Which engine is your critical engine?

[TogaLock]

Critical Engine Question

You are taking off in a large four engine jet with a crosswind from the left. Which engine is your critical engine?

I’ve heard for some time that this was a CX interview question, and recently I came across an explanation in “Flying the Big Jets” that was opposite to what had been explained to me. This got me thinking, and I’d like to hear other ideas on this, not for interview reasons – just for the sake of thought and discussion.

“Flying the Big Jets” says that your outboard left (upwind) engine is the critical engine in this situation. The book explains how the airplane wants to weathercock into the wind. The failure of an outboard upwind engine exacerbates the weather cocking and therefore is the most difficult to control – therefore your critical engine.

While I understand the points that “FTBJ” uses to come to the conclusion of the upwind engine, I think the book misses something important and that is that weather cocking is a phenomenon that relies on the aircraft being in contact with the ground. Once airborne weather cocking does not occur.

When considering the original question again (a takeoff with a crosswind from the left) this additional point is significant: during an all engine takeoff you will be holding right rudder to maintain directional control. If you had the left outboard engine fail, you’re already holding some right rudder input for the wind, you would merely need to increase the rudder you’re already holding to maintain control. On the other hand if your right outboard engine failed, you would actually need to reverse your rudder input, and this would be most dramatic just as the wheels leave the ground. Let’s also remember that it is not uncommon for winds to lift the upwind wing just as climb out begins, this would add further roll tendencies toward a failed right engine.

So with all that, the right outboard engine is the critical engine – any thoughts?

[the wizard of auz]

I'm guessing that the critical engine would be the one thats still going when the other one stops. ;)

[Mad (Flt) Scientist]

Well, since minimum control speed will have been determined in near-zero crosswind, it follows that any failure which causes the crosswind to be adverse - i.e. the (most) upwind engine - will be more serious. If the wind is strong enough and the failure occurs at a critical speed and a number of other factors are (mis)aligned, then you'll be in a situation where the "minimum control speed of the day" is somewhat higher than the assumed (certification) minimum control speed and you will be lucky to avoid a runway excursion. So in terms of engine failure during the takeoff roll, the upwind engine is certainly most critical. The assumption in the text below (underlined) is that control can be maintained, and this is not assured.

during an all engine takeoff you will be holding right rudder to maintain directional control. If you had the left outboard engine fail, you’re already holding some right rudder input for the wind, you would merely need to increase the rudder you’re already holding to maintain control. On the other hand if your right outboard engine failed, you would actually need to reverse your rudder input, and this would be most dramatic just as the wheels leave the ground. Let’s also remember that it is not uncommon for winds to lift the upwind wing just as climb out begins, this would add further roll tendencies toward a failed right engine.

The dynamic effect of either engine failing at rotation will be the same (assuming that the weathercocking effect was being held) - the aircraft yaw response will be similar in magnitude. One might also assume that the wings were being kept level-ish during the roll, and the increased tendency to roll as the plane leaves the ground (and exits the ground effect of the wind) will be a "known" for the pilot, so will not be a surprise to him or her. In any case, this would have the pilot predisposed towards a into-wind roll control, which is what the upwind failure also calls for.

I'd suggest the possibilty of being unable to control the aircraft directionally outweighs any other considerations; upwind is more critical.

[LOMCEVAK]

This is a classic case of one phrase having a specific meaning with respect to certification but a different one in a generic sense. The "Critical Engine" which is failed for demonstrating Vmc and Vmcg (with zero crosswind) is effectively the one that will cause the greatest contol problem when failed at a given speed (i.e will need the greatest control deflections). Note that if the failure of each of the engines requires the same control inputs then the "Critical Engine" is usually determined by the sytems operated by that engine, for example the engine that provides the hydraulics for raising the landing gear is a common one.

In the case of a crosswind during the take-off roll, the engine which, when failed, causes the greatest contol problem will not necessarily be the same one as used for determining certification minimum control speeds.