Re: B744 engine shutdown question

Date:         04 Jun 97 01:45:40 
From:         Tony Maddern <tmaddern@cse.unsw.edu.au>
Organization: University of New South Wales
References:   1
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FRANK TONG wrote:
>
> When working on B744 with RB211-524G/H engine powered aircraft, when
> shuting down engines, why does the engine control switch on overhead panel
> will turn to alternate and alternate ligth will be on for few seconds ?
>  Also, in the course note, it states that every time you shutdown the
> engine, FAFC channel will switch from A to B, or B to A. However, I never
> note this happen. Could somebody advice ?

The FAFC looks for a discrepancy between commanded thrust and actual
thrust. When the fuel control switch is moved to cut-off the FAFC sees
commanded N3 of zero but actual idle N3 so it shows a fail until N3
decays below a certain value (43% or 47%, my memory is vague on this)
then it resets.
By displaying the engine parameters on the lower EICAS through the CMC
you can see which is the active FAFC channel. The selection is fully
automatic and there is nothing the pilot can do about it so there is no
reason to display to the crew which channel is active.