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In case of engine failure above the ocean with multiple electrical failures and after having exhausted the batteries the fly by wire mechanical backup with available hydraulics if left with the THS and one spoiler per wing under the control of the control wheel, How in these conditions the pilot will manage the asymmetrical thrust?

Basically thrust asymmetry is normally managed with the rudder, the schematic shows in mechanical backup the rudder is not Available, while the remaining spoilers 4 and 11 are rather for roll control, what can do the pilot?

N.B. June 17, 2019: if it is difficult to imagine one engine failure + total electrical failure, please imagine an engine failure + Total FBW computers and controllers failure, the most important is to go into the mechanical mode as described here below with an engine failure: enter image description here

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    $\begingroup$ Related (not a 777, but about the main purpose of such mechanical backups): How are Airbus pilots trained for using the mechanical backup control systems? $\endgroup$ – ymb1 Jun 16 at 11:56
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    $\begingroup$ @ymb1, thank you very much, so it looks for Airbus it is part of the mandatory training, if you find something similar for the B777, it would be very helpful as I imagine the mechanical backup is not done for nothing but for exceptionally rare cases, so what I need is a hint on how the pilots will fly it in such a case. $\endgroup$ – user40476 Jun 16 at 14:08
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    $\begingroup$ I don't understand your scenario. If you have only a single engine failure, the other engine will produce enough electricity via its IDG. You only lose all electrical power for both engines failed, APU failed, RAT broken and batteries exhausted. $\endgroup$ – Bianfable Jun 17 at 7:00
  • $\begingroup$ @Bianfable, the scenario is total loss of electrical power in order to match with the mechanical backup. However to reach this tragedy I am assuming an engine failure and multiple other troubles since more than an hour to exhaust the batteries. The second engines generators, The RAT, the APU, troubles etc, to match the total loss of power. What is important is how will the pilot fly the aircraft In mechanical backup mode. $\endgroup$ – user40476 Jun 17 at 7:25
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    $\begingroup$ @Bianfable, as you know each FADEC has its own miniature generators on its own engine, however if you prefer consider one engine failure with Flight Control Computers failure, the important is to go into the MECHANICHAL CONTROL mode, mode imagined by Boeing, not by me $\endgroup$ – user40476 Jun 17 at 9:49
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Your question is kind of confusing. I think you're asking "If the rudder isn't working, then how can a multi-engined airplane manage the asymmetric thrust from an engine failure?" The answer to that question is "by rolling toward the good engine". This produces a turning moment opposite to that produced by the engines.

If that isn't enough, then there's nothing the pilot can do to fly in a straight line. The pilot may be able to control the radius of the turn by rolling and/or adjusting the throttles, and so make slow progress in a series of loops toward some airport, but the airplane is going to turn in this situation.

If an engine fails, flying with a slight bank toward the good engine is in fact the recommended procedure for most multi-engine airplanes, even if you have full rudder control. There's actually a mnemonic for it: "Raise the dead", because the dead engine is going to be higher than the good one. It's more fuel efficient and produces less strain on the airframe then trying to correct with rudder, because the fuselage is lined up with the airflow. As for passenger comfort, well, you're going to need a minimum of four separate failures to get into this situation, so by that point, passenger comfort is well down the priority list.

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    $\begingroup$ I think your confusion, @user40476, comes from this incorrect assumption: "Should we loose all electrical power one may assume having lost one engine in addition to other troubles". Losing one engine does not cause loss of all electrical power. You'd have to lose both engines, the APU and the RAT would have to fail, then you'd have to exhaust the batteries. With one engine operable, both the current answers provide info on how the plane would be handled. $\endgroup$ – FreeMan Jun 17 at 13:00
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    $\begingroup$ @user40476 It's possible to lose a generator without losing the associated engine. So, no, you cannot just assume that electrical issues mean engine failure, just like you can't assume that engine failure means electrical issues. So far as I can tell, your question boils down to "What do you do if the rudder isn't available, and you've lost an engine?", which I've already answered. I guess you could also be asking "Why doesn't the B777 have a mechanical backup for the rudder, in case of engine loss?", to which the answer is "because rolling toward the good engine works just fine". $\endgroup$ – HiddenWindshield Jun 17 at 15:25
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    $\begingroup$ @user40476 I'll add that to my answer, along with some other info. $\endgroup$ – HiddenWindshield Jun 17 at 20:17
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    $\begingroup$ @HiddenWindshield, yes indeed make it an answer and I will accept it, so irrespective of the scenario, you are answering to the question given in the title: « B777: In emergency mechanical back up how does flight control manage engine failure? » $\endgroup$ – user40476 Jun 17 at 21:23
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    $\begingroup$ @user40476 I've edited my answer. $\endgroup$ – HiddenWindshield Jun 18 at 5:33
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In your scenario there would likely still be hydraulic power and electrical power from the one running engine and the RAT (Ram Air Turbine) generator, which can power everything except the autopilot. The TAC (Thrust Asymmetry Compensation) system would compensate for the asymmetric thrust.

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  • $\begingroup$ @ Juan Jimenez, In the question I am saying « available hydraulics » but no electrical power. The mechanical back up is designed for the cases of total electrical loss otherwise it wouldn’t have been implemented for nothing. Nevertheless,Please don’t change the scenario. If you think it is probabilistically impossible, you may sayit as a comment but not answering to an other scenario. $\endgroup$ – user40476 Jun 16 at 9:16
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    $\begingroup$ If you want to create wild imaginary scenarios, at least try to think up something reasonable. It is almost impossible for the RAT to not work as designed, and hence almost impossible for there to be zero electrical power. You drop it out and wind produces electricity. That's what it is designed to do. Please do not try to change reality. The mechanical backup is not designed just because someone thinks there will be total electrical loss, it's also used in normal operations. $\endgroup$ – Juan Jimenez Jun 16 at 10:00
  • $\begingroup$ And yes, I know what you will do, edit the original question after the fact to support your late "argument" of why you don't like the answers given to you. That's why I will ignore this question from now on. $\endgroup$ – Juan Jimenez Jun 16 at 10:41
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    $\begingroup$ @ Juan Jimenez, Thank you, you are so kind $\endgroup$ – user40476 Jun 16 at 10:47

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