On aircraft with flaps, a general certification requirement is that it must not be possible for the aircraft to suffer a loss of lateral control secondary to an asymmetric-flap condition. This requirement can be satisfied in one of two ways (many aircraft go for both):
- Demonstrating that the aircraft remains controllable without requiring exceptional pilot strength or skill, even if the flap(s) on one side are fully up and those on the other side are fully down.
- Including a flap-asymmetry protection mechanism, which automatically cuts off power (hydraulic or electrical, whichever the aircraft uses) to the flap-drive motors if the flaps on both sides fail to move in synchrony.
The benefits of flap-asymmetry protection in flight are obvious; however, in addition to their roles as high-lift devices in flight and high-drag devices during landing and rejected takeoff, flaps are also used to allow airliner passengers and cabin crew to safely egress through the overwing exits in the event of an emergency evacuation on land1 (low-slung aircraft, like the 737 and most regional jets, simply have the passengers slide down the flaps to the ground, while taller aircraft have inflatable slides that deploy over the lowered flaps). In such a situation, as optimal use of the overwing exits depends on the flaps being fully down, having flap-asymmetry protection active would be counterproductive, as a jammed flap on one side would prevent a properly-functioning flap on the other side from providing a good evacuation path, and flap-asymmetry-induced lift asymmetry is harmless on the ground anyways (it produces a rolling moment to one side or the other, but an aircraft is physically incapable of rolling to either side when on the ground unless it’s already at or very close to flying speed, because the ground is in the way).
Is airliner flap-asymmetry protection inactive when on the ground (like many other systems protecting against things that are only a hazard when airborne, such as stall protection [stickshaker/stickpusher] and GPWS warnings) to prevent it from hindering an evacuation, or does it remain active even then?
1: In the case of a water evacuation, the buoyant characteristics of most airliners are such that the water generally comes all the way up to the wings, making the degree of flap deployment, or lack thereof, irrelevant.