Why alternator off during eng fail

[PULL UP PULL UP]

On a C172 I instruct (and was instructed) the crash drill:

FMAM

Fuel selector off
Mixture Idle cut off
Alternator off
Magnetos off

My question is, why to switch off the alternator? It is also done automatically after the last flap selection on short final, when you switch off the battery (because you cannot switch off the battery without switching off the alternator)...?

[Hairy Mary]

The alternator exciter circuit consumes battery power when it's energised, even if it's not turning.

It's just another unnecessary load on the battery.

[distaff_beancounter]

My question is, why to switch off the alternator?

To reduce the risk of fire, in the event of any escaping avgas?

[A and C]

Quote

"its just another unecessary load on the battery"

I don't think so!

The working alternator regulator will comand more power to supply the field of the dead alternator reducing the engine power avalable to the prop.

This will further limit the aircraft performance........ not by much but a light twin on one engine is ONLY just able fly away from the ground.

[Oktas8]

Pull Up Pull Up-

I think that Cessna do not require you to turn off the alternator as part of an engine failure (landing) drill. Refer to Section 3 of your POH or AFM to check your particular model.

If I'm correct, the best answer to your question is "no particular reason". As you say, you turn off both Alt & Master before impact anyway.

There are lots of "I've always done it that way" home-made procedures around in the GA world; I've been guilty of it myself on occasion. However, it's wise not to depart too far from what Mr Cessna expects, unless you have non-standard aircraft or a particular type of operation that requires a non-std procedure.

A and C - regarding light twins and EFATO -

Let's assume the dead engine alternator field is supplied with 5A at 12V because it was not turned off after feathering. This represents 60W of power from the live engine, or about 1/13th of a horsepower, or about 1/2500 of the live engine power output. Exactly how much are you going to clear that obstacle by, whether you turn off the alternator or not?

[A and C]

Not much in terms of aircraft performance but a lot in terms of understanding how the systems work.

As an instructor I repeatedly find the technical side of flying instruction is very poor.

[foxmoth]

When you get an engine failure the other bit that usually suffers at least a partial failure is the brain, it is easier to just remember to turn everything off (except master until last flap if electric flaps) rather than try to remember what to turn off and what to leave on.

[FlyingForFun]

When you get an engine failure the other bit that usually suffers at least a partial failure is the brain, it is easier to just remember to turn everything off (except master until last flap if electric flaps) rather than try to remember what to turn off and what to leave on

True.... but with that partial brain failure, will you remember which part of the two-part switch is the alternator, and which is the battery master? The effects of getting the wrong bit are worse than the marginal performance gain of getting the right bit. At least, that's my view.

FFF
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[foxmoth]

On those types I usually teach to turn them both off together, either during the checks with manual flaps or after last flap selection with electric, in fact with electric I would say this is even more important, the last thing you want to be doing at this stage is to try to work out which half of a 2 part switch you need to move which way!