B777 FIRE CARGO FWD/AFT checklist

[extricate]

Hi,

Does anyone know what's the reason for selecting "LDG ALT Selector" to....PULL ON, set 8000 and to push off LDG ALT Selector when leaving cruise altitude in the FIRE CARGO FWD/AFT checklist?

I've heard something in relation to ensuring positive pressurization in order to remove smoke asap, but I doubt it. I don't think just by setting landing alt to 8000, the cabin altitude controller will do anything since cabin altitude controller will still be in cruise mode (assuming the non-normal happens in cruise).

Can someone enlighten me?

Thanks

[wiggy]

I must admit I thought selecting the LDG ALT selector back off (i.e, returning the system back to normal scheduling ) was done to ensure you have still have positive flow “outwards” as you descend the aircraft below 8000’....

Standing by for a more considered answer.

[cashash]

Have not got time to look any further at the moment but when you select FWD or AFT ARMED on the Fire Control Panel that signals both outflow valves to 50% to ensure that any smoke will be sucked out of the Cargo Bay and not into the Cabin.

[Sidestick_n_Rudder]

I seem to remember, that my Boeing instructor's explanation was that selecting the landing alt to 8000' was meant to increase the cabin altitude, thus decrease diff. pressure in order to reduce the leak rate of the extinguishing agent out of the cargo compartment.

I think it has to do with ETOPS and suppressing the fire in cargo holds for long diversions.

[extricate]

Thanks for the reply. Now it rings a bell