Engine failure, Vmc, and Max 5 degrees Banking Angle

[AeroTech]

Extract from FAR 25.149:
Vmc is the calibrated airspeed at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the airplane with that engine still inoperative and maintain straight flight with an angle of bank of not more than 5 degrees.

Does it mean that the rudder and the aileron are oversized to cope with engine failure so that the banking angle will not exceed 5 degrees?

Feedback appreciated.
Regards

[STBYRUD]

Yup, this counts especially for the rudder - thats why the rudders on modern transport aircraft are as large as they are (and why flight crews have to be very careful when using them in flight)...

[Mad (Flt) Scientist]

Actually they aren't oversized - they are sized for the design case, which is usually the VMC condition.

The reason crews should not be cavalier about rudder use has nothing to do with the sizing of the rudder for the VMC case; it's all to do with the structural design requirements and the defined design manoeuvres, which do not include aggressive use of the rudder. So any such aggressive use will exceed the design loads in all likelihood, and if persisted with then structural failure is likely.

Thats even the case where the rudder design has nothing to do with VMC, such as on a single engine aircraft.

[rudderrudderrat]

Hi Aerotech,

maintain straight flight with an angle of bank of not more than 5 degrees.

When a manufacturer establishes the VMCA, they can not use more than 5 degrees of bank into the live engines. With level wings - the VMCA increases considerably (e.g. 707 increased by 20 kts).

[Mad (Flt) Scientist]

Reading the OP more carefully:

Extract from FAR 25.149:
...

Does it mean that the rudder and the aileron are oversized to cope with engine failure so that the banking angle will not exceed 5 degrees?

No.

The VMC rule you quoted cites a 5 degree bank angle to be used for the demonstration of straight flight. It does not mean that an engine failure will always result in a bank of 5 degrees or less.

25.143 also addresses engine failure, and the following is extracted from AC25-7A which provides guidance on interpretation of the regulation:

c Controllability Following Engine Failure. Section 25.143(b)(1) requires the airplane tbe controllable following the sudden failure of the critical engine.

...

Additionally, the airplane will be considered to have reached an unacceptable attitude if the bank angle exceeds 45 degrees during the recovery.

So 25.143 allows up to 45 degrees bank following an engine failure.

Additionally, a "dynamic VMCA" test is required as part of compliance to 25.149. The guidance for conduct of this test specifies heading control, but does not constrain the bank angle directly.