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Many aviation discussions of the TransAsia GE235 accident include comments like "that was a Vmc roll". What is a Vmc roll and how should you avoid it and/or recover from it?

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I'm sure someone else will come along and give you a better answer with illustrations and everything, but until then, here goes :)

Multi engine aircraft usually have their engines off to either side of the fuselage. This means that if one engine fails, the other engine will produce what's called asymmetric thrust (one engine is stronger than the other). This in turn, because it's off to the side of the aircraft will cause it to yaw in the direction of the dead engine. Should the dead engine also have its prop windmilling, it'll cause more drag than if the prop blades were turned parallel to the wind and thus aggravate the situation by not only not producing thrust but actually acting as a speed brake.

To counteract this and continue flying straight you'd have to apply the aircraft's rudder. The rudder is an airfoil sticking out into the wind in order to yaw the aircraft, thus if the amount of air flowing over it is reduced, the effectiveness of the rudder is reduced. This has the effect that there's a minimum airspeed (i.e. minimum amount of airflow over the rudder) that's enough to allow the rudder to counteract the asymmetric thrust as well as some other properties like engine torque. This speed is called $v_{mc}$, or minimum controllable airspeed (there are actually two of these, one airborne and one on the ground). If speed drops below this airspeed, the plane will start to increase the yaw away from the live engine, to the point where eventually the outer wing produces more lift than the inner wing and the aircraft rolls over.

That's the $v_{mc}$ roll. Avoiding it is a matter of keeping the airspeed up to maintain rudder authority, and/or reducing thrust to the live engine while regaining airspeed. Further steps, like identifying the failed engine and feathering its prop, as well as flying with a slight bank towards the live engine should be undertaken as well as part of the recovery.

Furthermore it's advisable to never turn into the dead engine, as the live engine will make it harder to stop the turn. Turning into the live engine will make it harder to start the turn, but the engine will help you get out of it.

Since the engines usually spin in the same direction, the effect of torque differs, the torque of one engine aggravates the situation, where the torque of the others alleviates it, thus $v_{mc}$ is specified as the minimum controllable speed if the latter (known as the critical engine) fails and the prop continues to windmill, i.e. the worst possible aerodynamic scenario in the case of a single engine failure barring an uncontained failure causing damage or otherwise changing the aerodynamic properties of the aircraft.

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    $\begingroup$ Kevin precisely answers the question, you describe it well for laymen. If we could just get the two answers combined into one... $\endgroup$ – FreeMan Feb 6 '15 at 16:32
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    $\begingroup$ @FreeMan this is SE, there's nothing wrong with more than one answer ;) $\endgroup$ – falstro Feb 6 '15 at 16:34
  • $\begingroup$ That's true @falstro, sadly, you won't both get credit for great answers. $\endgroup$ – FreeMan Feb 6 '15 at 17:10
  • $\begingroup$ @FreeMan True but the 15-point bonus for having the accepted answer is only a vote and a half's worth of reputation. I agree it would often be nice to accept two answers, though. (More than two would probably be getting a little much.) $\endgroup$ – David Richerby Feb 7 '15 at 11:56
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It's more correct to say this is an unintended roll induced by flying at a speed below Vmc.

Vmc (V=Velocity, mc = minimum controllable) is the minimum speed a multi-engine aircraft can remain in control in case of a single engine failure. When an engine quits, thrust is no longer symmetrical. Rudder and a little bit of aileron are used to keep the plane level. E.g. If the left engine quits, the plane has a tendency to yaw left and roll left.

Effectiveness of control surfaces increases with speed. When the speed drops too low, the control surfaces may be unable to balance the plane even at full deflection.

Pilots are trained to keep the speed above Vmc at all times. For example, during the takeoff roll, it is a bad idea to lift off the runway before the airspeed reaches Vmc. If the plane pitches up too much during a climb, its speed may also fall below Vmc.

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  • $\begingroup$ Maybe to provide further info, when the engine quits the plane has direct tendency to yaw left, and compensates using the rudder at the vertical plane. This deflection creates the tendency to roll. Just to provide some extra mechanics :) $\endgroup$ – Trebia Project. Feb 6 '15 at 18:15
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The airplane has an engine failure on the left side. The wing on the right side has high velocity air blown over the wing providing more lift and keeping the boundary layer from slowing down and separating. The pilot counteracts the the rolling moment from the high lift right wing by using right aileron. Now the aileron on the left side is deflected down, increasing the effective angle of attack of the left wing, a wing that has nothing to sweep away the slowing boundary layer. Now he is using right rudder to try to keep the plane going straight. His mind is thinking he has the plane under control, wings level and plane going straight. But the plane is side slipping. This is the equivalent of bottom rudder. The skidding causes the plane to slow down because it can hardly fly when it it isn't skidding. The slowing down causes the rudder to be less effective, the skidding increases, that means more aileron is needed due to the dihedral effect. Finally the effective angle of attack of the left wing is high enough for the stall to begin and the wing starts down. The right ring is rock solid, it will never stall. The left wing dropping increases the angle of attack more and the rollover is rapid.

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The Vmc roll, specifically, is a rapid roll (not yaw) below Vmc. It occurs primarily because a thrusting engine located on a wing decreases the stall speed significantly. Therefore, as airspeed decreases, a moment occurs when the wing with non-thrusting engine stalls, while the other wing continues to produce lift. At that point the airplane quickly rolls.

Note that 1) for this asymmetry to happen you need a rather small amounts of thrust, and 2) the stall speed of one wing grows. So, if you think that you made it once the trottle is cut and the rudder deflection is way down, and try to fly a usual landing, then you will die. See this: https://www.youtube.com/watch?v=YZIzEtHzbNU

This is why all the yaw-centered explanations of Vmc roll are hazardous and you should reject them as soon as possible. It's not yaw that will kill you, it's the roll -- and it can happen from a minimal yaw.

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    $\begingroup$ There's a lot of vmc talk in the comments of that YouTube video, but to be honest, that looks like a classic base to final bottom rudder stall/spin, not an engine failure. Do you have a link to the accident report? $\endgroup$ – falstro Feb 7 '15 at 8:02
  • $\begingroup$ @falstro - A small amount of detective work revealed this: aviation-safety.net/wikibase/wiki.php?id=145223. No NTSB report (broken link) $\endgroup$ – Steve V. Feb 7 '15 at 16:04

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