Critical engine in the event of engine failure

[john_king]

I am completing my CPL Multi IR, I am familiar with the critical engiine on twins but want to understand if Jets have a critical engine, I appreciate this will not be for the same reasons as on the Twin Props.
Thanks for you help

[mutt]

No, however there is a train of thought that the upwind engine should be considered the critical engine during a crosswind takeoff.

Mutt.

Thanks for the correction.

[Bellerophon]

Mutt

A typo perhaps?

...the downwind engine should be considered the critical engine during a crosswind takeoff...

Surely the Upwind engine?

Most jet aircraft have a tendency to weather-cock into wind during a cross-wind take-off.

Following an engine failure during the take-off roll, this weather-cocking tendency will tend to increase the total yawing moment following the loss of the upwind engine, but reduce the total yawing moment following the loss of the downwind engine.

Thus a failure of the upwind engine, during a cross-wind take-off roll, will generally result in more rudder being required to keep the aircraft straight, than a similar failure of the downwind engine.

Or were you getting at something else?

Regards

Bellerophon

[Capt Fathom]

Perhaps what Mutt was referring to is that at the start of the takeoff roll whan advancing the thrustlevers, the downwind engine is prone to surging due to the disturbed airflow. (Although have not had a problem myself).

With an engine failure at or after V1, in very strong crosswinds, the upwind engine is considered the critical engine. With say a crosswind from the right, as you rotate, you will have in right aileron and be applying left rudder to maintain wings level and centerline tracking. The amount of left rudder remaining to counter the yaw resulting from the engine failure will be minimal.

In fact, you will not be able to maintain the centerline, and the certification allows for this divergence from the centerline. The theory being, if you get the technique right, you will become airborne as you approach the runway edge! Not a lot of room for mishandling.

[john_tullamarine]

Several points ..

(a) critical engine has little to do with crosswind. Under FAA rules Vmcg is determined for nil wind, the old BCARs used 7 knots ...

(b) the centreline divergence is stick and rudder related without any consideration of wind other than for (a)

(c) the effect of crosswind on realworld Vmcg is significant and will vary with Type .. typical effect 0.5 to something in excess of 1.0 kt/kt.

(d) the onset of control difficulty (at least in those aircraft whose low speed handling I have observed) can occur within a very narrow speed band around Vmcg .. ie now you stay on the runway .. now you don't ....

(e) if you get caught with a minV1 takeoff in a strong crosswind on a narrow runway ... don't necessarily expect to stay on the seal if you lose the upwind engine .. the aircraft's heading, essentially, will be outside your control (unless you close the operating throttles rapidly)

(f) if the case is a very short runway and you are forced to low weight and minV1, then there is not much you can do other than delay until the wind abates. However, if the runway is longish and you just happen to be light (eg ferry or positioning) consider using a higher speed schedule (but within the limiting runway RTOW figures) to obtain a pad for mum and the kids ....

(g) simulators often don't replicate this problem and looking at the problem in the sim may result in an unjustified complacency. In only one case I have observed (where the revised rudder equations were implemented for a 737-200) was the near Vmcg divergence anything like what I would have expected to see on the real article.


... then, again, there is the case of lightweight, minV2, aft cg, failures during the rotation .. that is a different, but very interesting, animal ... especially if the task is to handle the failure while backtracking the localiser. With a bit of practice it is not an overly difficult task .. and puts a whole lot of polish onto stick and rudder skills ...