Engine start with external air - APU inop or cost saving ?

[Volume]

Hi all,

during my last 4 domestic flights (same carrier, different type of a/c, different airports) we had external air usage for engine startup, and no air conditioning while boarding 3 times.
Are APUs so unreliable that this could be three times an APU inop ? Or is it so expensive to use the APU during short flights (less than 1 hour) that this is purely for economic reasons ?
Is there some AD on some types of APU that do not allow to operate them at the moment ?
Do you have some operational restrictions on the A32X or 737 if the APU is inop ?

[Notso Fantastic]

APUs suffer from ´significant´unreliability problems. There may be a shortage of spares as the summer season starts. These aircraft may be waiting for downtime to carry out repairs. I don´t think any operators would not use an APU in preference to starting manually-it´s a pain using external means.

[mogley]

It also costs $$$$$$ to get groung pnuenatics

[Flap 5]

There is no problem in despaching a flight with a u/s APU. Therefore Operations do not consider it to be a problem. Therefore there is no urgency to fix the APU. I have known an APU to be u/s on an aircraft for days before it is fixed. It is a hassle for the aircrew and hot for the passengers if external air conditioning is not available at your destination. You just need external power and external (pressure) air to start one engine. You then bleed air from the started engine to the other engine to start it (a 'cross bleed start'), and off you go.

[Boss Raptor]

Many noise sensitive airports dont allow (prolonged...) running of APU's as they are very noisy and not that enviromentally friendly, as such fixed ground power provision (and AC feed) is often made at the jetway/stand as opposed to a GPU

APU's are also expensive in terms of running costs, fuel/mx. and also have a high failure rate particularly in certain climatic/environmental conditions...in the case of an aircraft where the APU is enabled for inflight use in emergency, 777 for example, it seems better to keep it safe for that purpose as a preference.

Most airlines/scenarios I have come across at major airport operation will use ground power and/or a combined GPU/Air Start in preference to APU

Should add that with Boeing on some of their non ETOPS acft types the APU is a customer ordered option and certainly with the B737-300/400/500 series the APU can be removed long term at customer request/requirement and a specific Boeing supplied blanking kit seals everything off - some operators do this to save weight and/or mx. cost - again it revolves around the customers wants and his infrastructure at the places they serve and have seen the APU permanently removed on pax (737-300 and BAe 146 family) and freighter acft many times

[seat 0A]

Well, Boss, I have to disagree.
In my daily work I never see a plane do a groundair start and crossbleed unless the APU is U/S.
It`s a nuisance for ATC and all other staff around the A/C.

We always use our APU`s, because it is more comfortable for the pax, the groundstaff and it is safer.

[Silvertop]

And it adds the best part of ten minutes to the turn around, also the reliability of the GPU's at many places is dodgy!

[West Coast]

Can't see removing an APU, especially on a two holer. It may be an expense, but its worth its worth it to have it when you really need it. A deferred main gen on an aircraft with an APU removed might convince the bean counters otherwise. Unless that is they are dispatching on one gen alone, and then I don't think I would want to be on the plane.

[Flap 5]

West Coast has a good point as the APU runs a third generator. Although you can not start the APU at altitude you can when you descend below around 25,000 feet allowing you to bring the APU geny on line if you have lost one of the main geny's. This is not a consideration in the MEL though because you would have departed with two serviceable generators. Therefore from the point of view of the company Operations department (who are overseen by accountants) it is not a problem to depart with a u/s APU. The APU therefore only gets fixed when it is convenient to so, and safety yet again takes second place to economics.

[matkat]

Flap 5 i must disagree with you here,the APU being inoperative is a maintenance function and ops have nether input nor a clue about maintenance on any aircraft system.Also if the APU is inop it comes under the same MEL restrictions as any other part of the aircraft and must be repaired in the time-scale prescribed.
matkat

[West Coast]

How can a deferred APU bethe province of MX alone? Ops needs to be in the loop. At higher elevation airports the bleeds open hit is significant, transating to fewer pax/cargo. Again that can be mitigated to a degree by an unpressurized T/O, but it loses its value at places like Denver and alike.

I've seen APU's fall under 10 day limits per the MEL. Thats a lot of potential lost revenue. Someone will be on MX's ass in a hurry.

[Flap 5]

matkat,

Not sure what angle you are getting at here. Operations are informed of the serviceability of all the aircraft in the fleet otherwise how can they allocate aircraft to fly the next day? Some aircraft can not fly at all due to maintenance, some can night stop away from base, some need to return to base for maintenance. Operations clearly need to know the maintenance status of all aircraft to do their job. As the APU being u/s is not a groundable state for an aircraft, but only a possible problem for a night stop away from base, the aircraft is serviceable to fly as far as operations are concerned and will be programmed as such for the next days flying.

Yes there is supposed to be a maximum time for an APU to be u/s but I think you will find that this will often be taken to the limit and then extended further as allowed.