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# Aviation/Flying/VMC speed multiengines

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: Steve, your questions are much to hypothetical to answer. The relationship between Vy, V2, Vmc can be different for different types of aircraft.

Being below Vmc does not represent a total loss of control. It only means that an aircraft may be yawing in the direction of the dead engine. It could be slight yaw or a lot more depending speeds.

The advice to use a slight roll towards the good engine is to increase aerodynamics. If you don't use the slight roll you have to control heading with a lot of rudder. Using the roll decreases the amount of rudder you need and reduces drag.

Ned

---------- FOLLOW-UP ----------

QUESTION: I've also read about fin stall in these engine-out operating conditions. Do you know anything about it?

Hi Steve, this is beginning to sound like I am doing your homework! I hope not.

I am not really familiar with fin stall. I think it only occurs on prop airplanes, either two or four engine, and can occur when turning into the dead engine. Such as a situation at low altitude where directional control requires a turn to avoid obstacles. Such a turn would increase Vmc by quite a bit.

I don't feel that civilian operations would normally run into this condition as airport operations are predicated on having obstacle free zones for engine out conditions during takeoff.

Sorry I don't have any more details for you.

Ned

Aviation/Flying

Volunteer

#### Ned Dolan

##### Expertise

38 years as a professional pilot. Worked as an instructor for a major airline, as a tow pilot towing gliders and flew international routes until 1998. Do not know much about the maintenance end of the business.

##### Experience

Teaching new Captains the skills and knowledge needed to occupy the position. Helping develop procedures for use in a new airplane type. (B757)