Sign in to follow this  
Guest bobsk8

APU Usage

Recommended Posts

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?

Share this post


Link to post
Help AVSIM continue to serve you!
Please donate today!

good question, wondering this one myselfanyone?jason diaz

Share this post


Link to post

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

Share this post


Link to post

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

Share this post


Link to post

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

Share this post


Link to post

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

Share this post


Link to post

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...

Share this post


Link to post

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

Share this post


Link to post

"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

Share this post


Link to post

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

Share this post


Link to post

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?

Share this post


Link to post

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

Share this post


Link to post

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.

Share this post


Link to post

"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.

Share this post


Link to post

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

Share this post


Link to post

Bonjour,As you I am not clear of the use of APU during TO.It seems that when maximum power is needed for takeoff, only the left pack is used and air is provided by the APU not by the engines.Claude Troncy

Share this post


Link to post

ClaudeThis is termed a 'bleeds off takeoff' and is only normally used when you are heavily loaded or on a short runway which limits your performance. The APU is used to supply air to one pack and the engine bleed valves are turned off. Once airborne the system is returned to normal ensuring that you never back pressure the APU as this has pretty disastrous affects on the APU. Hope that helpskris

Share this post


Link to post

"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!"Sorry, Kris... I'm still confused...On the ground or in the air, if both packs feed a mix manifold on the Classic (presumably they do) and this cool air is distributed evenly throughout the cockpit and cabin, why should turning off one pack affect only one area. Surely with one pack off, you would get 1/2 cooling in all areas, rather than proper cooling in one area (e.g. cockpit) and no cooling in another? I'm not familiar with the shape of the mix manifold on a 737... Perhaps the Classic mix manifold is shaped in such a way that one pack favours the cockpit? (In schematics it is always depicted as a box, rather than a complex shape). The 737 NG mix manifold is sort of (upright) cyclindrical shaped with the cool air going into the manifold from 3 directions (2 packs and a ground connection) at the bottom of the cylinder and cold air going out at the top through pipes to the various areas.Hope this makes sense.Cheers.Ian.

Share this post


Link to post

IanOn the Classic there is a direct feed from the Left pack to the cockpit via the cockpit supply duct so effectively 50% of the air created by the Left pack goes direct to the pointy end, the other 50% goes to the mix manifold and is distributed accordingly. Thus by tuirning on the Left pack we get nice cold air almost straight from the pack without any warming affect from the mix manifold. If the right pack is used the mix manifold gets the lot and we just get the remnants of what is not used in the cabin as the flight deck outlet is pretty much at the end of the distribution ducting as I understand it. As to its physical shape we leave that to the gingers and live in our own little happy world also known as Bill Bulfers guideKris

Share this post


Link to post

There is a bit of airline procedure involved, but too certain conditions require the apu- ETOPS in a 737 requires the apu to be running (ask Aloha Airlines- they fly 737's from Hawaii to the mainland- the longest scheduled ETOPS 737 flights)Or on the ERJ the APU is running for every takoff and landing- at least on American Eagle. Gives more margin in case of an engine faliure or goaround.Tim

Share this post


Link to post

TimAs a non-ETOPS operator I wouldn't know the exact procedures however I would suspect that for ETOPS the APU must be 'servicable' rather than operating for the entire flight. Running the APU for a long range flight would accomplish only one thing... turning valuable fuel into noise as the APU would not be running any services during the normal operation of the aircraft, only in an emergency or malfunction would the APU be required to provide electrical and/or bleed airBear in mind that even the NG APU is limited to the following:For 2 bus electrical power ONLY max alt 41,000ftFor 1 pack OR both transfer buses max alt 17,000ftFor 1 pack AND both transfer buses max alt 10,000ftAPU start is not guaranteed above 25,000ft on the classic (not sure about the NG)So you see running the APU would simply waste fuel for a long cruise.in the case of operating the APU during takeoff and landing, this is a prudent course of action and I suspect it is a protective comapany procedure rather than an aircraft requirement (although I have no experience with the ERJ). We operate the B737 calssic with the APU on for take off and landing in poor weather but there is no need in fair weather as you are less reliant on the instruments and more on the mk.1 Eyeball. In poor weather if you lost a gen you COULD lose a bus if the bus transfer system doesn't work correctly, by having the APU on and running it means you can very rapidly restore the inoperable bus by moving one switch allowing you to concentrate on flying the aircraft.Hope that muddy's the waters some more, you can't beat a good discussion on trivia!Kris

Share this post


Link to post

Hi Bob,I realize you were probably looking for APU info during takeoff and you have a fair number of good responses in that area on this thread already.One other area where the APU is used in flight is during ETOPS operations. Most airlines (or perhaps it's an ETOPS requirement) must have the APU running during ETOPS flights. The following is an excerpt from an AOM.APU UsageAn operational APU is required for ETOPS flights. Due to the limited time thatthe B737 can operate on standby electrical power it is required that the APU berun continuously in ETOPS airspace. The policy for ETOPS flights is to startthe APU on the ground and leave it running until past the Extended Range ExitPoint (C-EXP). Continued operation of the APU beyond the C-EXP is at thediscretion of the Captain.Operation of the APU on non-ETOPS LRN flights is not required.

Share this post


Link to post

I stand corrected! This must be due to the lack of a RAT on the B737Kris

Share this post


Link to post

Geez, I am eating this up!!!! Thanks to everyone for the wonderful detail. Here's one vote for this level of discussion.Sorry to be dense, but what is ETOPS??Thanks so much.

Share this post


Link to post

ETOPS = Extended Twin OPerationSAlso known as EROPS - Engines Refuse to Operate Please Swim (or rather Extended Range OPerationS)Kris

Share this post


Link to post

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this