In early 1960, the Swiss physicist and educator, Auguste Piccard visited NAFEC and gave a talk to the FAA research establishment.  He was most famous for having set the world's altitude record in May 1931, two years before I was born.  Piccard went aloft aboard a balloon of his own design, which featured the first pressurized cabin used in flight.  During his lecture, Professor Piccard made a pointed recommendation that would permanently affect air traffic control.

For aircraft flying at elevations above the highest mountains, he observed, the only purpose for measuring altitude is to assure aircraft separation for one another, not for terrain avoidance.  Moreover, the instrument for determining altitude is itself subject to errors.  Altimeters are really barometers, measuring air pressure.  They must be constantly corrected by hand.  Each pilot is required to obtain the local barometric pressure by radio from the ground and enter a precise number in the altimeter's "Kollsman Window," a procedure made cumbersome by jet speeds.

Piccard's recommendation was simple: mandate that all aircraft flying at 18,000 feet or higher set their altimeters at the same -- standard -- setting (29.92 inches of mercury).  Thus was the phrase "flight level" created later as a term of distinction, replacing "altitude" for high flying turbojets.  Flight level 290, for example, corresponds to an altitude of 29,000 feet above mean sea level -- but only on a "standard day" (defined as one during which the local barometric pressure happens to be 29.92 inches of mercury).

Piccard's lecture at NAFEC was especially timely, for I was struggling with a technical problem with the transponder -- how to provide altitude data from the aircraft to the ground by telemetry.  During the applause, following his talk, I made my way to the lectern and introduced myself to Piccard, a tall man in tweed coat and spectacles.  Wisps of silvery hair fringed his pate.

"The airplane knows how high it is," I began.  "Air traffic controllers do not -- unless they ask the pilot over the radio."

Piccard placed his papers in his battered briefcase.  He anticipated my question.  "You vant ze altimeter to talk weez controllers by radio pulses?" he asked.  "Such instrument I haff myself already design-ed."

"Did you modify a standard altimeter?" I asked eagerly.

"Not suitable, not suitable," said he, with a wave of his hand.  "I show you."

While others from the meeting waited to greet him, Piccard drew a sketch on the blackboard and explained altimetry to me as if I were the only person in the room.  It was sufficient for my purpose merely to establish feasibility for the concept.

Ah, but I got much more in Piccard's answer than I bargained for with my question.  The venerable physicist anticipated a problem I had not considered.

"You must no allow ze pilot to adjust zis device," he cautioned.  "Fix standard aneroid pressure in factory.  Yes?"

"Local atmospheric pressure, Professor Piccard -- how do you correct for that?"

"Give ze controller a barometer," he said with a smile that lifted his face.  Someone pushed past me and grabbed his hand.  Piccard could see my puzzled expression.  "A barometer," he repeated.  "And a computer."

It was palm-to-forehead time.  "Insight is accompanied by the sudden release of the tensions of inquiry," said Saint Thomas Aquinas nearly seven centuries ago, describing the experience of that moment.

The matter was settled: my proposal for the peace-time transponder would incorporate "altitude reporting" -- the missing third dimension.

John Navaux wrote: 

In doing some research for a related project I came across your material. With regard to  the "invention " of the flight level concept in 1960 by Auguste Piccard,  with all due respect to you both, the concept of flight levels was already well in place and published in ICAO manuals (Circular 26-AN/23, to be specific) published in 1956 which was the revised edition of the original of 1953. 

Thank you for your informative message.  I am eager to set the record straight.  Prompted by your comments, my own searches on the Internet turned up many references, including this one... 

http://www.icao.int/eurnat/history08.htm 

....with relevant history dating back to 1958, but Circular 26-AN/23 seems not to be accessible nor do I find references to it.  Moreover, I have not found contemporaneous reports for the events you describe.  For example, at the International Civil Aviation Organization (ICAO) website, the expression "flight level" appears in undated reports by anonymous sources as routine terminology already established in their respective working vocabularies. 

A moment of invention marks the beginning -- not the end -- of a challenging journey.  The protracted effort of getting the idea put into practice is just beginning.  This is especially true in the environment of complex institutions with reluctant stakeholders.  The most serious competition for a new technology is the status quo.

If, as your researches show, the need for -- and the discovery of -- the flight-level invention dates back to 1953 and its benefits had not yet been realized by a reluctant industry until 1960, then my speculation is that in the speeches he gave during his sunset years at the fledging FAA, Piccard was in effect lobbying for its appropration
by the U.S. aviation industry, perhaps on behalf of ICAO, come to think of it. 

Mr. Navaux, inventions are made by people not institutions, and it would not surprise me to learn that Piccard was indeed the inventor in 1953 and was in 1960 still slogging along the pathway to assure its adoption, offering his fame as imprimatur and more -- unabashedly motivated by pride of inventorship. 

Best regards, 
Paul Niquette 

Thank you for your reply...

Circular 26 has no doubt been archived many  years ago. I do not doubt that Dr Piccard may have had a hand in developing this from the beginning.

The concept of "flight levels" and the proposed implementation of it has unfortunately varied widely world wide. In the US & Canada for example the transition altitude (to the flight level regime) was set at 18,000 ft, while in Europe and other parts it was implemented based on terrain height surrounding the airport, or the runway elevation. Consequently Europe (counting Eastern EUR) has some 400 or so different TAs varying from 1,350 ft to 16,000 and all altitudes in between, which makes it very confusing. 

Had they followed Dr.Piccard's advice, as you noted, they would have avoided the mess they have now .