push the envelope

This is an aviation term. It means to fly an aircraft beyond the limits it was designed for, especially in the context of flight testing. Envelope has several secondary definitions in mathematical and engineering jargon referring to the area covered by a series of curves. So in the world of aeronautical engineering the envelope is the collection of curves that describe the maximum performance of an aircraft. To push the envelope is to take the aircraft to the edge of what it was designed to do and try and take it beyond.

The mathematical sense of envelope dates to the 19th century. Isaac Todhunter’s 1871 Differential Calculus:

The locus of the ultimate intersections of a series of curves is called the envelop [sic] of the series of curves.

The use in aeronautical engineering dates to the mid-20th century. From the 1944 Journal of the Royal Aeronautical Society:

The best known of the envelope cases is the “flight envelope,” which is in general use in this country and in the United States...The “flight envelope” covers all probable conditions of symmetrical manoeuvring flight.

The phrase push the envelope is recorded in 1978, but it is probably older than that in informal use. From Aviation Week & Space Technology, 3 July 1978:

The aircraft’s altitude envelope must be expanded to permit a ferry flight across the nation. NASA pilots were to push the envelope to 10,000 ft.

Push the envelope is used prominently in Tom Wolfe’s 1979 book The Right Stuff, about test pilots and the Mercury space program, and in the 1983 movie based on the book. Because of this, the term broke out of engineering jargon to become familiar to the general public. From Wolfe’s book:

One of the phrases that kept running through the conversation was “pushing the outside of the envelope.” The “envelope” was a flight-test term referring to the limits of a particular aircraft’s performance, how tight a turn it could make at such-and-such speed, and so on. “Pushing the outside,” probing the outer limits, of the envelope seemed to be the great challenge and satisfaction of flight test. At first “pushing the outside of the envelope” was not a particularly terrifying phrase to hear. It sounded once more as if the boys were just talking about sports.

(Source: Oxford English Dictionary, 3rd Edition)

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