De-icing during Engines NOT Running B738
 

My company tells us to switch ENGINE BLEEDS to OFF during de-icing with Engines SHUT DOWN? This does not make sense to me.

If I am correct the Engine bleed valve on the B738 is DC controlled and pressure operated, meaning that if the engine is shut down the bleed valve will always be closed?

Can somebody confirm this, and also explain to me why my company does it like this?

Switching them off just makes it easier to forget to put them back on again. Of course if the engines are running it makes sense to switch them off.

/pt

Same with ours - supposed to be common with the regular deicing procedure I guess. Youre right, the valves wont move, it wont make the least of a difference, but thats what you do :ok:

Probably done to have similarity in procedure for all situations.

From a maint perspective. If you start the engine with the bleed off it may give the engine an opportunity to dry out the compressor of de-ice fluid before you turn the bleed back on after engine start?

>Nepotisim

Could make sense but the strange this is our procedure tells us to switch the ENGINE BLEEDS ON again BEFORE starting the engine.. Hmm

Well there goes my theory. Sorry not that familiar with the 737 bleed system.:(

Perhaps the 738 is similar to the 744. Sometimes the valves remain slightly open after engine shutdown (The OFF light doesn't illuminate in the overhead switch). When you switch off the bleeds (with the overhead switches), 99 times out of 100, the valve will close completely. This seems to happen irrespective of bleed duct pressure. The electrical close command and the springs in the valve must give the valve that extra bit of incentive to close(?). Or perhaps there is enough air pressure trapped somewhere in the system to assist the complete closure when the electrical command is given.

Regards
NSEU