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Aviation/Flying/VMC speed multieng

Question
QUESTION: In the book Aerodynamics for Naval Aviator it says that for each degree of bank angle towards the good engine your Vmc reduces by approximately 3 kts.

That is, banking towards inoperative engine, your Vmc might get pretty high, right?

Reading some VMC theory, I don't get it how VMC changes so much if you bank. I assume that is if you want to maintain straight and level flight, right and it doesn't apply if you want to turn. That is, if after takeoff, climbing at Vy, you are in a turn 30 degrees bank angle and your top engine failed, your VMC is likely above or at actual speed and thus resulting in an out of control scenario? You can't roll level using opposite aileron or you just can't maintain straight and level using that bank angle? I don't get it at all.

Other example, when your engine fails, you may not be able to arrest the initial roll, that means if the airplane rolls, let's say 20 degrees towards the dead engine, when it fails, your actual Vmc equals your actual speed considering this scenario after takeoff when you are usually about 20 kts above published Vmc. So, again you may lose control at all if the bank angle - Vmc relation is correct.

I also watched some videos where in single engine operation, light twins were banked normally during low and slow flight in single engine flying a normal traffic pattern. If so, they would lose control if VMC would be really increased when banking.

This theory also feeds the old never bank into dead engine rule. I'm not a ME rated pilot, but I hope will start my ME training soon, so at least I would like to be ready at least on the theory part of the issue which actually is the one which confuses me.

So what's the truth?

Thank so much!

ANSWER: Vmca is what we call a regulatory speed and in practical terms it is an indication of the effectiveness of the rudder and aileron to control the yaw associated with the critical engine inoperative.  To establish a standard the regulators have decided on 5 degrees wing down as being the nominal testing point for this maneuver.

In reality we know that as you increase speed the flight controls become more effective and the reverse happens as you lose speed.  We know that the yaw associated with an engine out can be handle using a combination of wing down and rudder to maintain directional control.  Therefore there is a threshold speed where if you fly slower you will not be able to maintain directional control even using full rudder and full aileron input.

Knowing Vmca is important when developing procedure for operating in an out of airports so that speeds are kept sufficiently high to ensure positive control in the event of an engine failure at the most critical time, while meeting the turning radius and climb gradients required so you miss the obstacles that might be in the way.

Remember control is speed related.  Losing an engine does not mean an immediate loss of speed if your reaction to the failure are correct.  Rolling the wings level and reducing the pitch attitude should be an initial reaction while increasing power on the live engine.  All these actions properly initiated will keep you safe.

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QUESTION: Ok, but you are usually just 10-20 kts above Vmc when you takeoff in a light twin and when the engines fails, it is almost an instant roll, you can't arrest it in the first second because it is impossible to have such a quick reaction, so Vmc increases that 20 kts which you initially had above, and if you don't have authority you might lose it at all, what do you think?

Yes there is some roll when you loss the engine but there is more yaw.  The speed does not drop off that quickly unless you have your nose too high to start with.  The aircraft are designed to be forgiving in this instance and pilot normal reaction time is taken into consideration when certifying a design.  The loss of an engine in a climb requires the pilot to arrest the roll and yaw while lowering the pitch attitude to maintain speed.  These actions can be done within a few seconds by a competent pilot.  During test flights we would fail the engine at the edge of the envelope and judge the ability of the aircraft to maintain controllability with normal pilot input.  Your right that its difficult for a pilot to react in the first second but part of flying a multi-engine aircraft close to the ground or anywhere else  is being ready for the possibility of engine failure.  If the aircraft could not be handled with a critical engine failure in a normal climb attitude it would not be certified for flight.

Aviation/Flying

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Kevin Parker

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I have 30 years experience in Commercial Type Rating training, program development, human factors, instructor training, flight simulation and flying large aircraft in general. Operated L188, DC8, B747, B757, A320, A330 and A340 aircraft. Involved in cockpit design, flight testing and type certification.

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Aviation college, military flight instructor, ATPL, check airman, maintenance test pilot, Simulator Test and Evaluation Pilot, Production Chief Pilot