B747-400 Engine Idle During Reverse Thrust

[NSEU]

During reverse thrust ops, a CF6 engine will be put into high (approach) idle.

The reverser system energises electrical relays (in the equipment centre) which send a signal to the CF6 ECU and this triggers high idle.

I was wondering if the RB211 or the PW4000 go into high idle during reverse thrust. The relays which control high/low idle on aircraft with these particular engines don't appear to have any input from the reverser system.

If the idle changes, is it done with direct reverse lever input into the EEC/ECU's?

Incidentally, why is the engine put into high idle during this period? For additional pneumatic pressure to drive the reversers? Or simply for faster spoolup time?

Thanks for any insight

Cheers
NSEU

[main_dog]

I don't believe our RB211-524s nor PW4056/4062s go into high idle during reverse, though they do go into an unannunciated alternate EEC mode. All our FCOM 1 says is:

Approach idle decreases acceleration time for go-around. Approach idle is maintained until 5 seconds after touchdown, when minimum idle is selected.

Hope that helps.

MD

[MarkerInbound]

Our GE powered FCOM stays the same thing thing as Main Dog's with the extra line "Approach idea is selected during thrust reverser operation." I'm not sure it has a chance to downshift after touchdown before the reversers are engaged, I guess it would be a matter of how soon you go to reverse, < or > 5 seconds.

[NSEU]

Originally Posted by main_dog

I don't believe our RB211-524s nor PW4056/4062s go into high idle during reverse

Thanks, guys.

Our CF6 and RB211 FCOMs are also different in this respect. I thought at first it might be an oversight, but I see now that there are differences in both the FCOM and the Maintenance Manual.

The question "Why?" still remains, however Why does the CF6 need those extra rpm during reverser ops.

Cheers
NSEU

[N1 Vibes]

main dog,

EEC ALT mode is because during reverse thrust the RB211 EPR reading becomes unreliable, due to turbulent air/back pressure in the fan area and the engine reverts to an EPR/N1 model.

Best Regards,

N1 Vibes