I've seen many tutorials and checklists on the startup of these aircraft (737-800/900) and most procedures match up closely with the exception of when to turn on and off these (pack,isolation,trim air, recirculation fans) switches...particularly before APU startup, and after shutting down after a flight. I've yet to see when to turn on and off the trim air switch. I know about when to turn off and on the bleed switches but like I said, there are a lot of different ideas out there on the other switches. There are a lot of good tutorials out there and I've seen some great checklists but all vary differently somewhat concerning these. I know there are a lot of scenerios but the below 3 are what I'm most interested in. Would prefer some expertise advice on these:taking into account that you should only start up the APU 5-10 minutes before engine startup, what position should these switches be in the 30-60 minutes beforehand?Lets say after a flight the aircraft you've just flown is going to be heading out again within 1-3 hours. What position should these switches be in in this case?What if the aircraft is done for the day and you've just shut down the engines. How soon and which swithes should be in what position during this period?Thanks for your inputChris

I've seen many tutorials and checklists on the startup of these aircraft (737-800/900) and most procedures match up closely with the exception of when to turn on and off these (pack,isolation,trim air, recirculation fans) switches...particularly before APU startup, and after shutting down after a flight. I've yet to see when to turn on and off the trim air switch. I know about when to turn off and on the bleed switches but like I said, there are a lot of different ideas out there on the other switches. There are a lot of good tutorials out there and I've seen some great checklists but all vary differently somewhat concerning these. I know there are a lot of scenerios but the below 3 are what I'm most interested in. Would prefer some expertise advice on these:taking into account that you should only start up the APU 5-10 minutes before engine startup, what position should these switches be in the 30-60 minutes beforehand?Lets say after a flight the aircraft you've just flown is going to be heading out again within 1-3 hours. What position should these switches be in in this case?What if the aircraft is done for the day and you've just shut down the engines. How soon and which swithes should be in what position during this period?Thanks for your inputChris

Bear with me - I've been out of the plane for 6 months. . .Don't mess with the TRIM AIR switch unless you're dealing with a ZONE TEMP caution. It's always ON unless you're following the QRH or an MEL. The pneumatics panel rules seem to change every so often, and every airline has a few differences.30-60 minutes before hand, you hopefully have good PCA (pre-conditioned air) and the packs and bleeds will be off (the engine bleeds can be on - no biggie - but at least APU bleed off). The rampies will probably kill the PCA before you want them too, or they will disappear when you need them to pull the air so you can turn on the packs. When you do get the APU running, after 1 minute, PACKS - AUTOAPU BLEED - ON ISO VALVE - OPENBefore StartPACKS - OFFISO VALVE - AUTOAfter StartPACKS - AUTOAPU BLEED - OFFAs far as the other situations, it's kinda up to the airline. If its a really hot or cold place, the company may have you run the APU. The FAA wants you to use PCA if it's available to prevent the plane from getting to hot (a la TWA 800).At the end of the day, maintenance may want to work on the plane, so I typically would call and ask how "shut down" do they want the plane. They might be taking it to the hangar as soon as you're out, so they might not want to put another cycle on the APU.Basically: Don't mess with the TRIM AIRENGINE BLEEDS can stay on unless you're doing a BLEEDS OFF T/O.RECIRCS - as REQ'D. (There's a note in my manual that says RECIRCS recommened off for cooling, but I don't know about that one.)Hope that helps a little!Matt Colles

That help a lot. One question though, before starting the engines, if you have the engine bleeds on like you mentioned was ok, should you turn them off or does it matter?Just trying to keep things as realistic as I can :)Thanks,Chris

That help a lot. One question though, before starting the engines, if you have the engine bleeds on like you mentioned was ok, should you turn them off or does it matter?Just trying to keep things as realistic as I can :)Thanks,Chris

They'll be on. Another time you turn off engine bleeds is during deice. Besides a BLEEDS OFF T/O (which, now that I think about it, I don't think you do in an NG), and QRH stuff, they're pretty much always on.A big concern is not to have DUAL BLEED (APU & ENG) on with the engines above idle. Matt Colles

They'll be on. Another time you turn off engine bleeds is during deice. Besides a BLEEDS OFF T/O (which, now that I think about it, I don't think you do in an NG), and QRH stuff, they're pretty much always on.A big concern is not to have DUAL BLEED (APU & ENG) on with the engines above idle. Matt Colles

Hi Matt,Great explanations. Just to add, I dont think the 1 minute wait to open the APU bleed valve after APU start is there anymore on the NG. It was there on the Classics. But I could be wrong. Besides, almost all the times when entering the a/c, maintenance has already got the APU running and on-line, or it has been left on line by the previous crew.With regards to BLEEDS OFF T/O, it is still performed on the NG, of course circumstances prevailing which require it. Cheers,Dinshaw.

Hi Matt,Great explanations. Just to add, I dont think the 1 minute wait to open the APU bleed valve after APU start is there anymore on the NG. It was there on the Classics. But I could be wrong. Besides, almost all the times when entering the a/c, maintenance has already got the APU running and on-line, or it has been left on line by the previous crew.With regards to BLEEDS OFF T/O, it is still performed on the NG, of course circumstances prevailing which require it. Cheers,Dinshaw.

According to my books, the restriction on 1 minute is still there for the NG, but if you turn it on before that, the "smarter" NG APU won't actually move the valve until the 1 minute has elapsed. Since my airline operates both classics and NG, I think they just keep it 1 minute for the sake of consistency.As far as having the APU running at all times, my airline put a stop to that when gas went up to $140/barrell. They actually have an "APU Sherrif" who walks around counseling crews on how much money they could save the company if they leave the APU off and rely on PCA instead.~Matt Colles

I was hoping Matt would chime in on this. Thanks.

An APU sheriff. I bet that is a job that doesn't get many volunteers, I suppose if you have to be "reminded" more than twice it goes in your permanent record.