Engine Bleed Air Switch in Boeing-737 NG

[Robin Jain]

hi friends
Can anybody explain me why engine bleed switches are kept all time ON in Boeing 737-NG i.e. during engine shut down condition also. What are the consequences if we keep it off.

Regards
Robin

[spannersatcx]

With the bleed off, should you get a tail pipe fire on shutdown, there is no air available to immediately motor the engine to blow it out (hopefully).

[Bobcanfixit]

The valve solenoids are energised with the switch in the off position. The switches are left on to save overheating or burning these out!

No adverse affect in the on position, so that's why it's so. Any motoring requirement would likely come from the APU.

[spannersatcx]

Do you not have to have the bleed switches in to motor/start an engine on a 737 (I don't know I work on 747's)?:eek:

[Robin Jain]

Dear ASFKAP..
You are right to extent that Bobcanfixit is confusing engine bleed s/w with HYD EDP & not EMDP as you said. I asked the ques with colleagues of mine who answered that no consequence if keep it off but to avoid one more checklist of putting it ON from OFF is the reason to keep it ON. Any comment on this..But still i m not satisfied with the answers..
Waiting for more replies..
Cheers!!!
Robin

[Clipped]

I'm pretty sure it is to enable engine bleed operations after the start cycle, with the crew not having to make another selection.

That is, when engine start is initiated and the start valve opens the Bleed Air Regulator vents control air from the Bleed Valve - PRSOV and it closes under its spring force pressure. When the start valve closes the BAR solenoid allows control air to open the PRSOV ( this can only happen if the ENG BLEED switch is ON ).

Therefore, had the Bleed switches been OFF, the crew would have to make those selections manually to enable engine bleed air.

[HAWK21M]

ENG BLEED L & R switches are left in on position to reduce the need to switch it on.The Dual bleed Amber lt will signal if APU bleed is being supplied.Nothing to do with solenoid usage as on EDP switch.
regds
MEL.