Poor APU reliability

[oceancrosser]

APUs appear to be a rather poor system reliability wise. Frequent failures and poor performance for decades while other systems have improved. What gives? In my fleet we talk about “spring fever” as temperatures rise and cooling demands increase, APUs start to fail more frequently. These machines (afaik) seem to be quite similar since first introduced in mainstream jet aircraft (727, 737, DC9 probably).

[Uplinker]

And are frequently abused.

I have often seen pilots starting APUs, and the second they come on line they bang on the APU bleed and both packs - a huge demand shock and so soon after the heat shock of the APU start.

I have also seen abuse of the start cycle and cool down timing.

In one airline all the APUs were sold and leased back. Not sure if any preventative maintenance happened after that.

[MechEngr]

Aren't APUs relatively small? I would suppose there is a tradeoff between a heavier, more costly, and more reliable APU with the increased fuel costs to cart it around and the current situation. Other systems have improved because they decrease fuel costs to do so.

I would expect a more even solution is to improve the operational monitoring of the APUs to detect when their performance is dropping off and make a schedule for replacement on that basis before it shows up as unacceptable operation in use. Clearly the decreased demand for cooling in the Winter could mask this decline and monitoring might catch it based on previous measurements through a year long cycle.

But abuse? Yup. Bane of my existence.
me: What does it say on the label? 400 pound limit. What's on the broken item now? 600 pounds. Why? Because it still worked OK at 550 and this is just a little more.

[Uplinker]

I believe the APU core on a B777 is a PT6 ?

Does size have any correlation with reliability - unless you mean that they are worked very close to, or at their maximum power capability all the time ?

As far as monitoring is concerned, did they still do that when the units were sold and leased back, I wonder. Not their problem any more, just phone up the new owners to send a other unit if one broke down. However, that attitude would not have helped us at all if we had an engine failure over the Atlantic, when having a 100% reliable APU fitted in the tail is quite important.

[MechEngr]

Yes - if there is 50% margin then the output should be fine until the guts come loose. If it has 5% margin, just getting some buildup might push it over the edge.

Example: monitoring could tell that the output is down 3% and knowing they stop being helpful at a loss of 5% so replace it before the next Atlantic flight. Or the oil temp is too high or fuel consumption has gone up or some other factor that is predictive.

[tdracer]

The 777 was the first time that Boeing went out of their way to ensure and validate the APU as a reliable system - because it was needed for ETOPS (especially 'early' ETOPS).
Prior to that - since you don't need an APU for dispatch at most airports (ground power and air available), an APU failure wasn't considered to be a big deal.

[aerobat77]

I believe the APU core on a B777 is a PT6 ?


nope , a tpe331

[Sidestick_n_Rudder]

Maybe there’s some improvement. When I flew the A320, the APU failures were frequent. Now I’ve been on the 787 for 5 years and can’t recall a single time I had the APU on the MEL. The only issue I remember was an APU shutdown during taxi in - and it happened once to me.

[CV880]

Re post #4 the L1011 APU by Hamilton Standard was based on a PT6. The only good thing about it was it was easy to replace!! I recall replacing one on a 2 hour turnaround at main base when it was difficult to get the airplane rescheduled to overnight at base. One of the best things about the 744 compared to earlier 747 models was the new APU which was vastly superior to the original Airesearch unit.

[tdracer]

Maybe there’s some improvement. When I flew the A320, the APU failures were frequent. Now I’ve been on the 787 for 5 years and can’t recall a single time I had the APU on the MEL. The only issue I remember was an APU shutdown during taxi in - and it happened once to me.
The 787 APU was the proverbial POS at Entry Into Service - despite getting serious attention during the design phase for ETOPS. A big part of that was the (incredibly stupid) decision to do a complete clean sheet APU design and installation - without referring to decades of lessons learned with previous Boeing APUs.
A few years after EIS, I was involved in a design review of the APU Controller (aka "APUC"). Some of the design decisions were mind-bogglingly bad. As just one of numerous examples, a standard design practice on the engine FADEC installations is that the FADEC is powered by some sort of dedicated generator/alternator on the gearbox, with aircraft power as a backup if the ded gen failed. So you effectively had power supply redundancy (the engine driven generator being required for dispatch). On the 787 APUC, the also used both ded gen and aircraft power, except that instead of redundancy, the design required both to work properly - so the loss of either one would cause the APU to shutdown :ugh:
Due to requirements of ETOPS, the 787 APU was one of many areas that received great attention to correct the many shortcomings that quickly became apparent after EIS.
Good to hear it apparently worked...

[Cough]

Been on the 787 for close to 9 years. Not a single dispatch without an APU, (others on my fleet have though!). No APU issues ever, and I've done multiple starts at FL390-430 at the end of the flight for ETOPS verification purposes without issue.

TD - Seemed to be worth it!!!

[hans brinker]

The 787 APU was the proverbial POS at Entry Into Service - despite getting serious attention during the design phase for ETOPS. A big part of that was the (incredibly stupid) decision to do a complete clean sheet APU design and installation - without referring to decades of lessons learned with previous Boeing APUs.
A few years after EIS, I was involved in a design review of the APU Controller (aka "APUC"). Some of the design decisions were mind-bogglingly bad. As just one of numerous examples, a standard design practice on the engine FADEC installations is that the FADEC is powered by some sort of dedicated generator/alternator on the gearbox, with aircraft power as a backup if the ded gen failed. So you effectively had power supply redundancy (the engine driven generator being required for dispatch). On the 787 APUC, the also used both ded gen and aircraft power, except that instead of redundancy, the design required both to work properly - so the loss of either one would cause the APU to shutdown :ugh:
Due to requirements of ETOPS, the 787 APU was one of many areas that received great attention to correct the many shortcomings that quickly became apparent after EIS.
Good to hear it apparently worked...

I do love reading your posts. Lots of common sense.

[Uplinker]

I've just worked out what POS means ! :)

[aeromech3]

"The 777 was the first time that Boeing went out of their way to ensure and validate the APU as a reliable system - because it was needed for ETOPS (especially 'early' ETOPS)."
Also with our intro of the B767-300ER and need for ETOPS we had to monitor the APU serviceability mostly, that it would start/run at altitude and oil uplifts were within limits. also a GTCP 331-200 and not the more powerful -400 unit.
Up until ETOPS the APU was considered a dead weight (unless you wanted MEL dispatch with a main geny out) and as such they were built rather light for the hard work expected on the ground..

[Check Airman]

I've just worked out what POS means ! :)

Something about Port of Spain, in Trinidad?

[Uplinker]

Piece of.............

[hans brinker]

Piece of.............

Point Of Sale, or cash register....

Used in many stores and restaurants . Still blows my mind nobody in management didn't realize the implications.