The term flight envelope refers collectively to the operating parameters and capabilities of a specific model or type of aircraft. The various parameters that make up a flight envelope include the aircraft’s maximum altitude, maximum and minimum speed, the maximum amount of g-forces the craft can withstand, climb rate, glide ratio and other factors that define the aircraft’s flight characteristics. Pilots are required to be familiar with an aircraft’s flight envelope before being certified to fly the aircraft in order to ensure its safe operation.
All aircraft have an approved flight manual, which contains the parameters. Common aspects of flight parameters found in the flight manual include speed, stall speed, thrust, drag, maximum altitude, and other factors based on the specific wing and engine design. The range of these parameters often leads to an aircraft being described as having small or large flight envelopes.
Flight parameters for commercial aircraft tend to be considered small, as these vehicles are intended to operate under moderate conditions efficiently. A small flight envelope means that the span of speeds and maneuvers the aircraft is capable of is fairly limited. The flight parameters for civilian and commercial aircraft are often also called restricted, which refers to the fact that the local aeronautics authority, such as the FAA in the United States, has reduced the authorized flight envelope to add a margin of safety. Military aircraft, such as jet fighters, tend to have very large flight parameters. Their envelopes are considered to be large because they are capable of a great range of speeds and maneuvers, and they need to function in extreme conditions.
Over time, a number of expressions have arisen relating to flight envelopes. For example, the expression "pushing the envelope" originally referred to military pilots taking their aircraft to the very limits of their performance capabilities, usually during combat. In testing the experimental aircraft of the 1950s and 1960s, pushing the envelope came to mean to increase the known capabilities of a test aircraft by demonstrating its new ability in flight. Proving an aircraft could go higher than it was previously thought to be capable of widened or pushed the aircraft’s known flight parameters to include the new capability. In the non-military world, the expression has come to mean doing something at the very edge of one's capability or thinking of a new way to do something that will exceed or surpass what are believed to be the absolute limits.