Has anyone noticed how the APU bleed switch switches itself off when you shut down the engine? (Couldn't figure out where the clicking noise was coming from :-))I realize that the APU bleed valve closes automatically when the APU is shut down with the bleed switch left ON, and opens automatically when the APU is started (and reaches a certain rpm), but the APU bleed switch is just a toggle switch and can't move by itself (or is the F/O picking up on my sloppy cockpit procedures :( ).And here's something else I learned today:During engine start, the bleed duct pressure increases when you turn the start knob, but only if you are using the APU to supply bleed air. If you are using an external source of bleed air, the pressure goes down. I'd say that the 737 is like the 747-400 where an electrical signal from the start switch goes directly to the APU controller to tell it to put out more air.Cheers.Ian.

Hi Ian,The APU switch movement has been noted before, I believe also in relation with problems of getting the APU switched off properly. And there are more moving switches. For instance, you can get the window heat switches to trip to off by removing their electrical power source. I can't find anything about this in my manuals so it is my impression that this isn't right.Leo Bakker

APU bleed.Older type of APU (-129 classic) have a pressure drop during engine start.New type (-280) and 737NG are demand controlled. Selecting eng. start = pressure increases.eker

Thanks for the tips, Eker :-) You never know when this might come in handy.Cheers.Ian.

"And there are more moving switches."I was surprised when I read in our -800 manual that the A/T switch is held in place by a solenoid (unlike the 767 and 747-400). :-)Cheers.Ian.