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When a dual engine flame-out is encountered, and the engines themselves are not damaged such as in the case of Air Transat Flight 236, why is the RAT deployed for emergency power instead of using the engine generators?

I imagine that the engines themselves will still be spinning sufficiently quickly in the wind to generate enough electrical power. Of course, I'm not saying that a RAT should be removed, because in a different scenario, both engines could simply explode and their generators would be useless. But I'm wondering why, in the case of fuel exhaustion, the engine generators are not used at all, and we rely only on the RAT / batteries. Is it because connecting a generator to windmilling engines would slow them down too much and causes unnecessary drag during the glide? Or is it because the engine generators are optimized for much higher rotational speeds?

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    $\begingroup$ The engine's generator is connected to the engine core, and the core can never windmill to the needed speed. It'll be a different story if (one of) the generators are mounted on the fan. If so then you could indeed get rid of the RAT because the fan windmills much faster. $\endgroup$
    – user3528438
    Mar 27 at 2:11
  • $\begingroup$ Isn't the fan directly connected to the core via the LP shaft? In that case, both the fan and the LP core rotate at the same angular speed (RPM), and could produce the same torque. $\endgroup$
    – natanijelvasic
    Mar 27 at 2:25
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    $\begingroup$ @natanijelvasic The fan is usually (not on RR Trent engines) connected to the LPC and LPT (low pressure compressor and turbine), but the generator is connected via the accessory gearbox to the HPC and HPT (high pressure compressor and turbine). $\endgroup$
    – Bianfable
    Mar 27 at 6:25
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    $\begingroup$ @0scar RR Trent engines have a 3 spool design: the fan is connected to the LPT, but not the LPC (which is powered by the IPT). Anyway, the accessory gearbox is still connected to HPC and HPT (the N3 stage for RR Trent). $\endgroup$
    – Bianfable
    Mar 27 at 11:33
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    $\begingroup$ @0scar You're right, somehow I thought they still call it low and high pressure compressor, but you're correct: I meant the IPC. $\endgroup$
    – Bianfable
    Mar 27 at 15:47

4 Answers 4

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There are a few things to consider:

In an Airbus (such was the case with Air Transat Flight 236) when both engines are lost the RAT will automatically deploy (see the answer to this question) so in the case of fuel exhaustion or any dual engine failure its happening regardless of pilot action.

That would be bad aircraft design. You shouldn't design an aircraft to rely on anything failing in a particular way. When designing a backup power system, and subsequent checklists for its use, the assumption is that there has been a critical failure of the whole component. What if the engines shear off? What if its a dual fire situation? Dual fan blade impact and they are not spinning? The RAT covers all these cases.

You could perhaps add this as a troubleshooting step somewhere in an additional flight manual.

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    $\begingroup$ You're completely right - the RAT should deploy regardless, and assume that the engines have failed completely. So I guess the reason why "scavenging" power from windmilling engines is a bad idea, is because a transition to a complete engine failure might put your plane in an unrecoverable configuration. For example, the pilot might extend flaps using the "scavenged" engine power, the engines might then fail completely leaving only the RAT, and the flaps can no longer be raised. $\endgroup$
    – natanijelvasic
    Mar 27 at 13:40
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    $\begingroup$ One should never plan on having an emergency backup of a critical component reliant on the successful operation of one of the systems the emergency backup supports. You want completely independent solutions that do not have any dependence on each other. $\endgroup$
    – Milwrdfan
    Mar 28 at 19:30
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Because why would you want to do that?

As you say in the question, the plane already needs a RAT, and the RAT and batteries are already sufficient to get you down safely (because you can't rely on anything else working). You don't need more electrical power. How does it help?

Because why would you want the crew to do that?

We can all agree that losing both engines qualifies as an emergency. In an emergency, the priority is flying and navigating the plane safely. Everything else is secondary, and the fewer extra tasks you have there, the better.

Humans have a limited processing capacity, and a number of emergencies/malfunctions have been converted into accidents purely by overloading the crew. You don't want your crew wasting time trying to figure out exactly why or how the engines failed, or assuming incorrectly that they can rely on the engines and then losing electrical power entirely, or misreading a complex fault-finding procedure and losing electrical power as a result, or trying to follow a complex fault-finding procedure and forgetting to fly the plane whilst doing that. That's all nonsense, and worse than that, it's entirely unnecessary nonsense which has literally killed people. I'm assuming your intention here isn't some sadistic "let's maximise casualties" mission? ;) If you want the crew and pax to have better odds, give the crew one solution which works, and they just do it and move on.

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  • $\begingroup$ Yes that makes sense. As I've commented above - we don't want the pilots to rely on partially failed components, only to discover later that they can't anymore, resulting in an unrecoverable configuration. $\endgroup$
    – natanijelvasic
    Mar 27 at 13:50
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    $\begingroup$ I think @natan's "how does it help" idea is that it might improve the glide ratio, something that matters in some dual-engine-out situations. The engines are going to be in the airstream anyway, so if increasing their drag (by the braking action of a generator) can extract sufficient power more efficiently than having the RAT out in the airstream, you win a slight reduction in drag vs. the RAT. Of course, even if it would work at all (generator on a part that windmills fast enough), it's probably not much of a drag difference, and as you say, crew task saturation is critically important. $\endgroup$
    – Peter Cordes
    Mar 28 at 2:58
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Most jets with a RAT that provide emergency AC power to the aircraft have constant speed drive generators. These generators spin as a certain RPM (say 12,000 RPM) to provide the requisite 400 Hz AC power for the electrical system. If the generators spin at a different speed the hertz will fluctuate and, in an AC powered airplane, that would cause additional issues.

An engine that is windmilling cannot create enough rotational torque to spin the AC generator at the required RPM for power.

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  • $\begingroup$ I haven't done the maths, but it does seem a bit odd that a small RAT can produce the required torque for the constant speed drive, and the windmilling engines can't. Do you think it's because the engine's constant speed drive is only designed to operate at a torque and RPM range under normal, powered operation? $\endgroup$
    – natanijelvasic
    Mar 27 at 2:37
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    $\begingroup$ No. The traditional setup uses the RAT to generate hydraulic power which feeds a separate hydraulic-powered generator to create electrical power, preventing the need for constant RAT speed. This includes the A300-A340, 747-8, 757, 767. The 777's RAT generator only supplies transformer/rectifiers (switching power supplies) which supplies DC. (The 777's backup engine generators feed a cycloconverter, so they aren't constant-speed either.) The A350, A380, and 787 have variable frequency AC systems, so like their engine-driven generators, their RAT-integrated generators are not speed regulated. $\endgroup$
    – user71659
    Mar 27 at 3:18
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The engine generators are designed to run at the engine idle speeds, which are higher than the speeds that the engines would be windmilling. Working with a larger range of engine speeds would add design challenges.

Another issue is that the generator is usually attached to the N2 high pressure spool while the fan is attached to the N1 low pressure spool. Windmilling would primarily be rotating the fan and not the spool that the generator is actually attached to.

It would certainly be theoretically possible to have an engine generator designed to run while the engine is windmilling. But as others have pointed out, this wouldn't help in all situations where the RAT is needed, and the RAT is more efficient and reliable being designed specifically for this purpose.

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  • $\begingroup$ Good point about the high pressure spool. Now I'm wondering what typical N1 and N2 values are during glide. $\endgroup$
    – natanijelvasic
    Mar 28 at 21:13

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