B-747-8 multi eng fail & "ELEC DRIVE" EICAS advisory

[RandomPerson8008]

Technical question about the 747, -8 specifically although it may also apply to the -400.

During a multi eng fail/stall scenario, first the memory items are accomplished which entail moving the affected engine fuel control switches from cuttoff to run.

After ~ 5 minutes or so, if the fuel control switches are left in run to facilitate restart, and the engines do not relight (stay failed), the ELEC DRIVE 1, 2, 3, or 4 advisory message will appear on the EICAS with the associated non normal electronic checklist put into the checklist queue.

My question: is this ELEC drive message spurrious as a result of an extended restart attempt? (i.e. the warning systems on the jet mistakently interpret low IDG RPM to be an IDG defect becuase the warning system isn't aware that the engine is failed/shutdown due to the fuel control switch position?). Should the ELEC DRIVE non-normal checklist then be overriden, or should it be accomplished with the associated irreversible dissconnection of the IDG after the drive disconnect switch is pressed?

Thank you.

[tdracer]

When an engine fails, the IDG will drop off-line (it needs ~50% N2 to operate). The message is inhibited if the fuel switches are in cutoff, but once you put them back to RUN, there is a five minute timer after which the associated ELEC DRIVE message(s) will be displayed if the engine is still not running. Normally the ELEC DRIVE message is inhibited if engine running is not TRUE, but there is a five minute timer that overrides the engine running logic (normally used for some functional checks).
The message (or messages) are essentially nuisance - an effect of the engine(s) not running - your focus should still be on engine re-light.

[RandomPerson8008]

Thank you for the explanation, tdracer.

[Klauss]

Hm. I think that if the airplane is flying, the engins will have more than 50%...unless at low speeds during an approach or so.
?

[tdracer]

Originally Posted by Klauss

Hm. I think that if the airplane is flying, the engins will have more than 50%...unless at low speeds during an approach or so.
?

The question in the original post was:

During a multi eng fail/stall scenario, first the memory items are accomplished which entail moving the affected engine fuel control switches from cuttoff to run.

Windmill speed of a failed engine N2 (N3 for RR) is nowhere near 50%... A stalled engine may be above 50%, but as soon as you cycle the fuel control switches to Cutoff, all bets are off.