Why are APU's so bad

[Gulfer]

I'm very happy to say, that "in 15 years of commercial flying, I haven’t suffered an engine failure or shut down" and long may it last !. .However, If I had a pound for every APU failure or shut I've had, the old pension fund would be looking a lot healthier.. .Surely an APU’s just a tiny engine, Why are they so crap ?

[criticalmass]

Because they are too small? With jet engines reliability seems to be directly proportional to size these days. Maybe an A&P would care to thrown some light on this?

[Blacksheep]

1. Because they are cheap and nasty.. .2. There's little competition. APUs are usually SFE * so the aircraft manufacturer is the customer rather than the end user.

* SFE = Seller Furnished Equipment. There is no choice of supplier, you get what comes with the aircraft. As opposed to BFE or Buyer Furnished Equipment where the customer selects from the available options and negotiates price and product support terms with the equipment vendor.

In some cases the APU in one model of aircraft is the engine core or gas generator of the Prime Mover in a smaller aircraft. Since the reliability of the small aircraft engine is then as expected for a main power plant, this seems to imply that lower quality standards do indeed apply to auxiliary equipment. Which confirms point number one above.

**********************************. .Through difficulties to the cinema

[The Vicar]

Most probably all of the above but also the lifestyle they suffer particularly on short haul operations.

They are most probably operating near maximum capability most of their life supplying Air Conditioning and Electrical requirements on the ground.

After engine start they are shut down and then, being located in the unheated part of the airframe, are subjected to very low temperatures for most of the flight. Then, after landing or just prior,they are fired up again. Can't help wondering how the engines on the wings would cope with frequent shut downs and then relight at very low temperatures like minus 40-50 Celsius.

I remember many years back to prove reliability of a particular APU for operation on a certain route it was required to be continuosly operated in flight for a period. No problems. When it was returned to normal operations the usual problems reoccurred.

Seemed an unusual coincidence that when continuous operation was reinstated for a further check..No problems!

But I agree.... they are a damned nuisance, although I suspect the more recent fitments are much better.

[ 28 January 2002: Message edited by: The Vicar ]</p>

[PA38]

If they are so bad why is the ground not littered with bits of BAE 146'S <img src="tongue.gif" border="0">

[gumbi]

What a wonderful idea did they have for the ATRs with the prop brake concept!!!

I've been flying the L1011 for five years and just converted to A310. L1011 is a great machine but what un underpowered APU does it carry. Now I'm in business on the 310, APUwise! <img src="wink.gif" border="0">