APU Usage

[Guest bobsk8]

It seems that I have read that the APU should be turned off once engines have started, and I have also read that APU should be left on until at cruising altitude. Which is correct, and if the APU is to be left on, what is the reasoning behind it being left on. Also, should it ever be turned on prior to landing?

[Guest JDiaz]

good question, wondering this one myselfanyone?jason diaz

[Guest Nickman]

I think different airlines have different procedures for the APU. The only reason I think they would keep the APU on until cruising altitude is if both engines fail, you would have something to at least give you the minimal power you need. Good questionNick B.Continental Airlines Virtual 737NG Pilothttp://www.precisionmanuals.com/images/forum/800driver.jpg--AMD Athlon XP 3200+ @ 2.2 Ghz (Equal to 2.8 ghz)400W Power supply3x 80 mm Case FansSoyo VIA KT600 Dragon PlusnVidia GeForce FX 5200 128 mb2 x 512 PC400100 GB Western DigitalMicrosoft Sidewinder Precision 2

[Guest Pilot3033]

From what I have experianced most airlines use ground power at the gate, they start the APU before pushback and the closeing of the main bording door. The leave it on for push and start, and turn it off after both engines has started, at which point they turn on the gens. That explains the quick loss of power in the plan for a second or two after PB. This all comes from flying as a pax and reading the manuals. I don't know for sure.-Matt

[Guest Nickman]

On all the flights I've been on (mostly CO), you would always hear (and sometimes see) them switching the generator to the engines. Usually you see the lights flicker, AC and seat IFE go off for a few seconds. Sometimes you can ever hear a humming sound.Tells me they switch to engine generatorsNick B.Continental Airlines Virtual 737NG Pilothttp://www.precisionmanuals.com/images/forum/800driver.jpg--AMD Athlon XP 3200+ @ 2.2 Ghz (Equal to 2.8 ghz)400W Power supply3x 80 mm Case FansSoyo VIA KT600 Dragon PlusnVidia GeForce FX 5200 128 mb2 x 512 PC400100 GB Western DigitalMicrosoft Sidewinder Precision 2

[gbalean 0]

Hi AllAPU usage depends on the operator. Most companies either turn the APU off after connecting the IDGens to the busses, or leave them on until turning them off post takeoff around the stage where the After Takeoff Checklist is completed.I'm not 100% clear on this, but I believe that this is because the APU can act as a source of bleed air required to relight the engines or something. Regardless, it is certainly as a backup measure.Correct me somebody as you see fit if i've made some mistakes,Geoffrey Balean YSCBVirgin Blue Virtual - VOZ9211ZAB & SSY Observer (OBS)http://www.vozvirtual.org/images/pmdgforumsignature.pngMSN - gbalean@hotmail.comCaution! Not a real ATC, but I do sit around all day in an airconditioned office :-dohP4 2.4gHz | ASUS P4G8X Deluxe | 512 DDR 333 | LG L1710S 17" LCD | GeForce MX440 MS | Soundblaster Audigy | Quantum Fireballp AS30.0 | Win XP Pro | CH Yoke & Rudder

[Guest rellehenk]

After pushback when engines are started and Engine Generators are connected, you do not need the APU anymore.After landing, the APU is being turned on and de busses will be connected to the APU. So when shutting down the engines, there is still a power source available (also battery of course...).After the ground power is connected you can shut down the APU.Do not use APU bleed and Engine bleed at the same time when using more than idle power...

[Guest boxjockey99]

GuysAPU is generally at the crews discretion unless an airfield says their use is limited for noise reasons. Our company uses them all the time as GPU's aren't necessarily available all the time. Also in hot places the aPU is nice to have on as it provides bleed air for the Air con packs! Generally the aPU is switched off prior to take off unless the weather is poor in which case the crew may decide to leave it onjust in case a gen was to fail as this would mean losing the failed sides EFIS. Usually it is not left on up to cruise alt because the little engine in the tail doens't really like the thin air and can cause problems at high levelAPU is generally started after landing as part of the F/O after landing scan and the busses are switched onto it once the blue APU GEN OFF BUS light comes on. THe APU is started in flight when either an emergency exists requiring its use or if you are doing a real CAT III approach in anger in which case again it is nice to have a backup gen if one of the main engine gens were to failHope that helpsKris

[Qavion 19]

"Generally the aPU is switched off prior to take off unless the weather is poor in which case the crew may decide to leave it onjust in case a gen was to fail as this would mean losing the failed sides EFIS."Why would you lose the EFIS, Kris? Wouldn't the Bus Tie(s) close automatically? If there was a serious short on the failed generator side preventing Bus Tie closure, wouldn't the APU have the same problem providing electrical power to that side of the bus?Thanks for any insight.Cheers.Ian

[Balint 18]

Hello There,As to my knowledge, there is a restriction by Boeing, I believe it being FL180 above which altitude APU can not be operated under any circumstances.Cheers

[ahaka 0]

Hello,Am I right that in 737NG the APU must be on to get the aircondition to work while on the ground with engines off? Since there's no switch or indication for external bleed air this must be the case?

[Guest boxjockey99]

IanYou are correct of course, the Bus tie's would close and prevent loss of EFIS, I was thinking of a full AC fail which results in Loss of EFIS and reversion to SBY instruments except on 2 of our fleet which do have emergency power to the no. 1 EFIS. The aPU is required for CAT III ops as 2 independent power systems are required for the use of the 2 a/p's so by having the APU running you can quickly switch it onto the bus re-engage the disconnected A/P and continue the Cat II approach.In answer to the other queries, The APU can be operated up to FL300 on our fleet (300 series). The restriction you mention may relate to the starting of the APU which is limited to FL180 I think (I'd have to check)Yes the APU is required to drive the Air Con packs while on the ground with engines off. The APU bleed air is used to drive a single pack and thus provide a/c on the ground. The only problem then is deciding who gets the air ....left for flight deck or right for PAX? Kris

[Guest FattyBear]

Yes the APU is on to use the A/C packs, if it is not on, an air cart can be used, connected to the mix manifold under the aircraft to supply bleed air for the packs and engine start. If you ever see those big yellow tubes with the black rings around them, those are used for external air source.

[Qavion 19]

"The APU bleed air is used to drive a single pack and thus provide a/c on the ground." Kris....The NG has a much improved APU and can handle two packs without any problems."The only problem then is deciding who gets the air ....left for flight deck or right for PAX?"You're joking (I hope)! Or are you really trying to tell us the pax on a Classic 737 would freeze/die of carbon dioxide poisoning if the Right pack failed in flight....unless, perhaps, you opened the cockpit door.Jokes aside.... please let's not confuse the PMDG NG users any more than some of them already are ;-)Cheers.Ian.

[Guest boxjockey99]

IanAs I said I am not on the NG and therefore have to exist with a 'gutless' aPU that can often struggle with one pack never mind 2, perhaps the boss will buy us some NG's in the near future!When saying which pack to choose I meant on the ground! Ie do we let the pax keep cool or do we keep all the cold air for ourselves in the pointy end! In the air the classic air con packs would switch to high flow should one fail for any reason and one single pack is more than enough to keep the cabin and flightdeck pressurised and cooled / heated. I didn't realise I was confusing anyone merely passing on my knowledge, yes the NG is different to the classic but to honest not by that much, after all I do have the NG on my license without even having sat in the thing. Kris