777 EICAS Eng Autostart

[felixthecat]

Just wondering why the Engine Autostart is a memory item?

When the EEC aborts the start it closes the spar valve and motors the engine for 30 secs. So why the hurry with the Fuel Cutoff memory item?

Also why do we then motor the engine for 30 seconds when the EEC already did it for us?

[BobNZ]

Autostart and EEC are different systems. (Both very clever at what they do)
An EICAS Autostart msg means it's (autostart) concluded that it's not working
correctly, and the fuel cut off you refer to will not necessarily occur, thus the
recall item. (once the fuel is cut off, by you, nothing major can go wrong)

[felixthecat]

The Autostart sequence is controlled by the EEC and if it fails the EEC aborts the start, closes the spar valve (i.e. cuts off the fuel) and motors the engine for 30 seconds.....at least that what it says it does....

[BobNZ]

B777 FCOM page 7.20.6
.....The proper sequencing of fuel and ignition is controlled by the autostart system....

[felixthecat]

FCOM 7.20
Autostart allows the EEC to control fuel and ignition.....
During autostart the EEC monitors EGT N2 and other engine parameters and will abort the start for ... x y z

On the ground if all attempts fail the EEC aborts the start. Fuel and ignition are shut off and the engine is motored for 30 seconds to clear residual fuel.

So my question remains

"When the EEC aborts the start it closes the spar valve and motors the engine for 30 secs. So why the hurry with the Fuel Cutoff memory item?

Also why do we then motor the engine for 30 seconds when the EEC already did it for us?"

[tdracer]

First off, autostart does not close the spar valve, it closes the engine (hydromechanical unit) fuel valve. Second, one of the potential reasons that autostart has failed is because the EEC can't properly control the fuel valve, which means it may not have been able to close it. Putting the fuel switch to Cutoff closes the engine fuel valve via an independent circuit. It also closes the spar valve (which autostart can't control).

If autostart already did the 30 second dry motor, there really isn't any reason to repeat it. However there are failure modes where the autostart system won't (or isn't able to) do the 30 second dry motor. Hence the instruction to dry motor.

BTW, on the GE90, autostart is resident in the EEC, so it's not really independent.

[felixthecat]

Hi Tracer...thanks for the info.
If you had had a EEC problem would you not get ENG CONTROL? Also if it was an incorrect valve position would you not get ENG FUEL VALVE?

I guess its just covering worst case and Im over thinking it :)

[tdracer]

If you had had a EEC problem would you not get ENG CONTROL? Also if it was an incorrect valve position would you not get ENG FUEL VALVE?

ENG AUTOSTART is something of a catch-all message covering all sorts of start failures, not all of which are faults in the autostart system (bum igniters or possible engine damage a couple of possibilities). Further, ENG CONTROL and FUEL VALVE are Advisory, AUTOSTART is a Caution with an associated aural.

[felixthecat]

Thanks for the input TD

[BobNZ]

My 7.20 quote was from RR Trent, Kenya Airways config.
(Don't know anything about PW versions), so could it be that
being all models/all engines of the 777 are a common type rating,
THAT recall item is there as a legacy carry over, covering the
older types, even though modern GE-90 machines look after
themselves?

[CCA]

Worst case, the EEC loses data during the engine start and for example doesn't cut the fuel off, but generates an advisory message, while the crew deals with that the engine cooks itself. $$$

Thus any autostart (as said encompassing many minor/major faults) message cut the fuel off just in case the EEC can't/doesn't.