If a modern commercial airplane such as a Boeing 787 would stall with only one functioning engine, is it possible for the pilots to right it or would it require two functioning engines?
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1$\begingroup$ Welcome to aviation.SE! $\endgroup$– PondlifeCommented Apr 20, 2021 at 22:36
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$\begingroup$ Does it matter whether it's on the ground? $\endgroup$– StrawberryCommented Apr 21, 2021 at 11:43
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12$\begingroup$ Think about it, can a glider stall? How does a glider recover from stall? $\endgroup$– Harper - Reinstate MonicaCommented Apr 21, 2021 at 20:02
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4 Answers
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Yes, but the challenge is managing the asymmetric thrust effects when applying power during the stall recovery.
Pilots are trained in the sim for two stall (actually, just stick shaker onset) situations: high altitude and low altitude. Low altitude stall recovery training is normally done by slowing until the stick shaker starts, then adding max thrust while lowering the pitch attitude moderately, only enough to keep the speed just above stick shaker onset.
The objective is to minimize altitude loss while accelerating. The training doesn't actually get the airplane to a full aerodynamic stall; pilots are trained to start the recovery as soon as the shaker goes, which is still a few degrees below the actual stall AOA (as a rule, only production or experimental test pilots will take airplanes past stick shaker right into the natural stall, or stick pusher activation if equipped with one).
I don't believe anybody actually trains for stall recovery while single engine (I never did when training on the CRJs). In any case, if you had to deal with that, you just have the added complication of managing the yawing motions caused by the asymmetric thrust, and the much more sluggish acceleration, so in addition to managing pitch, you are pretty busy with your feet to keep the airplane straight, and you would have more altitude loss.
At high altitude, it's similar, except that the recovery maneuver is much more aggressive in the thin air. With little to no reserve power, you pretty much have to dive to gain speed, and you cover a lot more distance through space to get the same indicated airspeed rise than at low altitude, so the indicated speed increase is slower.
So you pitch over a lot more, with no effort to minimize altitude loss and you let the airplane dive and wait for the speed to come up. If you were single engine (you would be descending anyway to single engine ceiling if you were) you would be doing the same sort of fancy footwork to allow for the asymmetric thrust and keep the plane straight, to the extent that you have thrust to work with to help accelerate.
So you could say a single engine stall is perfectly recoverable, but the extra workload increases the chance of secondary problems from mishandling (like secondary stall).
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$\begingroup$ Ugh; shouldn't they train for actual stall at least in high altitude because with blocked static ports, you won't get the stick shaker. $\endgroup$– JoshuaCommented Apr 22, 2021 at 1:43
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1$\begingroup$ The problems you're talking about is loss of airspeed from the ice crystal blocked pitot problem. If you blunder into an aerodynamic stall at 38000 ft, you are in a world of hurt in a lot of airplanes and in some non-FBW is unrecoverable. The training to deal with that is attitude control. All the OEMs have QRH procedures now for flying with loss of airspeed that is based on pre-determined pitch attitude vs power values to use for different configs. Pilots with really good airmanship skills don't really need that kind of training, but many can't cope. $\endgroup$– John KCommented Apr 22, 2021 at 1:59
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A stall recovery doesn't require engines (although they help, especially if altitude is an issue). To recover from a stall, you need to lower the angle of attack. You can do this by lowering the nose until airspeed picks up, no engines required provided you have altitude to spare.
answered Apr 20, 2021 at 22:41
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1$\begingroup$ This is exactly what I was thinking. If you have enough power to thrust your way out of a stall, is it really a stall? $\endgroup$ Commented Apr 21, 2021 at 15:02
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6$\begingroup$ @AndrewBrēza Yes. It even has a name: power-on stall :). Stalling refers to the aerodynamic configuration of flow over a lift generator or control surface. Presence of thrust is an orthogonal issue. Most planes have thrust-to-weight ratio (TWR) < 1, and thus you can always lose airspeed by pitching up sufficiently, and eventually stalling when the airspeed is insufficient to keep the flow attached to the wings or control surfaces. $\endgroup$ Commented Apr 21, 2021 at 18:35
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With enough height, any aircraft can recover from a stall without any engine power required. In fact, for under-wing engined aircraft like the 787, it is sometimes necessary to reduce the thrust, because of the pitch-up tendency this engine placement provides.
The technique in a stall recovery is always, always to reduce the angle of attack, which is easiest done by reducing the pitch attitude of the nose.
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$\begingroup$ Isn't the pitch-up stall issue related to the problems of the 737-MAX? $\endgroup$ Commented Apr 21, 2021 at 15:00
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3$\begingroup$ @AndrewBrēza All stalls are "pitch up" (or, more precisely, "high angle-of-attack.") So, anything to do with a stall or preventing one will be related to pitch (or, more precisely, angle-of-attack, which is the angle between the airflow around the aircraft and the wing's chord line.) The MCAS was designed to change the force feedback to the pilots on the controls at high AoA in order to prevent them from getting too close to stall in certain maneuvers, so, yes, it is vaguely related (though it's not really related to why the MCAS didn't work properly.) $\endgroup$– reirabCommented Apr 21, 2021 at 20:24
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Recovery with one operative engine is possible, requiring a reduced angle of attack to break the stall, then accelerating, and adding power in proportion to the increasing rudder authority. A slight bank toward the side with the operative engine would help, but only enough to null sideslip (center the ball) as thrust is increased.
Gradually accelerate and increase thrust. Until the plane is able to climb up and away from terrain. We're talking about seriously delicate flying. Milk that mouse, bruhs.
Such a situation would result in a huge altitude loss, but that is what it would take to accelerate without much engine thrust.
Pilot flying: DO NOT YAW THE PLANE WHILE IN THE STALL
Pilot flying: BEWARE OF VMC-ROLLING THE PLANE WITH TOO MUCH THRUST FOR THE AIRSPEED
