Hi!okay, I'm confused. Thats nothing really new, my wife keeps suggesting that I'm confused since she knows me :-doh, but here somebody certainly can help:When is the APU used? Sure, it's used to start the (first) engine, but afterwards? When is it shut down? Before take-off? Afterwards? When is it restarted (in flight, on ground)?. Can a 737 (or other Jet) be started without the APU, just with external power and bleed air (if a device providing this exists)? Is the APU restarted on approach or after landing? Does this depend on the weather and if so, how?I've tried to figure all this myself using the usual sources, but I've found odd and partly controversal information. One example is the "PMGD 737 Procedures" checklist, it says "APU off" in the "after start section" and again in the "climb and cruise section". Now what? :-hmmmOkay, hope my english is sufficient despite my considerable confusion. Maybe someone understood what I want to know and likes to illuminate my path :-)

Hey Robert,>okay, I'm confused. Thats nothing really new, my wife keeps>suggesting that I'm confused since she knows me :-doh, but>here somebody certainly can help:I know that feeling :)>When is the APU used? Sure, it's used to start the (first)>engine, but afterwards? When is it shut down? Before take-off?>Afterwards? The APU is used when on the ground when no external ground power is available or the GPU (Ground Power Unit) is unreliable. The other situations where you need the APU are usually the times that not everything is going to plan. Have a look throught the abnormal procedures manual for examples.Usually the APU is used to start both engines and is turned off when everything is up and running (I usually turn it off during taxi). >When is it restarted (in flight, on ground)?. I usually restart it as part of my after landing checklist, it can be started in the air aswell. >Can a 737 (or other Jet) be started without the APU, just with>external power and bleed air (if a device providing this>exists)? Yes, some GPU's can provide the bleed air required to get the engines turning.>Does this depend on the weather and if so, how?Not as far as I know, if your big engines are running then your little engine in the back can be running aswell (there are limits though, see the link below)>I've tried to figure all this myself using the usual sources,>but I've found odd and partly controversal information. One>example is the "PMGD 737 Procedures" checklist, it says "APU>off" in the "after start section" and again in the "climb and>cruise section". Now what? :-hmmmThe two entries are just for a double check, no other reason.>Okay, hope my english is sufficient despite my considerable>confusion. Maybe someone understood what I want to know and>likes to illuminate my path :-)I hope I've helped :)For more info on the APU and some of it's operating limits check out [a href=http://www.b737.org.uk/apu.htm]this[/a] page.CheersDavid