Most aircraft have high-lift devices on the wings (such as flaps, slats, droops, whatevs) to allow them to take off and climb out at a reasonable speed instead of having to accelerate to near cruising speed while still on the runway; this enables the aircraft to use shorter runways, but also places it at risk of stalling if these devices are retracted too early in the climb (example).

Given the danger of retracting high-lift devices at too low a speed, why don’t aircraft have interlocks to prevent this from happening?


Most modern airliners have some form or other of mechanism that does exactly that, mostly for the slats (which are more important in stall prevention, anyway).

E.g. on modern Airbusses, there is a Slat Alpha Lock function which prevents slat retraction at high angles of attack, on the Boeings there is a Slat Auto-Gap function extending slats from mid to gapped position at high angles of attack.

Military aircraft often have automatic slat deployment to aid with high angle of attack manoeuvres, and even some vintage aircraft did have aerodynamically activated slat-like devices for stall prevention.

Any sensible reference suggestions welcome for any of these... cannot post mine.

  • $\begingroup$ Airbus, Airbusses, Airbusi? :) $\endgroup$ Dec 13 '18 at 12:41
  • $\begingroup$ @AEhere Surely it must be Airbi... but sacrificing readability for correctness is not always the best tack ;) $\endgroup$ Dec 13 '18 at 12:45
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    $\begingroup$ To add something actually useful, iirc automatic slats were a thing as early as the Mig-3 (late versions), maybe earlier. Sadly I don't have a source at hand right now $\endgroup$ Dec 13 '18 at 12:53
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    $\begingroup$ @AEhere Yes, they had them on the Messerschmitt Bf-109, too. $\endgroup$ Dec 13 '18 at 13:35

While flaps do slightly reduce stall speed, that is not their primary purpose. Planes pass quickly through both Vs0 and Vs1 during takeoff or landing, and pilots are too busy to mess with flap settings during those few seconds anyway. The only reason we deliberately fly inside that range is to practice slow flight, stalls and spins, where such an interlock wouldn't be of any benefit.

During takeoff, low flaps may be used to increase lift and thereby shorten the takeoff distance on short runways or get into ground effect sooner on soft runways. In both cases, though, the climb afterward will be slower due to the added drag, so it's only done when necessary and then pulled out at best climb speed, i.e. well above stall speed.

During landing, high flaps are used to increase drag and thereby descend without gaining speed. You're not even close to stall speed until the flare, and raising flaps before then just means you'd land faster--not stall.

OTOH, while the benefit (if any) would be tiny, there is a very real risk that the device would fail somehow, such as preventing flaps from retracting at any speed, which is a serious safety problem. You would need a way to bypass it in such cases, and if you trust the pilot not to stupidly bypass it, then you might as well trust him/her not to stupidly need it in the first place.

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    $\begingroup$ "You're not even close to stall speed until the flare, and raising flaps before then just means you'd land faster--not stall." Not true for transport category aircraft. For example, the minimum safe airspeed for a 747 at landing flaps is more than 60 knots slower than the minimum safe airspeed with a clean no-flaps wing. If you were inside the outer marker of an ILS at flaps 25 or 30, the two typical flap settings for a 747-200, and at the proper speed for that flap setting and suddenly retracted the flaps, you would stall. Your first indication would be the stick shaker going off. $\endgroup$
    – Terry
    Dec 13 '18 at 7:22
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    $\begingroup$ I don't know about the 747, but can tell you that the smaller business jets I've flown typically have the Vref set at 20% to 30% above stall speed. $\endgroup$
    – user16289
    Dec 13 '18 at 13:43
  • $\begingroup$ @Terry I know the big iron has lots of calculations to determine V speeds based on weight, but the standard rule of thumb if no Vref is provided is Vref=1.3 * Vs0. Vs1 is typically only 10-15% higher than Vs0, so unless you're below Vref, pulling the flaps shouldn't result in a stall. $\endgroup$
    – StephenS
    Dec 13 '18 at 23:25

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