Fantom,

No, it is not ALL to do with prop wash hitting the fin. In fact it has almost nothing to do with it. It is a matter of thrust location/distance from the center of gravity of the aircraft, the farther away, the more trouble with engine failure on a twin/ multi-engine.

Sorry folks, there seem to be many mis-conceptions here.

The reason multi-engined propeller driven aircraft have a critical engine is due to the impact the rotating vortex trailing from each propeller makes on the wing and vertical surfaces behind, it while producing thrust: i.e. the angle of attack of the vortex on the wing and the vertical surfaces is different, depending upon the direction of rotation of the prop, the thrust being produced and the indicated airspeed of the aircraft.

The critical engine will be determined during flight testing for Vmca, which is defined as the minimum speed at which directional control can be maintained under defined circumstances (i.e. sudden power cut, maximum bank angle of 5 degrees, etc.).

On aircraft with propellers rotating in a clockwise direction, as viewed from behind, the critical engine will usually (but, not always) be the left engine. In other words, the failure of the left engine will cause loss of directional control at a higher speed than a failure of the right and therefore is more critical. On some aircraft, an engine failure at Vmca will result in a significant difference in performance between a left engine failure and a right engine failure.

Several observations ..

is due to the impact the rotating vortex trailing from each propeller makes on the wing and vertical surfaces behind

Probably a tad simplistic.

sudden power cut, maximum bank angle of 5 degrees, etc.

Generally the FT workup involves establishing what generally is referred to as the static Vmc .. bit like the pilot endorsement demonstration activity. Once this is established, checks will be made with sudden failures to make sure that the (you guessed it) dynamic Vmc situation doesn't provide any unpleasant surprises .. if it does, then the numbers will be revised to suit.

Important to keep in mind that real Vmc stuff potentially is VERY dangerous and one is best advised to stay some speed above the real world Vmc on the day.

an engine failure at Vmca will result in a significant difference in performance

True, depending on Type, but not overly relevant to anything. Vmc is concerned with handling rather than performance. Unless and until the pilot gets the aeroplane back up to a speed appropriate for OEI climb things generally are going to be untidy so far as climb is concerned ..

John:

All true and your cautions are wise.

Twochai