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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.

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    $\begingroup$ I'm not aware of any transport airplanes that have demonstrated control authority with flaps full and zero at the same time. You either have asymmetry detection (broken drive shaft detection) or you have a redundant drive line. The Dash 8 1 thru 300s use a redundant drive flex shaft that spans the entire flap drive line to drive the shafts outboard of a broken main shaft. They don't require asymmetry detection. All of the Canadair and Embraer regionals use ends of drive system asymmetric movement detection that is always active. $\endgroup$
    – John K
    Feb 20, 2021 at 2:45

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It is active whenever the flaps are being moved by the hydraulic system on the 737; it isn't available when using the backup (electrical) system.

Allowing asymmetric flap extension before takeoff could have consequences as serious as asymmetric extension in flight. Plus, tying the protection to an air/ground sensor is one more way that the system might fail.

Evacuations are pretty rare events; the chance of having one along with a malfunction as described in the OP is extremely remote. Such an unlikely occurrence doesn't overcome the reasons to keep asymmetry protection available full-time.

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    $\begingroup$ The lack of electric asymmetry detection is probably because an asymmetry event (broken drive line) following a hydraulic drive system failure would be a double failure event and doesn't need to be accounted for in the failure effect risk analysis. $\endgroup$
    – John K
    Feb 20, 2021 at 2:41
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    $\begingroup$ @JohnK Yeah, that'd be (incredibly rare) squared. The QRH tells you to watch the needles (L and R flap position) during electric extension, so you HAVE asymmetry protection - just not automated protection. Plus, the electric motors move the flaps pretty slowly compared to the hydraulics, so any asymmetry that developed would have a slow enough onset the PF could notice it well before it became significant. $\endgroup$
    – Ralph J ♦
    Feb 20, 2021 at 3:25
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    $\begingroup$ Are you just holding a spring loaded switch down to move them in that mode? $\endgroup$
    – John K
    Feb 20, 2021 at 3:37
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    $\begingroup$ @JohnK Yes, the PM holds the switch & watches the flap position indicator. When you get to the desired position, or if an asymmetry starts to develop, releasing the switch stops the motor. The same switch pushed in the opposite direction can retract them (though not the leading edge devices - they're hydraulically actuated only), such as in the event of a go-around. $\endgroup$
    – Ralph J ♦
    Feb 20, 2021 at 4:26

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