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Engine out lateral trim

Posted: Thu Apr 27, 2017 6:26 pm
by benbeny
I hope this is not a stupid question.
If I understand correctly, until now in case of engine out pilots keep fuel evenly balanced and trim with rudder and aileron. Why don'r anyone design fuel system as lateral trim in case of an engine out, so instead of adding drag from trim, fuel is utilized as trim instead? I believe airplanes might be more efficient with that.

Any explanations are greatly appreciated! Thank you

Re: Engine out lateral trim

Posted: Thu Apr 27, 2017 7:10 pm
by FlyHossD
One, I would not call your question stupid. Let me take a stab at an answer.

As I recall, all of the airliners that I flew (727, 737, 757, 767) used cross-feeding of fuel, rather than transferring fuel from one tank to another. So to use fuel to trim an airplane - which certainly could be done and is done fore and aft in some - would need another system. The additional pumps would have to be of relatively large capacity in order to be quickly effective as trim needs changed with the requirements of the various phases of flight; trim in an engine-out cruise would be different that final approach. This additional system would add weight - and how often would it be needed anyway?

Such a system could be created, but is it worth the bother and the weight?

Re: Engine out lateral trim

Posted: Thu Apr 27, 2017 11:46 pm
by Florianopolis
One could also imagine ways for things to fail that could take the lateral CG outside the control envelope.

And more importantly: your airplane would fly differently. A lateral CG would change the point your airplane rolls around, and how it responded to control inputs and gusts. The intertias would all be different, and how you stalled and spinned would be different.. The USAF crashed an F-15 in Libya a few years ago in part because its lateral CG was so screwywampus.

Re: Engine out lateral trim

Posted: Thu Apr 27, 2017 11:48 pm
by Florianopolis
Florianopolis wrote:
A lateral CG would change the point your airplane rolls around, and how it responded to control inputs and gusts.


A thought occurred to me: Could a clever FBW system take advantage of an asymmetrical airplane?

Re: Engine out lateral trim

Posted: Fri Apr 28, 2017 10:11 am
by unimproved
Because you'd have a lot of weight on one wing, which doesn't do much other than roll and change the CoG. What you need is yaw, which can't be trimmed by weight.

Let's say the right engine is out. Moving fuel to the other wing would move the CoG left. Because you try to fly as straight forward as possible the right wing still generates the same amount of lift so the CoP stays the same, rolling your a/c to the left. Another effect of moving the CoG is the arm of the dead engine getting longer, so the drag gets worse.

Re: Engine out lateral trim

Posted: Fri Apr 28, 2017 11:10 am
by Flow2706
Florianopolis wrote:
One could also imagine ways for things to fail that could take the lateral CG outside the control envelope.

This would not necessarily be an issue. The A320 (don't know about other aircraft) can be flown with one wing full, one wing empty. The imbalance limits in the FCOM are only applicable in "normal" situations (i.e. no fuel leak) and are due to long term structural considerations. The FCOM contains a note about the in the fuel leak procedure.
I do however agree that the weight of such a system would be prohibitive considering that it is only used maybe one or two hours during the whole service life of the airframe (might be an idea for ETOPS aircraft or Quads), but on a typical short-/mediumhaul two engine jet aircraft you usually won't spend much more than 30 minutes in the air after an engine failure (if an engine fails on a two engine aircraft you have to divert to the nearest airport).

Re: Engine out lateral trim

Posted: Fri Apr 28, 2017 1:21 pm
by zeke
Moving fuel around is just too slow for trim.

You are not going to be flying that long OEI to worry about trying to do it more efficiently.

Re: Engine out lateral trim

Posted: Fri Apr 28, 2017 2:43 pm
by 7BOEING7
Flow2706 wrote:

.... but on a typical short-/mediumhaul two engine jet aircraft you usually won't spend much more than 30 minutes in the air after an engine failure (if an engine fails on a two engine aircraft you have to divert to the nearest airport).


The terminology in the Boeing checklist is "Plan to land at the nearest suitable airport" which may or may not be the nearest airport.