The flight envelope (environmental envelope) is the space within which an aircraft can operate safely, that is to say within a range of speeds, altitudes, and temperature data.
For example, an Airbus A320 should not exceed a speed of Mach 0.82 (624 mph / 1,000 km/h), must not fly higher than 39,800 feet (12,130 m) and not take-off if the ground temperature is less than -36°F (-38°C) or above 130°F (55°C).
At its maximum altitude of 39,800 feet (12,130 m), the aircraft must remain within a temperature range of between -94°F (-70°C) and -18.5°F (-28°C). Boeing declines to disclose the flight envelope of the Boeing 737.
Before marketing a new aircraft, aircraft manufacturers’ test pilots will carry out several flight tests to determine the aircraft’s limits. After which a safety margin is applied for use by the airlines.