APU question

[WebMaximus 372]

Hi,What exactly is the right procedure for the APU during startup? I noticed that when starting up with a "warm & lit" cockpit the APU switch is in it's 'OFF' position when the engines are already running on the ground but when starting from a "cold & dark cockpit" and going through all the startup procedures yourself to fire up the engines (have followed Timothy Metzinger's tutorial) the APU is left in it's 'ON' position until you're already in the air and then it's switched off and that works fine. If I try to turn it 'OFF' before that while still on the ground all different kinds of warninglights illuminate on the overhead panel.Thanks in advance for any info on this subject,

[marcom 139]

Hi Richard,it seems as though you forget to switch on the generators before you shut down the APU! When you shut down the APU in your scenario on the ground, you are suddenly left with battery power only and therefore many warning lights come on. So why isn't this happening in the air?The 737NG has a unique feature where it will automatically engage both generators should you have taken off with APU and switch the APU off without manually switching the generators on. Note, this feature only works once in a flight!The correct procedure, assuming you don't need the APU for takeoff (on short runways you may need the APU for pressurization to get more power from the engines), you'll shut down the APU after both engines are running, you have pushed back and you have recieved a signal from your ground crew. Now you first activate both generators and then shut down the APU. Don't forget to set up your bleed air system now! You shouldn't get any warnings now!Regards,Mark

[WebMaximus 372]

Hi Mark,Thanks for your rapid reply, works fine now! Realize I could learn a lot more about both the APU but also the "bleed air" and "left and right packs" concepts which I honestly don't know that much about... :-roll When it comes to the APU there are five switches associated with it as far as I can tell - the APU switch itself and the two switches named "APU Gen" and then two more switches named "Gen 1" and "Gen 2"...

[marcom 139]

Directly associated with the APU are:APU switchAPU Genand APU Bleed.Gen 1/2 control the left/right engine driven generators. Basically, the generators supply the aircraft with electrical power (lets forget about battery and external power for a while). You have to select which of these sources supply the aircraft. If the APU Generator is supplying power, you have to select a different power source before switching off the APU, as otherwise you're left with no "real" power. (Again, battery is left out for now).Regarding the bleed air, again, there are three sources of air - the left engine, the right engine and the APU. I suggest you read the manual to find out more about it!Regards,Mark

[WebMaximus 372]

Thanks for all info provided Mark and as already said I think diving into the different manuals would be a good idea...;-)