Hole in the tail-end

Sorry about the title.

I have spent a lot of time waiting around in airports and on runways today and whilst looking at the back end of the planes I realised that they all seem to have an exhaust pipe coming out of the tail.

Now I understand about the workings of a jet engine, and I was under the impression tha all of the exhaust from those comes out the back of the engine. So why is it that there is an extra exhaust +/- engine actually in the plane? Surely you don't need one that big just for the aircon.

[preiffer]

It's called the APU (Auxillary Power Unit). Something's got to start those engines, provide ground power, lighting and aircon

It's basically a mini jet-engine.

If you take notice of the "Boeing vs Airbus" threads on other boards, they will tell you it is a 5th engine required to get "underpowered" A340 off the ground!

As preiffer says they are used for ground power, lighting and AC, and give enough pneumatic power I beleive to start up the first main engine.
I think in Hot and High airports the APU is also used during take off to run some of the Airconditioning packs whilst the main engines are busy doing the important stuff rather than driving the Aircon.

Last year I was stuck on a QF 744 at SIN bound for LHR. Having missed our slot we were stuck on board and on the ground for some 90+ minutes before we got another slot allocated. The crew informed us of the failiure of the APU. A GPU was used for start up that day, and an air-con trolley hooked up to us for the delay, which made no difference atall to be honest.

The APU is used to generate enough power onboard the aircraft to start the main engines. It is also used to power the aircraft when no ground power is available, as it has a much lower fuel burn than the main engines.

I work on regional jets, so I'm not sure about widebodies, but our RJ's cannot be started from ground power. We have a ground power unit that is capable of powering the aircraft, but the current draw during engine start is too great for a lot of ground power units. When the APU is inoperable, the only other method of startup is an air start/air bottle, which throws near 1,000 psi of compressed air into the turbine to turn it over.

On the CRJ 200 you'll notice a little door next to engine 1, just forward of the tail, on the top of the aircraft, this is the APU inlet, which opens when the APU is in use. Also on the 200, the APU outlet is on the right side of the aircraft, below engine #2. You can see the inlet door just over the engine in this photo.

On the 70 seater CRJ, and many other aircraft types (including widebodies), the APU outlet moves to the tail, which explains the description provided in the OT. The inlet door on the 70 seater moves just to right of the tail, on the back of the aircraft. This can be seen on ship 10182 in this photo.

Now for the VS fleet...

B744- APU Inlet, at the base of the "V", right side of the tail..

B744- APU Outlet, center of the fusalage, aft of the tail..

A346- APU Outlet, center of the fusalage, aft of the tail..

I couldn't find a good photo of the inlet on the 346, so if anyone has one, feel free to add on.