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I know there are planes whose APU (Auxiliary Power Unit) is located elsewhere, but for best sellers commercial jet aircraft (A320, B737, B747, A330,...), the APU is located in the tail. Is there a good reason for that (place, weight, airflow,...)?

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2 Answers 2

up vote 20 down vote accepted

Dead space... Not much room in the tail to put other stuff, and it fits. Plus it has a nice empty space just outside for the exhaust pipe to exit without the hot gasses hitting anything.

Put the APU in the nose and you need a long pipe somewhere to get rid of the exhaust. More weight, more space, etc.
Put it in the wing and you're looking at uneven weight distribution (you want it at or near the centerline).
Leaves the tail area as a logical place, and there just happens to be space there as well in most aircraft.

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2  
and most pressurized aircraft have most of the tail section as unpressurized so you don't have to compromise the structural strength to allow the APU to reside there (it make no sense for it to be in he pressurized section) –  ratchet freak Sep 3 at 9:08
    
@ratchetfreak true, but there's other places like the wing root that might work as well for that. –  jwenting Sep 3 at 9:29
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@ratchetfreak and on many the landing gear. Hence the dead space answer... Of course the PACKs could have been located in the tail and the APU in the wing root, if it made more sense to do that. –  jwenting Sep 3 at 9:35
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For anyone else wondering: PACK = Pneumatic Air Conditioning Kit or sometimes PACS = Pneumatic Air Conditioning System –  CGCampbell Sep 3 at 15:13
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@vasin1987 you'd maybe be surprised to learn that that fuel is used as a heatsink for the airco units... –  jwenting Sep 9 at 13:19

There are a few reasons that the APU is located in the tail in most modern aircraft.

As previously mentioned, the space in the tail is otherwise mostly unused. Aside from hydraulics and such for the control surfaces, there isn't much use for the space. Most aircraft don't have any other spaces like that. Alternate locations like the wing root are generally used for fuel tanks, landing gear, air conditioning packs, and other systems. This area is also outside the pressure vessel, which reduces the number of ducts and such that need to pass through the pressure vessel wall.

Another important benefit is fire protection. Having the APU in the tail places it as far away from the cabin and fuel tanks as possible, and being outside the pressure vessel makes it easier to isolate from those areas.

Having the exhaust exit the tail prevents it from interfering with any other parts of the airplane.

Not having it next to anything else also makes it easier to access for maintenance, generally through doors on the underside of the tail.

The drawbacks of having the APU in the tail include the need to run bleed ducts, fuel lines, and power supply wiring all the way back there. Also, the hydraulics and controls in the tail must be protected in case of a problem with the APU.

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