     

    The least useful acronyms are those which provide no mnemonic support for sequence: TTTTT (Time, Turn, Throttle, Tune, Talk) for the five things you are supposed to do at the outer marker (which I have replaced with "watchman power and radio chatter"); for emergencies, I replaced the traditional CCCC (Climb, Confess, Communicate, Comply) with my on credo: PADO (Pull-up, Admit that I need help, Describe my predicament, Obey instructions).
      Aviate, Avigate, Communicate -- in that order.
             -- Not so old aviation saying.
    clearance Loosely speaking, an agreement between ATC and the pilot of an aircraft, under the terms of which safe separation from other aircraft is assured in the event of communications failure. Simple clearances include permission to taxi somewhere on the airport, to take off, to land, to change frequencies.
     
    Under IFR, a clearance covers a chain of in-flight procedures, which must be taken down by the pilot in "aviation shorthand" from clearance delivery and read back verbatim.  Here is an example: C34914 CLRD TO SFO M3 EXP H IN 5MIN.  MRH TO X SLB R123, RT 270, RV TO LAX VOR.  FPR.  DEP 126.4, SQ3241.  Which sounds like this over the radio…

    "ATC Clears Cardinal Three Four Niner One Four to the Santa Francisco Airport. Maintain three thousand. Expect higher five minutes after departure. Maintain runway heading until crossing the Seal Beach one-two-three degree radial, right turn heading two-seven-zero for radar vectors to Los Angeles VOR. Flight plan route. Contact Departure Control, 126.4. Squawk 3241. Proceed with your readback."

    cockpit Where you control a plane from, which everybody knows. With the increasing presence of women in flight crews, the term is being displaced by 'flight deck.'

      As with other terms ('navigate,' 'dead reckoning,' 'rudder'), 'cockpit' came to aviation by way of maritime parlance, there being a lower space near the stern from whence some vessels are steered. Just so there is no misunderstanding, 'cock' derives originally from the fighting bird.


    cockpit, glass Collection of flat-screen, twenty-first century displays. Prepare to learn a whole new collection of TLDs, including FMS, SVS, and PFD.

    compass  A primitive instrument that indicates magnetic heading.  It has digits painted around a black cylinder that floats in a bath of mineral oil.  The thing is subject to many kinds of errors as the plane maneuvers -- so much so, that a pilot must rely instead on his or her "heading indicator," which is stabilized by a gyro  

    Both instruments are subject to the same design flaw.  The designer was obviously ignorant of human factors and suffered a misguided penchant for abbrev.
     
    For safety's sake, headings are invariably given over the radio as three digits.  That's called 'redundancy.'  Thus, North-West is said, "three-zero-zero."  On both compass and heading indicator, the least significant digit is deleted on every printed number, such that North West reads "30."   That's bad enough, but then leading zeroes are also deleted (must be to save paint).  That means North-East, which is said "zero-three-zero," reads just plain "3."  Now, 30 is exactly 270 degrees from 300, making the expression "right angle" a potentially fatal -- well, misnomer.  And yes, a high-time pilot I know personally has experienced the indicated inadvertence while aloft in IMC.
    course Intended direction of flight -- usually referenced to magnetic North, some place up in Canada where compasses point. Because of wind, your course generally differs from your heading.

    crab Angle between course and heading resulting from the influence of a crosswind component.

      Crab is especially onerous on landing. The pilot uses an intentional (side) slip to correct it out; otherwise the wheels will not be lined up with the runway at the instant of touchdown.
    crosswind landing, crosswind takeoff Runways are generally designed to be lined up with the wind.  Once the airport is built, though, it doesn't always work out that way.
     
    If a significant component of the wind is blowing across the runway, an airplane taking off tends to skid sideways just as the wheels leave the ground.  For a crosswind take-off, the pilot anticipates this tendency by prepositioning the aileron control for a turn into the wind before releasing the brakes.  Immediately after take-off the plane actually does turn into the wind and then commences to crab along the runway. Best to explain these matters to your passengers before starting the engine(s).

    Crosswind landings have the same problem in reverse, but not the same solution.  The preferred approach is to execute an intentional slip by banking into the wind and holding opposite rudder to cancel out the effect of the crosswind and to keep the plane's landing gear lined up with the runway.  Of course, with the plane banked, one wheel will touch-down first, but -- hey, nobody ever said crosswind landings have to be pretty.

    crosswind leg See pattern.

    cruise-climb Conventional maneuver in the early stages of each flight, using the indicated airspeed recommended  in the owner's manual for the aircraft, calling for a pitch angle that affords good forward visibility and an air-flow that assures good engine cooling, distinguished from -- and faster than -- Best Angle of Climb speed and Best Rate of Climb speed. The abbreviation "V CC" in the V-Speeds is a coinage for this glossary. 

    CUT Coordinated Universal Time (see GMT.).

    curve, power See drag.

    dead reckoning An unclever contraction of 'deductive reckoning,' which uses speed and direction together with elapsed time to estimate one's present whereabouts from some previously known position. In aviation, the procedure is especially prone to errors, primarily because of winds, but it's anything but "dead." 

    The preferred term is pilotage. Likewise, the phrase 'positioning flight' seems to have replaced 'dead-head' in aviation parlance.
     
    There is some controversy about the derivation of the phrase. According to the Oxford English Dictionary, the 'dead reckoning' dates from Elizabethan times (1605-1615).  The popular etymology cited here is not documented in any historical dictionary.  Instead, 'dead reckoning' is navigation without celestial references. Whereas with stellar observation, you are, in some sense, live -- working with the stars and the movement of the planets.  Conversely, using mere compasses and clocks but no sky, you are working dead.


    delivery, clearance A position in the control tower responsible to obtain the authorized procedures for each flight under IFR and to provide them as an official clearance to the pilot over a dedicated radio frequency.

    density altitude An aviation expression used to describe the effects three factors that determine the local density of the air.

The three H's are invoked as a mnemonic: HHH for  High, Hot, and Humid. That adds up to 'high density-altitude' -- a perilously poor term. It really means 'low-density air.' Engines gasp, propellers flail inefficiently, and wings find scant support. To stay aloft in this rarefied stuff, you must fly over the ground faster than normal, though your airspeed indicator, which is thrown out of calibration, reads the same.  For an anecdote, see Maritime Air.

    DF Direction Finding, generally referring to a primitive radio receiver and instrument on the ground at an airport control tower or a Flight Service Station (FSS). 

    The use of DF for assisting a lost pilot requires two-way radio communications ("Which way, Amelia?").  Typically, the operator on the ground will say over the radio, "Give me a short count, please."  The pilot responds, "1-2-3-4-5-4-3-2-1, over." The DF needle indicates azimuth to the aircraft which can be transmitted by voice to the pilot (as soon as he or she remembers to release the microphone key). 

    You are invited to admire a world-class collection of radio direction finders here.

    ADF, Automatic Direction Finding is an onboard receiving instrument, with a needle that continuously indicates bearing to a selected radio beacon on the ground or commercial broadcast station.  The instrument on the right has been manually set using the knob so that the aircraft's heading, 340 degrees, appears under the lubber; meanwhile, the needle, which automatically points to the tuned-in radio station or beacon, is indicating that the aircraft is indeed homing on that heading with no apparent crosswind.
    ADF dates back to olden times, long before the VOR became a reality in aviation. Which explains what you hear even today in radio broadcasts, "We pause now for station identification [just in case there's some lost soul in the sky who's trying to use his or her ADF to figure out where the hell he or she is]." 

    Personal Note The ADF on the panel in Two-Four Fox was routinely kept tuned to KNX 1070 AM, a 50,000-watt 'clear channel' broadcast station located near Torrance Airport.  The needle always pointed home on flights all over the West, even from as far away as the tip of Baja California.

    DME Distance Measuring Equipment, an instrument that determines distance of the aircraft from a selectable navigation aid on the ground.

    The DME in the aircraft acts like a transponder, only in reverse, sending out a pulse of radio energy in all directions and measuring the time interval for the return signal at the speed of light from the DME on the ground.
      The facility on the ground is usually co-located with a VOR and is operated under a "frequency pairing plan."   The combination is called VORTAC, an acronymic portmanteau, which combines VOR with TAC, which is an abbreviated TLD for TACAN, which stands for Tactical Air Command Air Navigation.  Clear?

    DOF Degrees of Freedom, of which every solid object enjoys six.  Here are their names in aviation parlance alongside their corresponding (nautical terms in parentheses).
    1. Moving up and down ~~~~~~~~~~~~~~~ climbing/diving (heaving) 
    2. Moving left and right ~~~~~~~~~~~~~~~ slipping/skidding  (swaying) 
    3. Moving forward and backward ~~~~~ accelerating/decelerating (surging) 
    4. Tilting forward and backward ~~~~~~~~~~~~~~~ pitching (pitching) 
    5. Turning left and right ~~~~~~~~~~~~~~~~~~~~ yawing (yawing) 
    6. Tilting side to side ~~~~~~~~~~~~~~~~~~~~~~ rolling (rolling).
    downwind leg See pattern.

    drag Force acting to retard the motion of the plane or to keep people from having fun at a party.

      All things that move through fluids experience drag. Airplanes are distinguished by the fact that for them, there are two kinds of drag. Common old ordinary drag is called 'parasite' drag, which becomes greater with increasing speed, the same as for barracuda and boats, bikes and blimps, bullets and buses.  Airplanes also experience a pernicious form of drag, which paradoxically increases at low airspeeds. It is called 'induced' drag -- induced by the act of flying.

      Added together and graphed against airspeed, these two drags form a U-shaped curve. Corresponding to the bottom of the curve is the 'minimum drag speed,' usually about 30% above the airplane's stall speed. In still air, this is the most efficient airspeed, giving the greatest distance for the least fuel consumption.

      By the way, since drag is overcome by thrust (or 'power'), an airplane exhibits a U-shaped 'power curve.' Hence the expression that has been so rudely appropriated by management and politicians, "getting behind the power curve." It means flying so slow that no amount of power can prevent a stall.

    drift meter Telescopic instrument (obsolete) used in dead reckoning, that enabled an aircraft navigator to peer down through the aircraft's fuselage and measure the lateral motion of objects on the ground thereby enabling adjustments of the aircraft's heading to correct for crosswind. 
    One obvious limitation: clouds that obscure the ground.  For flying over open water, whitecaps can be tracked; however, for that, a wind at the surface must be greater than about 10 knots (see Which way, Amelia?)
    DUAT FAA's Direct User Access Terminal, a free service for qualified pilots, providing self-service weather briefing, flight planning and filing.

    E6B Computer A circular slide-rule used to perform in-flight calculations, such as time-distance-speed, fuel consumption, and wind-triangle.

    Doesn't have batteries. That's neat.  Not only that but, unlike GPS, the E6B is not disabled by solar flares in the daytime.

    elevator The movable surface at the trailing edge of the stabilizer (the horizontal part of the empennage). Operated by pulling or pushing the control wheel (or stick), the elevator controls pitch (animation).  Many planes have a single, movable surface ('stabilator').
     
    'Elevator' is a misleading term. Within limits, the throttle controls 'elevation.' The elevator -- through pitch -- controls airspeed.

    While it is true that pulling back on the control wheel, which pushes down on the tail, imparts an upward motion to the plane, the long-term effect is to slow your airspeed. Enough of that and you achieve a stall. Old saying: "To go up, pull back. To go down, pull all the way back."

    ELT Emergency Locator Transmitter, a radio beacon mounted in the aircraft usually near the tail, activated by an accelerometer in the event of a crash.  The signal is picked up by low-earth orbiting satellites for use in search and rescue operations. 

    empennage  The tail assembly of an airplane, including rudder and elevator.

      Empennage is one of the few pure aviation terms and was derived from the French word (tail feathers of an arrow). Other French contributions: aileron, fuselage, and longeron.

    engine, critical The most important flight parameter in multi-engine flying is Vmc, which represents the expression "minimum controllable airspeed with the most critical engine out." 
    The most critical engine for a twin is the one on the left wing, which is determined by the P-factor.  For a single-engine aircraft, the most critical engine is... never mind.
    engine, General Aviation The most common powerplant for light aircraft may be described as [deep breath here]...
    ...internal combustion engine, gasoline-powered, spark-ignition, with four-stroke reciprocating-pistons, dual-magnetos, normally-aspirated carburetor or servo-managed fuel injection, with or without turbocharging, configured with either four or six horizontally-opposed cylinders, displacing 100 to 400 cubic inches of swept volume, air cooled or (rarely) liquid cooled, with inlets and exhausts managed by overhead poppet valves engaged by spring-return rocker arms activated by externally sleaved push-rods riding on gear-driven cams in the crank case, producing 100 to 300 brake-horsepower at sea-level, either directly driving or via reduction gear to spin a metal propeller with two- or three-blades, delivering thrust by fixed-pitch or by pilot-selectable, governor-controlled, constant-speed between 1,500 and 2,800 RPM.
    The two most prominent manufacturers are Teledyne Continental Motors and Textron Lycoming, with BRP Rotax taking a strong position in the LSA market.

    You can tell quite a lot about an airplane's powerplant by its model number, for example the Continental GTSIO-520C features...

    Geared propeller, Turbo-Supercharged, direct fuel Injection, horizontally Opposed, 520Cubic Inches; for number of cylinders, you have to know that, for the same displacement, Continental favors six and Lycoming four.


    envelope, flight
    A representation of limits that characterize the performance of an aircraft in varions operating modes, as exemplified for a light aircraft in level, non-maneuvering flight in the diagram on the right. Observe...
    • Stall Speed sets the lower boundary on airspeed at all altitudes dominated by induced drag and is seen to increase with altitude: This edge of the envelope mandates that one must move faster through the air the higher one flies to keep the plane from stalling.
    • Drag Limit sets the upper boundary on airspeed at lower altitudes dominated by parasite drag and is seen to increase with altitude, as the rarified air reduces its resistance to flight, allowing the plane to fly higher and faster until the...
    • Engine Limit sets the upper boundary on airspeed at higher altitudes dominated by the volumetric efficiency of the engine in developing power for thrust, decreasing with altitude and thus slowing the plane until reaching...
    • Maximum Altitude (Service Ceiling) at which the Engine Limit boundary intersects the Stall Speed boundary at the top of the...
    • Level, Non-Maneuvering Flight Envelope, such that an aircraft flying anywhere within the green area can fly at a constant altitude with extra power available for climbing to a higher altitude, increasing airspeed, maneuvering in turns, and overcoming turbulence.
    • Maximum Speed for level flight is seen to be defined at the intersection of the Engine Limit and the Drag Limit.
    • Minimum Drag is characterized by induced drag and parasite drag being approximately equal, which occurs typically at 1.3 times stall speed, and is seen to determine the
    • Maximum Distance the flight can achieve in the absence of headwind.
      Unlike weight-and-balance, which imposes boundaries that must never be crossed, flight outside the envelope depicted here may be safely accomplished by maneuvers from inside the green areaZooming, for example, is a transient maneuver which trades airspeed for altitude.  Within  redline limits, diving trades altitude for airspeed and can be sustained so long as there is plenty of altitude AGL to spare. See "pushing the envelope."


    ETA Estimated Time of Arrival at a fix or destination (preferably GMT). Distinguished from ATA, Actual Time of Arrival.

    ETE Estimated Time Enroute to next fix (usually in minutes) or to destination (hours and minutes), critical for estimating fuel requirements. Distinguished from ATE, Actual Time Enroute.

    FAA Federal Aviation Administration, U.S. Government institution responsible for regulating all aspects of aviation. 

    FADEC Full Authority Digital Engine Control. 

    Just the thing for 21st century flying, especially in light aircraft with diesel engines that burn jet fuel.  FADECs monitor and simplifies engine operation (throttle, propeller, mixture), enable single lever power control, support improved engine starting , reduce fuel consumption through parameter optimization, and perform real-time data logging.
    fail-safe Does not mean the same as fail-proof.  Everything made by man fails (same for all other things, come to think of it).
     
    A fail-safe system incorporates redundancy ('back-up'), so that an isolated failure (‘single-point-of-failure’) shall not result in catastrophe. Moreover, a single-point-of-failure must not go undetected (‘unrevealed’ is my term for it).   That would obviate redundancy. 

    The popular juxtaposition ‘safety-and-reliability’ has meaning only by virtue of systematic measures that assure the joint unlikelihood of simultaneous multiple failures.

    Piston engines have dual ignition systems, both independent of the plane's electrical system, with two spark plugs in each cylinder energized by separate magnetos that are individually tested before take-off.  Planes also have two wings; however,...never mind.

    FARs Federal Aviation Regulations ("regs").

    FBO Fixed Base Operator, an airport concessionaire (sometimes also the airport owner) who offers vital services, including fuel, repair, and ground transportation.

      Some FBOs run charter services and sell or rent airplanes. The etymology of FBO is unknown to this author. I like to suppose that it was originally a term of distinction: the early 'barnstormers,' always on the move, would not have qualified for any term with 'fixed' in it. {Egg Plant on Wheels}

    FBW Fly-By-Wire, flight controlled from the cockpit by electronic signals in place of direct mechanical connections to stick and rudder pedals via cables and hydraulic lines, augmented by computer processing of sensor information to fulfill pilot commands efficiently and safely.

    FIKI Flight into known icing.  Not recommended -- forbidden, actually -- except for aircraft equipped with de-icing features and a pilot who knows how to use them.

    final, (final approach) See pattern.

    fix Location in space at a particular time reckoned by the pilot or determined automatically (see GPSLORAN) during flight. 

    The term 'position' (the P in GPS) is most frequently used to describe a flight's location with respect to a feature on the ground.  The word 'waypoint' refers to a 'position' along a planned course.
    flap Surface on the in-board trailing edge of each wing, which when extended deepens the effective thickness ('camber') of the wing, changing the relationship between lift and drag.

      Strictly speaking, flaps do not increase lift, which in steady flight equals the weight of the aircraft. With the flaps extended, stall speed is lessened, permitting slower flight for landing. (Cowl flaps are a different thing; they control the cooling of the engine. Marital flaps are cockpit discussions resulting from both partners holding pilots' licenses.)

    flare Increasing pitch, nose up, just prior to landing, a rounding-out maneuver intended to transition from descent on the glide path to horizontal motion -- on the runway, of course...
    There are three simple rules for making a smooth landing.  Unfortunately no one knows what they are.
        -- Old Aviation Saying
    Sitting in the right seat of a Skylane, I watched with admiration while the pilot executed a perfect approach in a stiff but steady wind at Flagstaff, Arizona, flaring just right so that, with the plane nearly stopped, the wheels might 'smooch' the pavement without even squealing -- except for one thing: I could see the shadow of the right main gear wheel on the runway four feet below the right main gear wheel. The pilot peered over the instrument panel and grinned, preparing to taxi off the runway and accept my applause. Then all at once: ka-thunk. Glad I was there.

    flight level (FL)  Assignment by ATC for IFR altitude separation of flights above transition altitude of 18,000 feet MSL, which is safely higher (AGL) than highest terrain in the U.S.

      FL is given in multiples of 500 feet using standard sea-level atmospheric pressure of 29.92 inches of mercury.  History of the concept provided by Altitude Telemetry.   

    flight log A document created  before take-off and updated by the pilot while aloft to keep track of information essential for the safe execution of the flight. Here is an example of a modern flight planning form, specifically designed to fit on a knee board
     

    Conventional abbreviations are abundant, including ALT altitude (MSL), ATA actual time of arrival, ATE actual time enroute, Dev (magnetic) deviation, ETA estimated time of arrival,  ETE estimated time enroute, LEG leg (distance or fuel consumed between waypoints), MH magnetic heading, REM (distance or fuel) remaining, TC true course, TH true heading, Var (magnetic) variation, WCA wind correction angle.

    flight, perfect That perfect flight repays the requisite diligence deserves an artful treatment.

    flight plan A document prepared by the pilot and submitted to Flight Service (in person, by phone, or by radio).
     

    Under VFR, filing a flight plan is optional. The pilot 'opens' the flight plan shortly after take-off and 'closes' it just prior to landing (or by telephone after landing). An overdue flight triggers search-and-rescue operations along the flight plan route. Under IFR, a flight plan is required and forms the basis of a clearance.

    Flight Service Part of the services offered to all pilots by the FAA. Briefers at hundreds of locations reply to pilot requests for weather information -- information, not advice -- by telephone and radio. They operate a network over which flight planning information is communicated.  Also see DUAT.

      I'd rather be down here wishing I were up there than up there wishing I were down here.
          -- Old Aviation Saying
    flying, real What birds and light airplanes do (see Doggerel in the Sky).

    FMS Flight Management System, subset of glass cockpit, flat-screen depicting an ensemble of flight and navigation instruments plus other panel gauges.

    FOD Foreign Object Damage,  can result in a "turbine stall" as the air flow is disturbed by the ingestion of snow, sludge or, ugh, a bird in a jet engine.

    As with a stall, the remedy in your owner's manual may be counter-intuitive: Immediately pull back on the thrust lever for the affected engine.  FOD will become increasingly relevant with the growth of the VLJ fleet. 
    fugoid See phugoid.

    fuselage The central body portion of an aircraft designed to accommodate the crew and the passengers or cargo.

      Fuselage is one of the few pure aviation terms and was derived from the French word (spindle-shaped). Other French contributions: aileron, empennage, and longeron.

    gate, approach Imaginary point in space that marks the beginning of the common path to a given runway, most commonly on an ILS approach.
    The term was appropriated by air traffic control in the 1950s from railroad parlance, wherein gates are marked by wayside signals to protect trains from one another in approach to an "interlockings" (yes, with an 's').  See "Rails in the Sky."
    GCA Ground Control Approach. The GCA has two high-resolution radars that track the plane inbound, one for the approach course, the other for the glide-slope.

      The requisite airborne equipment is nil. All you need is a two-way radio. Thus, GCA can be considered a last resort for getting one's backside on the ground when everything airborne has run amuck.

      The special GCA radar facilities are rare -- most have been de-commissioned. Unless there's an emergency, you have to make an appointment to get some practice.

    GCT Greenwich (England) Civil Time, historic predicessor to GMT.
     
    At the time of Amelia Earhart's last flight in 1937, worldwide agreement had not yet been reached for referencing time zones to a common meridian.  Indeed, the radiomen aboard USCGC Itaca, the picket ship loitering hard by Howland Island awaiting the arrival of the flight, were using a local time zone displaced on a half-hour from GCT (+11:30)!  The primitive radio protocols of the 1930s with their staggered transmitting and receiving times ("transmitting quarter past the hour, listening quarter to the hour") resulted in blocked two-way communications, with tragic consequences.

    General Aviation What's left over when you subtract out military and airline operations. You have your corporate jets, your crop-dusters, and -- well, please don't ask, "Oh, you mean like Piper Cubs?"

    The Piper Cub is the safest airplane in the world; it can just barely kill you.
         --  Max Stanley, Northrop test pilot
    Along with Beech and other companies, Piper manufactures many fine General Aviation aircraft today -- always with their wings on the bottom, by the way.  Piper Cubs are history.  The most popular private planes are made by Cessna, and like the venerable Cubs, they have their wings (and petrol tanks) on the top, which means no fuel pumps to fail, no interferences with spectacular views out the windows for passengers, and no ungraceful entries for ladies in miniskirts.

    All low-wing birds must be extinct.
        -- Old Cessna Pilot
      Private flying is a scarce privilege, almost uniquely American. Nobody, though -- not even an aviation enthusiast -- would say that private flying is the most pleasurable activity in the world. That terminology, by convention, is reserved for something else. For many persons, a trip in the sky aboard a light airplane ranks as a close second. Not everybody agrees.

    GMT Greenwich Mean Time, also called 'Zulu.' Noon in November in New York is 'one-seven-zero-zero Zulu.'  Being gradually displaced by CUT (Coordinated Universal Time).

    GPS Global Positioning System, precise, feature-rich navigation system using signals received from a fleet of low-orbiting satellites.  Also see FMS.

    ground effect Increased pressure underneath the wings produced by flying close to the ground. An overloaded airplane taking off on a hot humid day may not be able to fly without it. Upon reaching the airport boundary, such a hapless airplane can experience another kind of ground effect.

    groundspeed The speed (in knots usually) at which the airplane moves ('makes good') over the ground. Distinguished from airspeed. With a 'tailwind,' the groundspeed is faster than the airspeed. With a 'headwind,' the groundspeed is, alas, slower.

    GUMP Abbreviated pre-landing checklist: Gas (fuel selector on the fullest tank), Undercarriage (that's "landing gear, Old Chap"), Mixture (full rich), Propeller (highest RPM).
     

    On an instrument approach, the pilot has plenty of things to do.  The author concocted "Check freak killers and Miss High Time's Speed" for pre-landing checklist: set up approach frequency (freak); study approach chart for obstacles and terrain hazards (killers), recite the missed approach procedure (miss), review decision altitude (high), set timer for missed approach limit (time), and slow to approach speed (speed). After all that, GUMP.

    glide Un-powered descent (see throttle).

      For a typical light plane, gliding at 65 knots, the descent rate is 600 feet per minute (about 6 knots in the downward direction).

    gyro A spinning mass inside certain instruments vital to the safe conduct of flight under IMC. There are three such instruments.

      The 'artificial horizon' or 'attitude indicator' is the primary gyro instrument. Because of the idiosyncratic behavior of the magnetic compass, the pilot relies instead on the 'direction gyro' or 'heading indicator.' Finally, there's the old 'needle-and-ball' or the new 'turn coordinator' that tells the pilot the rate at which the plane is turning and whether the turn is 'coordinated' (yaw in balance with roll).

    hangar An enclosure for housing aircraft. From Medieval Latin angarium, shed for shoeing horses.

    heading The direction the plane is pointed with respect to True North (true heading) or Magnetic North (magnetic heading).  Because of wind, heading does not necessarily correspond to the intended course, the latter achieved by a 'cross-track correction' (see crab).

    headway Separation, measured either in nautical miles or time between successive aircraft flying in the same direction on a common airway at the same altitude.

    Headway has made its way from nautical terminology to aviation via railroading (see approach gate).  Leeway is another transportation  concept that ought to be brought into aviation (see "Headway vs Leeway").
    HIRL High-Intensity Runway Lights, a row of lights located at the approach to a runway that flash in rapid succession to provide a visual reference at the conclusion of an ILS approach.
    For the story of its invention, see "Flash in the Sky."
    holding pattern In planes -- unlike trains and automobiles -- one cannot just tell the vehicle to stop when there is congestion ahead. The holding pattern was invented to cover that case. It is part of the concept of a 'clearance limit' under IFR.
     
    The holding pattern is shaped like a race track in the sky.  Actually a stack of them, each separated by a thousand feet. One plane at a time can occupy any particular level. Think of a half dozen Indianapolis 500's piled up over a navigational fix. 

    The holding pattern comprises four parts, each requiring one minute.  (1) Upon reaching the assigned fix, you make a 180-degree turn at three degrees per second; (2) fly outbound for one minute on an assigned heading; (3) make a 180-degree turn back inbound at three degrees per second; (4) "home in" on the fix, like cruising along the straight-away at Indianapolis.

    hood A large plastic hat-like contraption with a drooping brim that blocks the pilot's view out the window, thereby simulating instrument meteorological conditions (IMC)..

      The IFR instructor acts as a safety pilot to watch for other airplanes. The weather might be perfectly clear, but the student 'under the hood' must conduct the whole flight by reference only to instruments. {Under the Hood}

    homing Steering toward a waypoint or destination, often guided by ADF tuned to a navigational signal such as a beacon
    As the aircraft approaches the homing source, navigation errors decrease, reaching zero overhead the beacon.  For any outbound course, of course, navigation errors increase with distance, much the same as for dead reckoning.
    The phrase honing in on may be an imperfect appropriation for popular usage.
    IFR Instrument Flight Rules, a rigorous set of principles and procedures that assure safe operation into and out of airports and enroute separation from other aircraft, in weather conditions that preclude operating under VFR, Visual Flight Rules.

      One requirement is called a 'clearance,' which constitutes a contract between the pilot and flight controllers that sets forth the de-fault procedures to be followed if a break-down occurs in air-to-ground communications.

      The informal expressions "flying on instruments" or "flying IFR" mean controlling the plane by reference only to gauges on the panel, especially those that incorporate gyros, a mode of flight necessitated by "IFR conditions," which is a phrase now generally replaced by "IMC" for "Instrument Meteorogical Conditions."

    ILS Instrument Landing System, electronic equipment that guides an airplane precisely onto the runway.

      Two of its subsystems, 'localizer' and 'glide slope,' are radio transmitters on the ground which define respectively the straight-in course over the ground and the vertically slanting path to the touch-down point. A pair of needles on the panel provide the requisite guidance. On my approaches, it looks like a sword fight.



    ILS Approach, a sequence of essential tasks performed by the pilot in transitioning from cruising flight through to touchdown using the ILS, typically including at least 18 steps.
     
    1. Fly level at initial approach speed on an assigned heading.
    2. Intercept the localizer (vertical needle).
    3. Turn to localizer/runway heading.
    4. Become "established" on the localizer, correcting for crosswind.
    5. Recognize "outer marker" beacon, report to control tower.
    6. Deploy landing gear and extend partial flaps.
    7. Intercept the glide-slope (horizontal needle).
    8. Reduce power to final approach spead
    9. Become "established" on the appropriate descent-angle.
    10. Recognize "middle marker" beacon.
    11. Extend full flaps and maintain glide-path 
    12. Descend to "minimum descent altitude."
    13. Apply power and maintain altitude if the runway is not in sight.
    14. Recognize runway approach lights
    15. Report to control tower "Landing assured."
    16. Reduce power, kick out crab-angle, flare-out, and touchdown.
    17. Retract flaps, apply brakes.
    18. Speed permitting, turn onto assigned taxiway.

    IMC Instrument Meteorlogical Conditions (see IFR).

    iniquity Flying under the influence, with passengers. Compare to stupidity.

    intersectionA fix or waypoint defined by the crossing of radials from two omni stations.

      A number of years ago, the FAA changed all intersection names to five-letter words, ostensibly pronounceable. Computers again, folks. Back when I started flying, there were no such constraints. Intersections were named for what underlay them, usually a city. Thus, Anaheim became AHEIM; Tustin, TUSTI; El Monte, ELMOO. Out in the ocean, we had Albacore, which became ALBAS, and Mermaid, which is now, alas, MERMA.
       
      My flying instructor John, who is featured in several chapters (Two-Four Fox, License to Learn) for his nonchalance about regulations, did  not himself have an instrument rating.  That did not stop him from filing a “pop-up” IFR request during one of my early lessons.  It was a night flight in “actual” IMC. John and I got vectored all over the Los Angeles Basin.

      “Two-Four Fox, report Alhambra intersection," said the voice on the speaker. 

      I was struggling with altitudes and headings, but I watched while John flicked his flashlight all around the LA Sectional in his lap. 

      “Here it is,” he said, “La Habra is 15 miles further.” 

      “Um, John, you mean ‘farther’.” 

      “That’s what I said.  Hold whatcha got for another 15 miles.”

      “For distances, the word is ‘farther’ not ‘further’,” said I, pulling his chain.  La Habra is 15 miles farther than Alhambra.” 

      John dismissed my pedantry with a shrug.

      “Besides, John, didn’t that guy tell us to report at the ‘Alhambra’ intersection not ‘La Habra’?”

      He shined his flashlight on the chart and handed me the mike. “Yeah, guess you better tell ‘im we’re there.”

      Being limited by computer protocols to five letters might not always be much of a hindrance after all (ALHAM vs LAHAB)

    knot Nautical mile per hour (MPH): 100 knots corresponds to 115 MPH.

    leeway Nautical terminology, which, alongside headway, has meaning in making the distinction between, say, the flexibility of General Aviation and scheduled airline operations.

    lift The upward force of the air upon the wings.

      In level flight, lift acts vertically (opposing weight) and produces -- well, level flight. When you bank the plane, part of the lift acts horizontally to cause a turn.

    "line-up and wait"  A clearance given by the Local Controller that authorizes a plane ready for departure to move onto the runway in preparation for "cleared for takeoff" (changed in 2010 from "taxi into position and hold").

    Local Controller The person in the Control Tower who authorizes takeoffs and landings.

    longeron A fore-and-aft framing member in an airplane.

      Longeron is one of the few pure aviation terms and was derived from the French word (to pass along). Other French contributions: aileron, empennage, and fuselage.

    LORAN Long Range Navigation system applies an onboard receiver to process signals from a set of ground-based, low-frequency transmitters, generally replaced now by GPS.

    LSA Light Sport Aircraft [Work in Process]

    mayday internationally recognized distress signal via radiotelephone (from the French m'aider), shortening of venez m'aider "come help me!"

    MFD Multifunction Display, a member of the expanding family of flat-screen, glass cockpit instruments.

    mile Two sizes: statute mile (sm 5,280 feet) and nautical mile (nm 6,080 feet), resulting in two different units for airspeed and winds aloft

    You can get from one to the other by multiplying or dividing by 1.1515, but there's seldom enough time for that. The nautical mile corresponds to one minute of latitude.

    Since the 1960s only nautical miles are used in aviation, which, like GMT, was a long-overdue standardization for safety (see Which Way, Amelia?). 

    MSL (Above) Mean Sea Level, the measurement of altitude provided by the airplane's altimeter. Compare to AGL, Above Ground Level.  Also see flight level.
    Illustrated on the right is the altimeter indication for a plane at 6,264 feet MSL.

    navigation Determining one's location and guiding the control of heading to achieve a desired course.
    Appropriated by aviation from nautical parlance, the term is gradually being replaced by avigation.
    Navigator's Definitions: 'Latitude' is where we are lost, and 'Longitude' is how long we have been lost there. 
           -- USAF Navi-guesser - Woo Hoo
    NDB Non-directional beacon.

    needle-and-ball Obsolete predecessor to the turn-coordinator.

    NOTAM NOtice To AirMen.

    Never mind the gender-specificity, NOTAMs are official bulletins distributed from the FAA by subscription and via up-to-the-minute communications over the radio from Flight Service, obligatory information generally for the safety of flight.
    north Direction toward the North Pole (true north) or toward a site located in Eastern Canada (magnetic north), a reference used to determine an aircraft's heading as measured by a compass.

    numbers, phonetic Officially, there are only twelve ('one-two') numbers used in radio communications: Zero, One, Two, Three (pronounced tree), Four (fow-er), Five (fife), Six, Seven, Eight, Niner, Hundred, Thousand (tou-send).
     

    An altitude of 12,500 feet is said "One-two tousand, fife hundred." You do hear pilots taking shortcuts ("twelve point five") but never controllers. By the way 'point' is supposed to be said "day-see-mal."

    The numbers ten, eleven, and twelve appear only preceding o'clock to designate the relative bearing of traffic or landmarks. 'Twelve o'clock' means straight ahead, 'eleven o'clock' means 30 degrees to the left of straight ahead, 'one o'clock' means 30 degrees to the right, and so forth. Pity pilots who have grown up with digital watches.

    In the expression, "inbound with your numbers," the term is used to summarize the current operating conditions and procedures at an airport to save repeated instructions from the controllers (see ATIS).

    "Landing on the numbers" means touching down on the painted numerals -- hey, at the beginning of the runway.

    omni Short for 'omnirange', the most common radio navigation device in current use (also called VOR). The term is used here for both the ground facilities and the flight instrument. Unlike ADF, reception of a VOR signal is limited to 'line of sight' from the station to the aircraft.

      There are hundreds of omni-range stations in the United States and elsewhere. Each defines a set of 360 invisible 'radials,' each resembling the spokes of a wheel and corresponding to the magnetic bearing of the plane's position (not the airplane's heading). Certain radials are shown on charts connected together forming the 'Victor' airway system.

      The pilot first tunes his or her omni receiver to the frequency of a nearby station. Then, by centering a needle, the magnetic course to or from the station can be determined.

    over Spoken at the end of a radio transmission -- only when required to inform the listener that a reply is expected. Movie-makers take note: the term does not routinely conclude every transmission.

    pattern, ("landing pattern") A rectangular flight path associated with each runway, one side of which is the runway. It forms the conventional framework for controlling the orderly flow of aircraft to and from an airport.

      There are five legs: upwind (straight ahead immediately after take-off), crosswind (a ninety degree turn at the departure end of the runway either left or right depending on the published pattern for that runway), downwind (parallel and to the side of the runway), base (perpendicular to the runway at the approach end), and final (lined up with the runway, descending for touch-down).

      Generally, you depart on a 45 degree angle from the upwind leg and enter the pattern on a 45 degree angle to the downwind leg ("on the forty-five"). At controlled airports, you may request such alternative procedures as a "straight-out" departure or a "straight-in" approach. And sometimes you will get cleared for a mouthful: "mid-field crossing downwind entry." Listen up.  

      "Cutlass Three Seven Romeo, Orange County Tower, continue on the forty-five, for initial approach to Runway One-Niner Right, remain west of the airport, turn right abeam of the tower, make mid-field crossing to a downwind entry for Runway One-Niner Left and report turning base." 

      "Yeah, I can do all that.  Thanks." 

       "I thought so.  You're welcome."

      In Canada, by the way, MFCDE is the standard procedure for pattern entry.  It has a significant advantage: altitude separation.  Since entry takes place directly above the runway, any other aircraft in the pattern at that location are rolling along the ground -- well below the entering aircraft's pattern altitude (typically 800 to 1,000 feet AGL).

    PFD Primary Flight Display, a flat-screen glass cockpit that combines FMS and SVS.

    P-factor A tendency for a single-engine aircraft to yaw to the left at high angle-of-attack, most pronounced immediately following take-off and during climb-out.  In a twin, the P-factor defines the 'critical engine'.
     

    As the wheels lift off the ground at the beginning of your first flying lesson, it will feel like your plane has hit a patch of ice.  Your instinct will be to turn the "steering wheel" to the right; however, that will only make matters worse (see adverse yaw).  At that moment -- and often thereafter -- your instructor smirks and hollers over the sound of the engine, "Get on that right rudder pedal!"

    The propeller customarily rotates clockwise as viewed from the rear. Drag in "air screw"  might be expected to bank the aircraft counterclockwise; however, the aelerons have much greater mechanical advantage in the roll axis than the propeller tips, making any banking effects negligible.  So why does the P-factor show up in yaw?

    Several explanations are offered in aviation literature. One suggests that the empennage is rigged in the factory to match airflow forces passing along the fuselage and optimized for cruise speed.  Those forces are not exactly balanced in a climbing maneuver.  At full climb power and with reductions in airspeed toward a stall, that effect would decrease not increase.  Another explanation has it that the vertical stabilizer gets pushed to the side by clockwise spiraling propwash.  However, that would also decrease with speed, as more time will be allowed for the spiraling component of airflow to dissipate before the rear of the plane can catch up to it.  Gyroscopic 'precession' of the engine and propeller may contribute to the P-factor, but that will manifest itself only as a transient torque during pitch rotation not as a steady-state condition while climbing.  Oh right, the gyroscopic torque acts in the opposite direction to the P-factor, yawing the aircraft to the right not the left.

    The best explanation is based on the observation that in a climb the upward inclination of the fuselage tilts the plane-of- rotation of the propeller away from its normal orientation -- perpendicular to the flight path.  Accordingly, the downward-moving blade on the righthand side of the aircraft is advancing at an angle into the relative wind and thus developing more thrust than the upward-moving blade on the lefthand side, which is retreating out of the relative wind.

    phugoid  Often spelled fugoid -- different ancient roots for the same word, which means "flight" in the sense of fleeing (think of fugitive).  The term is used in the phrase "phugoid maneuver" to describe the oscillations, most commonly in pitch, of an aircraft that is allowed to fly on its own accord (see diagram).
     

    A small atmospheric disturbance, can cause a plane, which has been trimmed for level flight hands off, to pitch nose downward, say.  Airspeed increases, which increases lift, raising the nose back up again.  With no corrections by the pilot, inertia produces an overshoot into a nose upward attitude.  Airspeed now slows, which decreases lift, lowering the nose back down again.  For a light aircraft the period of oscillation might be as long as 30 seconds.

    Pilot-in-Command (PIC) has ultimate authority over all aspects of flight. 
     

    In theory, the PIC can decline to comply with clearances given by Air Traffic Control (ATC).  Any assignment by ATC --  altitude, heading, route, runway, speed -- can be rejected by the PIC.
    There is one exception.
    Hold Short (of an active runway) may be the only mandate in aviation -- the only utterance by ATC -- that must be obeyed by the PIC.

    ATC and the PIC collaborate to assure safety.  Thus, as a practical matter, the PIC will negotiate with ATC to resolve clearance issues -- in flight.  On the ground, though, ATC's "hold short" holds primacy.

    pilotage Term preferred by pilots in place of dead reckoning.

    pitch Rotation of an aircraft about a horizontal axis perpendicular to the direction of flight (nose-up / nose-down), controlled by the elevator

    Pitch is also used to describe the 'bite' of the propeller.
    The propeller is just a big fan in front of the plane used to keep the pilot cool. When it stops, you can actually watch the pilot start sweating
         -- Old Aviation Saying
    pitot tube External pressure sensing device, an essential part of the pitot-static system that serves the airspeed indicator plus altimeter and vertical speed indicator.
    Named for its inventor, Henri Pitot (1695–1771) a French hydraulic engineer, the pitot tube has an external opening on an arm projecting into the windstream that registers "stagnation pressure" (also called "ram pressure") and a port on the side of the aircraft that registers "static pressure."  The difference between these two pressures indicates airspeed.  The altimeter is an aneroid barometer.  It uses the static pressure, which is not static of course but varies with altitude.  A third instrument, the "vertical speed indicator," measures the rate of change in static pressure.

    The pitot-static system is elegant in its simplicity, with no moving parts.  There are occasions, however, when faulty airspeed indications can result in tragedy (Colgan 3497 and AF 447).  The pilot must be prepared to reject indications that don't jibe with elementary realities of flight...
     

    Landing a rented Cessna 172 at Warner Springs in a strong crosswind from the left, I set up a radical side-slip.  With left-wing down at some 30 degrees bank angle, I throttled back.  Thus, though we would touch down on the left side first, the plane's wheels were all lined up with the runway and I was grinning.  Then I glanced down at the airspeed indicator.  It was showing the plane to be flying at under 20 knots!  Note the exclamatory punctuation.  Meanwhile, the needle on the vertical speed indicator was pointing down at more than 1,500 feet per minute!  Oh right, and the altimeter was reporting that the plane had already descended to an altitude below the runway elevation!

    After the landing I reminded myself that, to save manufacturing cost for the 172, the Cessna people install only one pitot-static port (a small hole) in the fuselage -- on the lefthand side.  In a side-slip to the left, the static port is picking up increased air pressure, reducing its difference from the ram-pressure and thus reducing the indication of airspeed.   Also, the increased static pressure tricks the instruments into [a] an erroneously low altitude and [b] an exceptionally high rate-of-descent.

    More advanced designs (Two-Four Fox, for example) have static ports on both sides of the fuselage, which balance out the effects of uncoordinated maneuvers.

    position (see fix).

    position report Message spoken by the pilot over the radio during flight.  Under IFR , position reports are mandatory, for they clear the block of protected sky behind your flight for another flight.  Under VFR, position reports are optional procedures for safety -- to reduce the search area (ugh) following a mishap in the sky (see anecdote in Taciturn). 
     

    The minimal format has a venerable mnemonic PTAND for Position Time Altitude ETA Next (fix) & Destination.  On July 2, 1937, Amelia Earhart might have given a position report as follows:
     
    USS Ontario, Electra NR16020, VFR from Lae, New Guinea, landing Howland Island.  Position abeam Nukumanu Island, 0618 GCT eight thousand; estimating overhead your location at 0955 GCT; estimating Howland 2022 GCT.  Sky clear above cumulus clouds; wind 090 degrees at 23 knots; making good 131 knots; standing by 6120 kilocycles.
    pushing the envelope The expression began in aviation.  With the presence of VLJs in the aviation fleet, private pilots may approach transonic regimes and will encounter forbidden corners of the flight envelope
     
    Every aircraft has permissible airspeed ranges and altitudes, redlines and ceilings -- the  limits that characterize performance in level flight or while maneuvering.  Plotted on a graph, these parameters form a set of intesecting lines and curves enclosing an area called the flight envelope, a graphical representation for constraints in the sky. 

    In commissioning each new aircraft, some test pilot must take a prototype aloft and execute exploratory maneuvers to ascertain the extremities of the design -- to find the  edges and corners of the flight envelope.  Test pilots have colloquialized the most hazardous flight test procedures in a phrase tinctured with bravado -- pushing the envelope

    Metaphorical appropriations, "pushing the envelope," from jet-age flight testing has achieved popularity for characterizing risky behavior in business and government, referring to the act of  introducing new ideas in readily established concepts. 

    An example would be a marketing campaign which advertised its product in a way never done before seen, or a video game which interacted with the player in a completely original fashion.

    ramp See tarmac.

      A pregnant pilot who is a parent of an only child would describe her condition as "one in the hangar, one on the ramp."

    range Normally a measure of distance but also used to indicate an azimuth or bearing, as in omnirange.
    Oldtime pilots will remember, without fondness, low-frequency radio range stations, which were decommissioned back in 1970.
    real-time Completion of essential tasks at a pace matched to that at which vital action is required. Often characterized by relentless data rates and unforgiving consequences.
    One of the most useless things for a pilot is one second ago.
         -- Old Aviation Saying
    regs Nickname for FARs, Federal Aviation Regulations, that govern all aspects of flight in the U.S. (see FAA).

    roger Spoken on the radio, it means "I understand and will comply." 'Affirmative' -- not roger -- means 'yes'.  Etymology: from Roger, circa 1941 communications code word for the letter r (now replaced by Romeo) -- used especially in radio and signaling to indicate that a message has been received and understood.

      The term is seldom used except to offer commiserations for flight delays or turbulence.
        "Gettin' a lot of chop up here at 7,000."

        "Roger that." 

      Unlike in the movies, pilots repeat back instructions, often in abbr. form.
       
      "Cardinal Niner One Four, two miles north of MAGIC, turn right to One Five Zero, maintain three thousand five hundred until established on the localizer; cleared for the ILS Runway Two-One approach, contact Watkins tower 121.1 at the outer marker."

      "Niner One Four, right, One Five Zero the heading, three point five 'til established, cleared for the approach, tower at the marker."

      Listen in on such conversations on Channel 9 next time you're bored in an airliner.

    roll Same as bank; both refer to the tilting of an aircraft about an axis along the direction of flight (left-wing-up/right-wing-up, see animation).
     
      "Rolling" is also an atavistic expression used by old pilots like me (pronounced "Rollin'") to acknowledge "Cleared for take-off," referring with joy to the action of the wheels along the runway.
RON Remain Over Night -- Best advice, under certain conditions (example).
    rotation The pitch-up maneuver performed on the runway during take-off, which increases the angle-of-attack causing the wings to provide lift. The time I say "Whoopee."

    RPM Revolutions per Minute, the speed at which an airplane engine swings those paddles around, measured by the tachometer.

    rudderThe movable surface on the trailing edge of the vertical fin. Operated by the action of the pilot's feet upon the rudder pedals, the rudder controls yaw. The fixed part is called the 'vertical stabilizer.'

    runway Numbered at each end according to the magnetic course of an airplane taking off or landing on it.

      The last digit of the course, of course, is rounded. Thus, Runway One Niner has a magnetic direction between 186 and 195 degrees. Movie-makers take note: there are no runways with numbers greater than 36.


    sextant Handheld instrument used in celestial navigation

    In addition to a sextant, the navigator needs a precise time piece (see chronometer), a nautical almanac, and a plotting table.  Then too, the aircraft needs a transparent dome, and the pilot needs a navigator.  All have been made obsolete by terrestrial radio systems (ADF, LORAN, VOR) and by satellite-enabled automation (GPS). 


    shag The verb 'to shag' enjoys a thoroughly attested origin in baseball, meaning to chase and catch fly balls in batting practice and a venerated use in work-ups dating back more than a half-century before it would be appropriated as a leering euphemism for...never mind. The term receives a personal meaning -- for an aspiring aviator -- in Everlasting Shagger.

    skid An uncoordinated maneuver which results from too much rudder (or not enough aileron, your choice) in a turn. Compare to slip.

    An airplane might also skid on the ground, much as any wheeled vehicle.
    slip An uncoordinated maneuver (possibly Freudian) which results from too little rudder in a turn. Compare to skid.
     
      Intentional slips are useful for making 'crosswind' landings (see crab) or for rapid loss of altitude ('forward slip').
    solo An early milestone on the way to becoming a licensed pilot, it simply means flying an airplane alone -- no instructor, no passengers. The ultimate in self-reliance, I like to say.
    For some pilots, a thousand hours of flying experience really means only one hour of flying experience over and over again a thousand times.
          -- Old Aviation Saying
    speed, maneuvering Maximum airspeed at which abrupt, full movements of the flight controls will not result in structural damage to the airframe -- a vital limit when the flight encounters severe turbulence.
    Maneuvering speed is not marked on the airspeed indicator.  Because of its dependence on gross weight, V A must be placarded on the instrument panel  For safety, heavier means faster.  That irony is explained by the fact that increasing weight increases the aircraft's stall speed.  One might want to read the previous sentence again.

    In stalling, the wings are politely released from what I call the "Grasp of Bernoulli."  With a lightly loaded airplane, one must slow down, so that a sudden gust of turbulence will intentionally cause a stall.  You may lose some altitude, which you can spare, but not your wings, of which you need both.

    Flying is not dangerous.  Crashing is what is dangerous.
         -- Old Aviation Saying
    spin An aerobatic maneuver produced by deliberately aggravating an unbalanced stall or a perilous consequence of inadvertently aggravating an unbalanced stall.  Think of only one wing producing lift.  Uh-oh!

    spoiler Narrow flat plates typically fitted along the upper surface of each wing. In normal flight, spoilers lie flat and have no effect on the aerodynamic performance of the wing. However, raised upward into the air flow over the upper surface of the wings, spoilers generate turbulence that reduces lift and increases drag.

    In airliners, spoilers are armed aloft for automatic deployment by "squat switches" in the landing gear to facilitate maximum braking in the runway roll-out.  Expect to have spoilers to operate in your personal VLJ.

    In sailplanes, spoilers are used in varying amounts to steepen the angle of descent as desired by the pilot during landing approach, much like the inverse of an engine throttle (see Two in One Day).

    squawk Post-flight complaint about malfunctioning aircraft equipment, obviously not quite fatal. Also see transponder.

    stack (see holding pattern)

    stall An aerodynamic condition that develops when the angle-of-attack reaches a high value (see stall warning). The air flowing over the top of the wing separates into turbulent eddies, and the wing becomes exceptionally inefficient (see more about stall here).

    Push the controls forward and the houses get bigger; pull back and they get smaller -- that is, unless you keep pulling all the way back, then the houses get bigger again. 
         -- Old Aviation Saying
      One way a stall can occur is flying too slow ("unable to obtain/maintain flying speed," the accident report will say). During training, the student goes aloft to practice stalls -- rather recovery from stalls. Two basic kinds: with power (departure stalls) and without power (approach-to-landing stalls).  The remedy for either is counter-intuitive: immediately push forward on the controls.

      For the airframe, ironically, a stall is the least stressful maneuver except for taxiing (see maneuvering speed).

      A 'full stall landing' is an intentional maneuver resulting in the cessation of flight at zero altitude (AGL) and the lowest possible airspeed.

      The aviation use of the word 'stall' has nothing to do with the engine.  Well, almost nothing (see FOD). 

      After more than a century of aviation, 'stall' is still one of the most poorly understood terms. Aviation should have coined a distinctive term long ago.  This author has proposed "turbulate."  Meanwhile, an aviation expert named Gary LaPook made the following recommendation to this author in 2011... 

    We should find the person who coined the word "stall" in English to denote the loss of lift when the wings are at too high an angle-of-attack, dig him up, and [expletive deleted] on his body. The French don't have this problem: décrocage is their word for 'stall', no confusion with la panne de moteur, engine failure.
    straight-and-level Flight at a constant heading and altitude.
    The definition is easy; the maneuver is not.  In apparent contradiction to Newton's Third Law of Motion, every action (of the pilot) does not produce an equal and opposite reaction (of the airplane).  Flight controls impose forces that are interconnected to each other in counter-intuitive ways.  You will get some idea about that by regarding straight-and-level as a coordinated turn with an infinite radius.
    stupidity Flying under the influence, alone. Compare to iniquity.
      The regs forbid flying less than eight hours after drinking ("Eight hours from bottle to throttle," the saying goes) or when 'hung-over', whatever that means. Objective experiments have shown that, depending upon all the familiar variables, alcohol can adversely affect the pilot's performance of functions essential to flight safety up to -- now get this -- 36 hours following its consumption.
       
        Never trade luck for skill.
             -- Old Aviation Saying

    SVS Synthetic Vision System, provides three-dimensional images derived from GPS databases, an optional feature of some glass cockpits.
    Old-time pilots will doubtless complain that SVS effectively reduces your aviation experiences to nothing more than flight simulator sessions on a laptop.
    tarmac Short for 'tarmacadam,' the paved area adjacent to a hangar or to the side of the runway (also called the ramp).

    TCA Terminal Control Area, positive controlled airspace surrounding the busiest airports, in the shape of an invisible inverted wedding cake -- intended to exclude all aircraft not under the radar control. 

    The concept is incomplete. Airplanes are considered outside the TCA even when they are inside the borders of a TCA. All they have to do is fly above the ceiling or below the floor of the TCA. 

    Flying is a three-dimensional activity. Radar surveillance is not. Without altitude information (see transponder) for all aircraft, there is no way for ATC to assure protection for any aircraft, inside -- or outside -- a TCA.

thermals See unstable air.

throttle Controls the flow of fuel into the engine and therefore the amount of power delivered by the engine. 

    Within limits, the throttle determines the vertical movement of the plane (see elevator). For a given airspeed (the elevator held in a fixed position), one will cause the plane to ascend or descend by changing the throttle setting. Closing the throttle produces a glide -- at an airspeed determined by the elevator. Confusing, huh?
thrust The forward, pulling (or pushing) force produced by the engine(s). It is thrust that overcomes drag.

TLD Triple-Letter Designator, "They talk in TLDs / 'space-speak' if you please."

ADF, AGL, ATA, ATC, CAP, CUT, DME, E6B, ELT, ETA, FAA, FAR, FBO, FBW, FMS, FOD, GCA, GCT, GMT, GPS, IFR, ILS, IMC, LSA, MFD, MSL, NDB, PFD, SVS,TCA, TLD, VFR, VLJ, VOR
top-of-the-green Colloquial expression which means pushing the plane to its maximum performance.
    Several instruments in an airplane (airspeed indicator, engine instruments) have green arcs which mark their safe operating ranges. Some also have yellow arcs or a redline that signify dangerous operating regimes or absolute limits.
traffic Aircraft sharing the same region of sky, often at great distances from one another.
     
    To non-flyers, the term connotes motor vehicles in close proximity, all at the same altitude, of course, accompanied by frustrating delays. 

    Except in formation flying, air traffic is typically separated by miles and thousands of feet. There can be delays, but the popular expression "crowded skies" is nevertheless misleading.

    Imagine vehicular traffic without signals. Or visit Shanghai. There, you will find a white-gloved 'traffic controller' on every corner. Throughout t
    the U.S., traffic control has been automated. Except in the sky.


transponder An airborne electronic device which automatically responds to querying pulses in synchrony with ground radar.  All transponders transmit an assigned ("squawk") code, which identifies the airplane to radar controllers. Only 'Mode C' (altitude encoding) transponders provide a measurement of the aircraft's altitude MSL. Both are displayed on the radar screen in a box near the plane's blip, also called 'primary echo.'
     
    To assure interoperability, two frequencies are used worldwide: 1,030 MHz for interrogation and 1,090 MHz for reply.  The flight crew sets the assigned squawk code into the instrument using four thumbwheel switches displaying octal digits, (limited to cyphers 0, 1, 2...7), capable of supporting no more than 4,096 codes.  Nota bene: code "0000" is not used.  Moreover, for declaring emergencies aloft, the pilot will use any of 64 codes starting with "77"; likewise "76" signals a radio failure to radar operators on the ground, "75" declares a skyjacking in progress, and "12" means the aircraft is operating under VFR.  Thus, a net of only 3,839 discrete codes are available for routine use under IFR..

    For background on the altitude-reporting transponder and its impact on ATC, see the Internet Version of Squawk 1200: A history of the next midair collision.


trim An auxiliary control that permits the pilot to balance out steady forces on the corresponding main control.

    All aircraft have elevator trim. Some also have rudder trim. A few have aileron trim. A perfectly trimmed airplane in smooth air can be flown 'hands-off.'
turn coordinator Modern replacement for the needle and ball.
The instrument on the right is indicating a right turn in progress -- a skidding right turn, actually, with too much rudder.

turbulate Preferred term for stall.  An original coinage published first here. 
An aircraft flying with an angle-of-attack greater than some critical value (15 degrees, typically) will be said to 'turbulate', as the air over the top of its wing breaks away into turbulent eddies, resulting in pronounced loss of lift.  During turbulation, the wings are politely released from what I call the "Grasp of Bernoulli."
turn, coordinated Preferred manuever for changing heading.  Exactly how to do it is not so obvious, as you will learn in your first flying lesson.
 
Say you want to make a right turn in level flight.  You use what looks like a steering wheel in front of you to apply right aileron, and the plane begins banking to the right.  At that moment, the left wing is developing more lift than the right wing and therefore more drag.  The unbalance causes your plane to yaw to the left -- opposite to your intended maneuver. That would be adverse yaw; however, you prevent it by simultaneously applying right rudder.  By the way, the plane has not yet started to turn.  For a few seconds, you continue to fly straight ahead with your wings going steadily more crooked in the sky. Eventually, you will have the 'angle-of-bank' that you intend to use for your turn.  You neutralize the ailerons to stop the roll.  At last, the plane starts its turn.

With the heading changing toward the right, the outside wing (the one on the left) is now moving through the air faster than the inside wing and developing more lift.  The result is that the plane will continue rolling to the right, increasing your angle-of-bank ("over-banking," it's called).  Naturally, you prevent that by applying opposite (left) aileron. In resisting that tendency for the plane to continue rolling toward the inside of the turn, you have intentionally increased the lift on the inside wing and unintentionally increased its drag, yawing the plane to the right.  That, too, would be regarded as adverse yaw, except it is acting in the same direction as the turn.  Still, to avoid over-banking you must apply left rudder.  The plane keeps turning steadily toward the right at a fixed angle-of-bank as you wanted.

Meanwhile, with the wings at an angle to the horizon, some of their lift is being subtracted from holding your plane aloft and used instead to produce the turn.  If you don't do something about that, your airplane will begin to pitch downward, losing altitude.  You prevent that by pulling back on the control wheel which operates the elevator to increase lift on both wings in unison.  By the way, that increases drag in both wings, slightly slowing the airspeed. The plane stays at the same altitude nice as you please; however, part of that extra lift is directed toward the right, simultaneously increasing your rate-of-turn and the tendency for the airplane to over-bank. To prevent that, you crank in more left aileron, which you must balance with more left rudder.  And so it goes.

There you are, all crossed up in the cockpit: banked right, applying left aileron and rudder, watching your heading change to the right while pulling back on the elevator to hold altitude.  Quite counter-intuitive and hardly 'coordinated' when you think about it -- which you soon learn not to do.  Nothing like driving a car, is it.  Oh right, and to stop the right turn, you must go back to the top of this exercise and reverse each action as if you want to make a left turn.  Almost.

Reminder: The purpose of a turn is to take up a new direction of flight, often on an assigned heading.  That imposes a timing requirement for stopping the turn -- a requirement you don't have when you're starting a turn.  Your car-driving intuition will suggest that you should start gradually rolling out level before reaching that new heading so as not to turn too much.  But watch out.

All during your right turn, you are holding left aeleron and rudder.  To stop the turn, you must increase left aileron so that the inside (right) wing develops more lift.  Its increased drag toward the right will resist your effort in stopping the turn, so you must apply more left rudder at the same time.  But how much more?  Too much left rudder will cause you to stop the turn short of the desired heading; too little and you can expect to overshoot it.  That's what you think, anyway.  In reality, the tendency will be to use too much left rudder -- rather to be tardy in reducing it. Here's why: In your perceptions, angle-of-bank is far more obvious than rate-of-turn.  While reducing angle-of-bank, you will unsuspectingly stop the turn almost immediately.  The effect is similar to the way overcoming adverse yaw delays the start of a turn.  Although you won't find this advice in any flight manual, you will discover that the most practical stratagem is to continue the turn until the plane just about arrives at the heading you want, then rack the plane level with plenty of aileron and rudder while holding that heading nearly constant.

With the turn abruptly stopped, the plane still takes a few seconds to finish rolling.  As the wings become level, their combined lift will no longer act in the direction of the turn but instead become fully devoted to holding the plane aloft.  While that is taking place, you must remind yourself to stop pulling back on the elevator, otherwise that extra lift you needed in the turn will cause the plane to nose up into a climb. Whoa!  Simply relaxing the elevator control will not be enough, however.  To regain the airspeed lost during the turn, you will need to push the control wheel forward for awhile.

Unicom Communications channels allocated for air-to-air coordination and air-to-ground service requests, including fuel service, ground transportation, and phone calls.

unstable air A condition of the atmosphere in which small vertical perturbations of the air produce runaway drafts.

"I would rather be down here wishing I were up there than being up there wishing I were down here.
      -- Old Aviation Saying

 
Two separate properties of the air collaborate to produce unstable air.

First, thermodynamics mandates a change in temperature whenever a parcel of air is moved vertically. Air blowing up the side of a mountain, say, experiences a decrease in pressure and consequently cools to a lower temperature ('lapse rate,' meteorologists call it).

Second, the measured temperature of non-moving air generally decreases with altitude ('temperature gradient'). It simply gets colder outside when you go higher up.

Consider what might happen when a parcel of air gets lifted slightly. Its temperature goes down. The surrounding air at the higher elevation is also cooler -- but probably not cooler by exactly the same amount, since there are two phenomena involved.   

  • Suppose that the lifted parcel cools off to a temperature lower than the surrounding air. The parcel will be slightly more dense and tend to sink back toward the elevation from which it was lifted. That's stable air.
  • Suppose, on the other hand, that the lifted parcel remains slightly warmer than the surrounding air higher up. Well, the lifted parcel would be less dense and, instead of going back down, it will be lifted still higher. The cause of the initial lifting (an up-slope breeze, perhaps) has been reinforced by the thermodynamics of the situation, and up goes our parcel of air like a bubble.
On a clear, summer afternoon. the sun shines through the atmosphere and warms the surface. The air aloft is affected hardly at all. It stays cool. The air adjacent to the ground, though, is warmed. The greater temperature difference -- you should excuse the expression -- gives rise to unstable air and the ascending currents of air called 'thermals.'

Best to avoid such convective frenzies. Fly in the morning.

upwind leg See pattern.

VASI Visual Approach Slope Indicators.  Two sets of stationary lights adjacent to the beginning of the runway.  With lenses and masks, the lights form beams, such that viewing them from above a specific angle (the glide slope for that runway) shows white lights and below that angle red lights. If the pilot can see red on the set farther down the runway and white on the closer set then he or she is on the glide slope. White-over-white means the plane is approaching too high and red-over-red too low.

Red-over-white, you're all right. White-over-white, you're high as a kite. Red-over-red, you're gonna be dead.
     -- Old Aviation Saying


Vertical Speed Indicator (VSI ) measures rate of climb or descent calibrated in feet per minute. The instrument on the right is indicating level flight -- level, not necessarily straight.  For that you might refer to your Turn Coordinator.

vertigo Disoriented condition, not knowing which way is up. Originally it meant dizzy. In aviation, vertigo can be induced in some people by turns without a visible horizon, in others by movement of the head during acceleration maneuvers.

 
During instrument flight training, the student spends many hours flying around 'under the hood.' The instructor periodically takes over the controls and asks the student to cover his or her eyes. Then the instructor puts the plane through a sequence of maneuvers -- some gradual, some not so gradual -- designed to obliterate or confuse sensory inputs. After establishing the plane in an unusual attitude, the instructor traditionally calls out, "Your airplane!" Hah, then the fun begins.

The student sits up, blinks, and analyzes the instruments, some of which might have been concealed by jar-lids. Everything goes askew. The plane is either twirling toward earth or clamoring for height. The task is to re-establish straight and level flight as quickly as possible.

That may not be the end of the exercise, however. Sometimes, after curing an unusual attitude on behalf of the aeroplane, the pilot becomes afflicted with a malady of his or her own, vertigo. The student feels like the plane is not level, even though it is, and must consciously struggle not to tilt the controls in obedience to counterfeit stimuli.

VFR Visual Flight Rules, a set of principles and procedures that permit flight without reference to instruments. VFR is dominated by the imperative "see and avoid" (formerly "see and be seen").
Stay out of clouds.  The silver lining everyone keeps talking about might be another airplane.  Reliable sources also report that mountains have been known to hide out in clouds.
     -- Old Aviation Saying
    Generally, to operate VFR, in-flight visibility is required to be greater than three miles. For landing and taking off, cloud 'ceilings' have to be higher than 1,000 feet AGL.
    In the past, 'VFR' was informally used to denote the conditions that permit operating under VFR (replaced by VMC).
VLJ Very Light Jet [Work in Process]

VMC Visual Meteorological Conditions (see VFR).

VOR VHF (Very High Frequency) Omni Range, or omni.

VORTAC See DME

Voyager In 1986, for its historic flight around the world non-stop and unrefueled, a unique, center-line thrust, General Aviation aircraft, the Voyager flew most of the 26,358 miles on its rear (pusher) engine, using its front (tractor) engine for the flight's one and only take-off and for climbing above mountains. Both powerplants were Continentals, with an O-240 developing 130 horsepower in front and a Liquid cooled IOL-200 producing 110 horsepower in back.

V-Speeds a collection of nearly three-dozen different indicated airspeeds pertinent to the safe conduct of flight.  Here are their abbreviated names and meanings (caution is advised: an 'M' can mean either maximum or minimum)... 

 
V A    ~~~~~~  design maneuvering speed.
V B    ~~~~~~  design speed for maximum gust intensity.
V C    ~~~~~~  design cruising speed.
V CC ~~~~~~~ recommended cruise-climb speed (coined here)
V D  ~~~~~~~  design diving speed.
V DF  ~~~~~~  demonstrated flight diving speed.
V EF ~~~~~~~  the speed at which critical engine is assumed to fail during takeoff.
V F   ~~~~~~~  design flap speed.
V FC  ~~~~~~  maximum speed for stability characteristics.
V FE   ~~~~~~  maximum flap extended speed.
V FTO  ~~~~~  final takeoff speed.
V H  ~~~~~~~  maximum speed in level flight with maximum continuous power.
V LE  ~~~~~~  maximum landing gear extended speed.
V LO  ~~~~~~  maximum landing gear operating speed.
V LOF  ~~~~~  lift-off speed.
V MFC   ~~~~  maximum speed for stability characteristics.
V MC   ~~~~~  minimum control speed with the critical engine inoperative.
V MO   ~~~~~  maximum operating limit speed.
V MU   ~~~~~  minimum unstick speed.
V NE    ~~~~~  never-exceed speed.
V NO   ~~~~~  maximum structural cruising speed.
V R  ~~~~~~~  rotation speed.
V REF  ~~~~~  reference landing speed.
V S    ~~~~~~  stalling speed 
V S0   ~~~~~~ stalling speed  in the landing configuration.
V S1    ~~~~~  stalling speed or in a specific configuration.
V SR   ~~~~~  reference stall speed.
V SRO  ~~~~  reference stall speed in the landing configuration.
V SR1   ~~~~  reference stall speed in a specific configuration.
V SW   ~~~~  speed at which onset of natural or artificial stall warning occurs.
V X    ~~~~~  speed for best angle of climb.
V Y    ~~~~~  speed for best rate of climb.
V 1   ~~~~~~  maximum speed in the takeoff at which the pilot must take the first 
                        action (apply brakes, reduce thrust, deploy speed brakes) to stop 
                        the airplane within the 'accelerate-stop distance'. 
V 1     ~~~~~  minimum speed in the takeoff, following a failure of the critical  
                        engine at VEF, at which the pilot can continue the takeoff and achieve
                        required height above the takeoff surface within the takeoff distance.
V 2  ~~~~~~  takeoff safety speed.
WAAS Wide Area Augmentation System, a system of satellites and ground stations that provide GPS signal corrections, giving the pilot a position accuracy of three meters 95 percent of the time. 

warning, stall.  An electric horn or buzzer energized by a wind-activated switch mounted on the leading edge of the wing, which signals an impending stall as the angle-of-attack reaches a critical angle (about 15 degrees for many airplanes).

waypoint (see fix).

weight-'n'-balance, pronounced as one word, although two decidedly different flight parameters are involved.
 

The load-carrying capacity of an aircraft is limited ultimately by the lift of its wings at take-off and the strength of its undercarriage during landing. It is incumbent upon the pilot to tally up the weight of passengers and luggage, fuel and -- oh yes -- airframe to assure that certified 'gross weight' not be exceeded.

The controllability of the aircraft is limited by the 'pitch-authority' of its elevator, which depends on how weight is distributed.

An aircraft is obviously heaviest at the moment of take-off and gets steadily lighter throughout the flight (the only exception is when icing is encountered, but -- hey, it's the pernicious effect of ice on the airfoil not its weight that can bring down a plane).

Not so obvious is the location of an aircraft's center-of-gravity, which can change during flight. 'Nose-heavy' and any hope of 'up' gets to be out of the question. 'Tail-heavy' and -- well, a stall may result, but recovery won't be physically possible.

winds aloft Forecasted as speed (in knots) and direction (true, not magnetic) for selected altitudes (MSL) based most often on measurements by radiosonde or empirically during flight by means of WCA Wind Correction Angle.

yaw Rotation of the airplane about a vertical axis perpendicular to the direction of flight (nose-left/nose-right) controlled by the rudder (animation).

    Yaw is just one of the three axes about which an airplane can be rotated (the other two: pitch and roll). Incidentally, yaw is the only control axis you have for an automobile or a boat. How dull.
yaw, adverse Consequence of an unbalance in wing drag when subjecting an airplane to a roll maneuver for a coordinated turn.

zero-zero Ceiling zero and visibility zero, the extreme opposite of CAVU. Inside a cloud, visibility is zero. At some non-zero elevation AGL, clouds can form a real ceiling, so 'zero-zero' is somewhat redundant, but the condition deserves to be emphasized.ed.
 

Equipped with modern gyro instruments, private aviators can indeed take off safely in a ground fog. At this writing, airliners cannot.

After lining up with the runway by reference to a couple of runway lights, looking through the windshield is a bad idea. Everything depends on the instruments. The most essential information on the take-off roll is the heading. Best to tweak the direction gyro slightly to one side so as not to confuse any of its degree markings with the reference line ('lubber,' in nautical terms).  It's an exciting experience, I can tell you...

Bring the power up smoothly while releasing the brakes. Apply slight back pressure on the elevator. Hold right rudder against the P-factor. Check engine instruments for full power. Feel the acceleration. Adjust heading with rudder. Check airspeed. Heading again. Airspeed for rotation. Refer to artificial horizon and lift nose wheel. Feel lift-off. Vibration in the spinning wheels can be a source of distraction: tap brakes. Wings level. Check heading. Now altitude. Trim for best rate-of-climb. Reach for microphone. "Airborne."

zoom To climb suddenly, trading airspeed for altitude. A good time to say, "Whoopee."

Take-offs are optional.  Landings are mandatory.
     -- Old Aviation Saying

 
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