View Full Version : B747 critical engine and crooswind

26th May 2004, 22:18
Question regarding the B747 series. Which engine is the critical engine, and how does crosswind effect it? Thankyou.

Notso Fantastic
26th May 2004, 23:30
An outboard is more critical than an inboard. The only reason one may be more critical is in the event of a crosswind. On a crosswind takeoff, to stop the aircraft weathercocking into wind, 'downwind' rudder must be applied. So if an upwind engine failed, there is less rudder authority to counter it as there is already some applied to counter the swing due to the crosswind. That's it. Not a serious problem at all, there is usually plenty of authority designed in to counter this. It's more theoretical than practical.

26th May 2004, 23:43
.. not (necessarily) so, NotSo ....

At "normal" V1s .. fine .. but Vmcg is determined for nil wind (FARs) and a minV1 takeoff in a strong crosswind does not address an engine failure in these conditions ... be prepared for a significant increase in real world Vmcg as compared to certification Vmcg (other conditions being kept constant).

27th May 2004, 10:45

UK designed aircraft used to be certified using 7 kts (I believe) of crosswind for takeoff VMCG calculations.
Present aircraft certification does not account for crosswinds. You may not have the buffer margin which you believe.


Notso Fantastic
27th May 2004, 12:00
I do know that to trim out an outboard engine failure, one never needs more than 4 divisions of rudder trim to hold it nicely (about 1/3 available on the indicator). Two engines out on one side can require full trim and a little bank to balance it out, but losing two on one side instantaneously is almost unheard of- even with things wrong, jets still produce power. Therefore more than adequate rudder authority is designed in by far more knowledgable people than I. Boeing produce good aircraft and I'm happy to accept their stamp of approval! Sometimes certain take-offs approach the limit (as in Min V1s), but still no problem if willing to take a turn instead of climbing straight ahead, and the danger patch is of very short duration.

Captain Airclues
27th May 2004, 12:13

The subject of Vmcg/V1/crosswind has been covered several times on PPRuNe (do a search under Vmcg). As mutt says, to have an upwind engine failure at V1 could ruin your whole day if the wind was strong. For this reason we used to calculate the 'real world' Vmcg in the days when we would fail engines during base training. The certification limits take into account the likelyhood of the failure occuring exactly at V1, whereas during base training it was pre-planned.
However, as far as handling is concerned, the most difficult failure to cope with is a failure of the downwind engine just after rotate. If you consider a strong crosswind from the right, the aircraft will rotate with left rudder and right aileron. In a perfect world, the pilot would slowly uncross the contols. If, however, the left outboard engine were to fail, a swing would develop in the same direction as the rudder which is already applied. The pilot would then have to rapidly change from left rudder to right rudder, both to balance the aircraft and to counter any swing which has developed. The right aileron will initially have to be increased but then rapidly centalised or even left aileron applied to counter the excess right rudder (the aces might be able to get it right first time but I never have).
All examiners should be aware of the varying difficulty of the types of failure, the timing of the failure and the handling characteristics of the aircraft, and should make appropriate allowances in a test situation.