Hi, folks....When a 737NG start switch is selected to GND, the start valve opens and the APU can then supply bleed air to the engine starter. When the start valve opens, an electrical signal is sent to the respective engine bleed system to close the main engine bleed valve (PRSOV). According to my books, closing this valve protects the engine bleed system from APU air (i.e. from reverse flow). However, the other engine bleed valve doesn't receive this signal (as far as I can see). If this is the case, what is protecting the other engine bleed system during start?Thanks.Cheers.Ian.

Ian,Well I'm far from an expert on this particualr bird, but based on how the other Boeings do it, here's my hypothesis. The PRSOV should be spring loaded Shut without muscle air available. If the engine is shutdown, it would be closed by default. I believe the reason they have that logic on the engine being started is to prevent the valve from opening during the Start as the Engine develops Pnue pressure from the Customer bleeds on the Engine. This would be similar to the Engine Speed card bleed logic on the 757 or 767 which prevents the bleed from coming on until the engine speed card signals "Engine Running".Disclaimer: this only a hypothesis and not approved data :-)RegardsPaul Gollnick :-cool Technical Operations/Customer Operational SupportPrecision Manuals Development Groupwww.precisionmanuals.comhttp://www.precisionmanuals.com/images/forum/devteam.jpg

"This would be similar to the Engine Speed card bleed logic on the 757 or 767 which prevents the bleed from coming on until the engine speed card signals "Engine Running"."Hi, Paul.The 767 (and I assume the 757) seems more logical to me as both PRSOV's (and, I recall, PRV's) are closed during (any) engine start (when using the APU). I don't understand why the NG doesn't do this. If you have, say, the right engine running (bleeds avail) on an NG and you start the left engine, the left engine is protected by system logic, but the right doesn't appear to be (If the Right Engine PRSOV is open (which it should be with the engine running), the APU is free to blast air back into the already running engine (this surely can't be good?).Perhaps the relative pressures of the APU Vs Engine and the spring loading help to keep the air flowing the right way (but if this is the case, why do they need the electrical close logic on the engine being started?).Thanks.Cheers.Ian.