I made some interesting discoveries, today, about the 737 Bleed Isolation Valve and the pnuematic system in general, both by reading the Boeing Maintenance Manuals and from seeing the real aircraft in action (c/o a fellow engineer). I'm sure it will be of interest to the PMDG developers ;-)I was overwhelmed by the simplicity of the logic behind the operation of the Bleed Isolation Valve:In AUTO, the Bleed Isolation Valve is controlled by the position of the Pack Switches(2) and the Engine Bleed Switches(2). If ALL of these four switches are in the AUTO, HIGH or ON positions, the Bleed Isolation Valve closes. If ANY of these switches are in the OFF position, the Bleed Isolation Valve opens. Easy, huh?I had a chance to look at the bleed switch configuration on two different airlines today, during quick turnarounds, and this is what I saw on both.http://members.ozemail.com.au/~b744er/737/BleedPanel.jpgThe engines, of course, were switched off. Both packs were running. All engine bleeds were selected ON, even though the engines were not running. The APU bleed was also selected ON (the APU was running). This is the interesting part.... The Bleed Isolation Valve was in the ON position. IF the Bleed Isolation switch had been selected to AUTO, the basic logic would have cut in and closed the Isolation Valve.... depriving the Right Bleed Duct of any pressure... and forcing the Right Pack to shut down.Note that the pressures in both ducts in the photo are equal (although I did see a split of about 2 psi on a really old 737, yesterday... perhaps an indication error?).Anyway, hope this helps.Cheers.Ian.P.S. Message edited due to errors grammatical :-)

Hi Ian,Were both 737s you saw NGs? The reason I ask is that in my 737 manuals the procedure is to put one pack on in high and leave the bleed isolation valve in auto, while the APU is providing air. So when I did just that in the KLM 738 sim, I was told that on the ground they always keep the isolation valve in the open position and have two packs on, just like you describe. Now, my manuals are for the 3/4/5, so I wonder if there might be a difference between the Classic and the NG? Or is it simply a matter of choice, a company option so to say?Interesting also to note is the apparent simplicity behind the logic of the DUAL BLEED light. It is on in your pictures, even though obviously there is no real chance of a dual bleed while the engines are off. My Boeing manual says this: it is illuminated when the APU bleed air valve is open and the engine 1 bleed air switch is on; or engine 2 bleed air switch on, APU bleed air valve and isolation valve open. In other words, it is not dependent on actual air flow, it is solely based on the position of the engine bleed air switches and the isolation valve.Leo Bakker

"Were both 737s you saw NGs?"I didn't check the writing on the side of the fuselage, Leo, but both had bendy wings ;-)" Now, my manuals are for the 3/4/5, so I wonder if there might be a difference between the Classic and the NG? Or is it simply a matter of choice, a company option so to say?"Now that you mention it, I did notice a different Isolation Valve selection on an old 737 on an overnighter. It may have something to do with the power output of the APU bleed (Two packs being too much strain).... or perhaps it's a simply issue of economy ($$$).... Having two packs on, must use up extra fuel. Two packs are also much noisier than one for the ground engineer ;-)"In other words, it is not dependent on actual air flow, it is solely based on the position of the engine bleed air switches and the isolation valve."This seems to a common design element on Boeings (Valve position dictating indication, rather than actual airflow).Thanks again!Cheers.Ian.

>"Were both 737s you saw NGs?">>I didn't check the writing on the side of the fuselage, Leo,>but both had bendy wings ;-)737NG - just check the flightlevel dialed in!>" Now, my manuals are for the 3/4/5, so I wonder if there>might be a difference between the Classic and the NG? Or is it>simply a matter of choice, a company option so to say?"No difference. One pack only on ground = saving fuel and money.Only difference in bleed press - APU source. Classic with -129 APU - over 34PSI. Classic with -280 and NG 131 APU bleed are biased by demand and deliver lower press as shown in pic.eker