Structural loads in flight - Aircraft flight area

Load calculations play an essential role in the certification process for civil aircraft. It links the aircraft's general – architectural – specification to structural dimensioning, a term to be taken in its broadest sense. Structural design is fundamentally conditioned by "classic" mechanical loads: external aerodynamics (maneuvering, gust, turbulence), internal in-flight loads (pressurization, interfaces, thermal loads) and ground loads (dynamic landing, taxiing). Contemporary calculations also incorporate loads due to the integration of systems into structures, and the dynamic effects of fly-by-wire controls.

In this context, it is essential first of all to address the fundamental elements of the regulations, drawn from long experience in civil aviation, to ensure robust calculation of the envelope of in-flight loads – defined by the flight envelope – authorizing qualification of structures for static loading and lift-related fatigue.

This is the purpose of this article, which sets out the general process and the regulatory definitions involved. Starting from the general framework of civil certification, the stochastic definition of limit and extreme loads leads to the concept of fail-safe structure. The parameters defining an aircraft's flight envelope are reviewed, leading to the definition of three standard envelopes, associated with maneuver and gust cases. These elements are decisive for the dimensioning of the aircraft's primary structures: load-bearing surfaces and interfaces with the rest of the airframe.