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What Is An APU?  
User currently offlineZona8 From United States of America, joined Oct 2003, 128 posts, RR: 0
Posted (10 years 3 months 3 weeks 3 days 22 hours ago) and read 14399 times:

I am sorry if this is a stupid question, but I keep hearing the term APU, what does it mean and what does it do? Thanks for the help and Happy Holidays!

33 replies: All unread, showing first 25:
 
User currently offlineAkjetblue From United States of America, joined Dec 2003, 790 posts, RR: 5
Reply 1, posted (10 years 3 months 3 weeks 3 days 22 hours ago) and read 14384 times:

Auxiliary Power Unit: Internal engine on aircraft that provides pneumatic air for engine start, a/c, hydraulics, and power while the engines are off. Am I missing anything?


Save a horse! Ride a Cowboy!
User currently offlineQ330 From Australia, joined Dec 2003, 1460 posts, RR: 22
Reply 2, posted (10 years 3 months 3 weeks 3 days 22 hours ago) and read 14379 times:

An APU is an Auxiliary Power Unit. It is used to provide power to a plane's systems when the engines are not running. APUs are located in the tail cone of most airliners.

-Q



Long live the A330!
User currently offlineRaven111 From United States of America, joined Jul 2003, 106 posts, RR: 2
Reply 3, posted (10 years 3 months 3 weeks 3 days 22 hours ago) and read 14371 times:

APU stands for Auxilary Power Unit.

The APU runs everything on an aircraft while the main engines are off.



"The secret to my success is that I always managed to live to fly another day." Chuck Yeager
User currently offlineJmc1975 From Israel, joined Sep 2000, 3242 posts, RR: 15
Reply 4, posted (10 years 3 months 3 weeks 3 days 21 hours ago) and read 14356 times:

Auxiliary Power Unit - it uses fuel to produce electricity for the aircraft without the need of using the engines.


.......
User currently offlineQ330 From Australia, joined Dec 2003, 1460 posts, RR: 22
Reply 5, posted (10 years 3 months 3 weeks 3 days 21 hours ago) and read 14350 times:

only on a.net can you get three replies within seconds of posting a topic! Big grin

-Q



Long live the A330!
User currently offlineA330Jamaica From Canada, joined Dec 2003, 59 posts, RR: 0
Reply 6, posted (10 years 3 months 3 weeks 3 days 21 hours ago) and read 14309 times:

As was said, an APU is an auxiliary power unit used to supply power when the main engines are down. It is really a smaller version of the main turbine engines that turn alternating current electric generators instead of a fan or propeller. There has been research into using other energy converting devices like fuel cells but for now they remain small turbine engines. If you are ever at an open airport i.e one where you are within reasonable proximity outside to a large commercial airliner, the APU is what makes that hissing sound when it is clear that the main engines are off.

User currently offlineFlairport From , joined Dec 1969, posts, RR:
Reply 7, posted (10 years 3 months 3 weeks 3 days 21 hours ago) and read 14256 times:

evenin jetways, you hear the hissing sounds (some airports are better than others in reducing... at ATL, the terminal is virtually soundproff except when there are arrving planes, but FLL is much less and you clearly hear the AP in the terminal! i=I lovehat sound!

User currently offlineFlpuck6 From United States of America, joined Jun 1999, 2122 posts, RR: 30
Reply 8, posted (10 years 3 months 3 weeks 3 days 21 hours ago) and read 14219 times:

NOT good when the APU is not functioning ... the need for an air start (external machine to push air into the engines) can be potentially annoying, since the air start machines never work when you need them to LOL.


Bonjour Chef!
User currently offlineAJ From Australia, joined Nov 1999, 2381 posts, RR: 25
Reply 9, posted (10 years 3 months 3 weeks 3 days 21 hours ago) and read 14216 times:

The hissing sound is often the result of the air conditioning packs running. As they are mounted in the belly of many jet airliners the noise attenuates on the nearby ground exaggerating the noise.

User currently offlineModesto2 From United States of America, joined Jul 2000, 2767 posts, RR: 6
Reply 10, posted (10 years 3 months 3 weeks 3 days 8 hours ago) and read 14128 times:

Similarly, a ground power unit (GPU) can be connected to the aircraft to serve the same purpose of the APU. Keep in mind that the APU is usually deactivated after engine start.

User currently offlineOPNLguy From , joined Dec 1969, posts, RR:
Reply 11, posted (10 years 3 months 3 weeks 3 days 8 hours ago) and read 14122 times:

Another thing to keep in mind, in addition to whatthe others have mentioned, is that the APU can be used in-flight on most aircraft (the 727 being an exception). With so many of the world's airliners being twins, the APU provides a third generator for redundancy.

The APU is usually located in the tail of the aircraft. In the picture below, you can see the air inlet to the left of the aft service door.


View Large View Medium
Click here for bigger photo!

Photo © Marlo Plate - IBERIAN SPOTTERS



The APU exhaust is right at the end of the aircraft, as seen here:


View Large View Medium
Click here for bigger photo!

Photo © Islam Chen



For a detailed description of what's what, see:

http://www.b737.org.uk/apu.htm

Happy holidays....!


User currently offlinePilotpip From United States of America, joined Sep 2003, 3139 posts, RR: 11
Reply 12, posted (10 years 3 months 3 weeks 3 days 6 hours ago) and read 14102 times:

In addition to electrical power, they also provide pneumatic power for start and can be a suppliment/backup in some aircraft for both systems in flight. The 727 is the only exception becuase the apu is in the wheelwell instead of the tail like other aircraft. They are pretty small too. The APU on the ERJ-145 is comparable in size to a couple large bags of potato chips. Most are made by Allied-Signal.

You can tell that an APU is running on an aircraft when you hear a small jet engine running, that is what it is. It may be harder to hear from the left side because most of the aircraft out there position the business end on the right side of the aircraft, or at the tail.



DMI
User currently offlineCanadianNorth From Canada, joined Aug 2002, 3387 posts, RR: 9
Reply 13, posted (10 years 3 months 3 weeks 3 days ago) and read 14089 times:

APU: Auxiliary Power Unit.

It provides power to the aircraft when the engines are shut off, and can provide the power to start the engines. On some aircraft, it also doubles as backup power incase of engine failure. On most aircraft the exaust outlets under the tail are easy to notice; and i believe Boeing's 727 was the first jetliner to feature an APU, but i may be rong.


CanadianNorth



What could possibly go wrong?
User currently offlineRayChuang From United States of America, joined Jun 2000, 7928 posts, RR: 5
Reply 14, posted (10 years 3 months 3 weeks 2 days 19 hours ago) and read 14067 times:

It should be noted that APU's are essentially miniature jet engines that provide power to an airliner when the jet engines are shut down (this allows APU's to use the same fuel supply as the main jet engines). Frequently, when the aircraft is seriously delayed on ground hold, the flight crew switches the airplane to be powered by APU power instead of power from the running main jet engines.

User currently offlineL-188 From United States of America, joined Jul 1999, 29688 posts, RR: 59
Reply 15, posted (10 years 3 months 3 weeks 2 days 18 hours ago) and read 14048 times:

Not all APU's are turbines, although they pretty much universally are today.

The B-17 used a Briggs and Stratton Lawnmower engine driving a generator for an APU. The B-29 has a bigger two cylinder jobber, I think the motor powering that was a McCullah (spl? My chainsaw is made by these guys)


Those APU's could only supply electrical power. The turbine ones today supply both electrical power but bleed air, which is used by the air conditioning packs and the start motors on the engines.



OBAMA-WORST PRESIDENT EVER....Even SKOORB would be better.
User currently offlineDC-10Tech From United States of America, joined Jun 2001, 298 posts, RR: 2
Reply 16, posted (10 years 3 months 3 weeks 2 days 13 hours ago) and read 14036 times:

Please, could we possibly get one more know-it-all to chime in with 'APU=Auxilliary Power Unit'.

I'm not sure if I fully understand what it is yet.

Thanks.



Forums.AMTCentral.com
User currently offlineWietse From Netherlands, joined Oct 2001, 3809 posts, RR: 56
Reply 17, posted (10 years 3 months 3 weeks 2 days 13 hours ago) and read 14023 times:

APU=Auxilliary Power Unit

There, got it know DC-10Tech?  Wink/being sarcastic

Wietse



Wietse de Graaf
User currently offlineB727-200 From Australia, joined Nov 1999, 1051 posts, RR: 3
Reply 18, posted (10 years 3 months 3 weeks 2 days 12 hours ago) and read 14020 times:


OK DC-10Tech, an APU is an Auxilliary Power Unit. Anything is possible  Big thumbs up

I believe British Aerospace uses four of these under the wings of the BAe146 range of commercial jetliners, although some would say they are actually lawnmower engines as used on the B-17 (re: L-188 above).

Seriously, quite an important piece of equipment. Here in Australia for instance, there are some airports in the "Outback" that do not have ground power, and have very few flights per day (some only one flight per day). As these remote areas are often very hot, airlines could not afford to send an aircraft with a non-functioning APU (air conditioning requirements on the ground). This can make scheduling quite interesting.

Some of the stats of the APU are phenomenal. I am sure someone can correct me with exact figures here, but some can generate enough electricity to run a small town, and spin at about 40,000 RPM.


User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 19, posted (10 years 3 months 3 weeks 2 days 10 hours ago) and read 14005 times:

AJ,
the packs need pressurised air to function. From a ground source, engines (making the packs hard to hear) or the APU.

Many (most?) APUs have an altitude restriction making them unavailable in cruise.

BTW, DC10-Tech, it's Auxiliary Power Unit.  Big grin

It'll have to be a very small town if you are to run it off an APU!

Cheers,
Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineOPNLguy From , joined Dec 1969, posts, RR:
Reply 20, posted (10 years 3 months 3 weeks 2 days 8 hours ago) and read 13985 times:

>>>Many (most?) APUs have an altitude restriction making them unavailable in cruise.

Just to clarify, some do have altitude limits, but the APU -is- available if you're cruising -below- that limiting altitude.

Cheers...


User currently offlineSlamclick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 21, posted (10 years 3 months 3 weeks 2 days 7 hours ago) and read 13988 times:

re: The B-727. That APU was an afterthought. The -100 came out without one. As small cities were obviously going to be a huge market for the 727 the APU was a smart alternative to having each little airport have to buy ground power units. It was added into the main gear well, forward of the wheels and the pneumatic air was originally plumbed through part of the wing anti-ice ducting. On the -200 it got its own piping.

The 727 APU is usable only on the ground. The exhaust discharges over the top of the right wing root and "torching" during start has caused more than one passenger-initiated evacuation.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineAirplay From , joined Dec 1969, posts, RR:
Reply 22, posted (10 years 3 months 3 weeks 2 days 7 hours ago) and read 13975 times:

Apu runs the "Quicky Mart" in Springfield. Although I'm sure he is quite important, the town can do without him for days at a time....

User currently offlineAJ From Australia, joined Nov 1999, 2381 posts, RR: 25
Reply 23, posted (10 years 3 months 3 weeks 1 day 13 hours ago) and read 13946 times:

Thanks FredT, I'm very familiar with the air conditioning packs. My point was the noise heard by passengers boarding via an aerobridge is the packs operating, turn the packs off and the APU is audible, but not nearly as noisy.

User currently offlineModesto2 From United States of America, joined Jul 2000, 2767 posts, RR: 6
Reply 24, posted (10 years 3 months 3 weeks 1 day 6 hours ago) and read 13915 times:

One can also determine if the APU is running by looking at the tail cones of most aircraft. If it's running, one can see the exhaust distort your vision.

25 A/c train : I dont believe its taken 24 replys to answer waht an APU is. Airplay got it right anyways ! this is the saught of topic that was looking for flowerin
26 Bio15 : Though a bit late I thought I might add a note: 747-400 APU's are capable of providing enough air to start two engines at the same time. That is a pre
27 Brons2 : Apu runs the "Quicky Mart" in Springfield. Although I'm sure he is quite important, the town can do without him for days at a time.... This post made
28 Fritzi : If I recall correctly, the VC-25s (US presidential 747-200) are fitted with 2 APU's, in case anybody wants to know.
29 CRJDispatchKid : Let's not forget how warm an APU can keep you on those cold winter nights when you are working the ramp. I sure don't miss those days.
30 Post contains links NoUFO : To add my 2 Cent (Euro): Here's an article on APUs: http://flug-revue.rotor.com/FRHeft/FRHeft04/FRH0404/FR0404d.htm
31 VC-10 : Let's not forget how warm an APU can keep you on those cold winter nights when you are working the ramp. I sure don't miss those days. During the nigh
32 747srule : What's the diff between a 747 and an A340? An A340 has 5 APU's!
33 Post contains images Starlionblue : I think you mean an Avro100/BAe-146 has 5 APUs...
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