Why are APU's INOP so often?

Andrew_VT

Andrew_VT

Well-Known Member
I'm just reading through the Turbine Pilot's Flight Manual (getting ready for class!) and when I got the the APU section couldn't help but think of the 100's of instances I've heard on JC and elsewhere of APU's not working.

My question is why are they broken so often?

Theoretically it's a turbine engine just like the main engines, and we all know the propulsion turbines are very reliable. Is the difference APU's aren't held to the same quality control/maintenance schedules as the main engines? Are the problems mostly not with the turbines themselves but with hardware they power such as the alternators/generators/voltage regulators/inverters/flux capacitors (I just guessed at this list as you can probably tell) ect?

Thanks in advance (especially to Max who I know works at Honeywell:))
 
APUs are a fairly simple device, however there is a lot of automation in it. Starting, generator contols, etc. It is all the bells and whistles that usually go TU not the actual engine itself. They are also usually D items on the MEL list meaning they can be defered for 120 days.
 
Its also "cost saving" by deffering it, and waiting to fix it. That way the airline doesnt have to cancel or delay a flight. But the sad thing-it can take days before the plane might be taken out of MX rotation schedules to actually fix the problem.
 
APUs are like any other mechanical thing, occasionally they break. Airlines want to save money, so they wait as long as possible before fixing them.
 
They don't start is the primary reason. The original APUs on the 737-400 had a fancy computerized FADEC on them that looked at something like 27 parameters. If one was out of wack, the darn thing wouldn't start. Additionally, if you get an inlet door cycling problem, the APU won't start.

The second biggest reason is usually a problem associated with the APU generator or pneumatic output. If either or both of these is out of limits, the APU goes on MEL. Even if its a one time deal, if someone writes it up, its MELed for 10 days or until the maintenance department gets time to fix it.
 
An APU doesn't burn fuel when it's inop!

On the other hand, how much productivity is lost when ground crews are scrambling to find a huffer cart?
 
My experience so far has been that they are INOP once ever few hundred hours of flying the line.
 
WAFlyBoy said:
An APU doesn't burn fuel when it's inop!

On the other hand, how much productivity is lost when ground crews are scrambling to find a huffer cart?
Click to expand...


Ahem! The preferred nomenclature is "Lewinsky"! ;) :sarcasm:
 
Andrew_VT said:
Wow thats alot less than what I had assumed based on what I've read.
Click to expand...

'cause nobody says "Hey, the APU was working all day today!" It's an expected thing.

The problems come along when it's broken and people complain. Which they kinda have a right to do if their company doesn't fix INOP equipment until they have to.
 
On the CRJ-200 the common issue is that the door motor wont work (can't open the inlet) or you get the dreaded ECU FAIL message that means the computer is sick... Both seem to be the common reason for the APU MEL.. I would say once every 200-300 hours is common to experience the problem with the APU.
 
Yeeeeeup...my company builds most of the APU's that are flying in commerical aircraft.

To answer your question, the over all resounding answer from several A&P's is heat.

Summertime apparently is more especially brutal on APU's. In fact summer starts our busy season here at the R&O (Rebuild & Overhaul).

We are absolutely slammed here at work with "recips" (receipts) from customers, wanting their APU's serviced, like yesterday! Were averaging 10-20 recips a day.

We have tons of work stacked up outside (APU's in crates) that we can't even work yet, because were so understaffed here, and the company won't lift the hiring freeze (do more with less).

Hope that helps!
 
Had an INOP APU this morning. Lovely! Thankfully it wasn't too warm at first and we only had it for 3 legs. Feel bad for the other crews though as the day winds on and the delays start happening :(

I don't see it too often, but we all really complain when it is either uber hot or cold outside!
 
It's usulaly no big deal from fall to spring but when the heat of the summer arrives, it turnes into a pain in the ass! From my experience most APU problems in the CRJ is fuel control parts and electric components. due to it being a "customer service" item we(Comair) try to get them fixed as quickly as possible. It's hard sometimes when the aircraft is RON at oustations for several days and there isn't enough time to work on it when it has a mx base turn. It gets even harder when there's no spares to swap lines with.
 
It's a pain in the arse when the APU is MEL'd. At XJT it seems like they do try to fix them quickly. As someone stated above, sometimes (at least on our ERJs) the FADEC on the APU doesn't like something causing the dreaded "APU FAIL" EICAS. Then sometime later when maintenance tries to check it out it works and you get your "Ops Check OK." The cycle then repeats itself a month later. ;-)

Maybe it seems like they are broken frequently because life moves slowly...It gets Hot/Cold, the quick turn you were trying to do gets spoiled by a broken down Huffer cart. Finding good places to do a crossbleed start etc....And if you keep the plane all day, after 3-5 legs it gets old.
 
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