A330Jamaica From Canada, joined Dec 2003, 59 posts, RR: 0 Reply 6, posted (9 years 11 months 3 weeks 1 day 17 hours ago) and read 13486 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.
Flairport From , joined Dec 1969, posts, RR: Reply 7, posted (9 years 11 months 3 weeks 1 day 17 hours ago) and read 13433 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!
Flpuck6 From United States of America, joined Jun 1999, 2120 posts, RR: 32 Reply 8, posted (9 years 11 months 3 weeks 1 day 16 hours ago) and read 13396 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.
OPNLguy From , joined Dec 1969, posts, RR: Reply 11, posted (9 years 11 months 3 weeks 1 day 3 hours ago) and read 13299 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.
Pilotpip From United States of America, joined Sep 2003, 3117 posts, RR: 11 Reply 12, posted (9 years 11 months 3 weeks 1 day 1 hour ago) and read 13279 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.
CanadianNorth From Canada, joined Aug 2002, 3377 posts, RR: 9 Reply 13, posted (9 years 11 months 3 weeks 20 hours ago) and read 13266 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.
RayChuang From United States of America, joined Jun 2000, 7864 posts, RR: 5 Reply 14, posted (9 years 11 months 3 weeks 15 hours ago) and read 13244 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.
L-188 From United States of America, joined Jul 1999, 29513 posts, RR: 59 Reply 15, posted (9 years 11 months 3 weeks 14 hours ago) and read 13225 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.
B727-200 From Australia, joined Nov 1999, 1051 posts, RR: 3 Reply 18, posted (9 years 11 months 3 weeks 8 hours ago) and read 13197 times:
OK DC-10Tech, an APU is an Auxilliary Power Unit. Anything is possible
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.
Slamclick From United States of America, joined Nov 2003, 10062 posts, RR: 69 Reply 21, posted (9 years 11 months 3 weeks 3 hours ago) and read 13165 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.
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AJ From Australia, joined Nov 1999, 2380 posts, RR: 26 Reply 23, posted (9 years 11 months 2 weeks 6 days 9 hours ago) and read 13123 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.