RELIABILITY OF THE OLD AND RECENT APU's???

[AeroTech]

Hi,

Just few questions about APU:
Did the recent APU's are less reliable than the old one?

The reason I am asking this question because the APU in B 737 NG is provided (or may be protected to be correct) with 21 automatic shutdowns, compared to 4 for the old APU' (like B 737-200). So I tought that the recent APU's are more prone (may be) to shutdown while trying to save from probable damage during detected APU malfunctions.

Why APU's (even the recent) cannot provide the pneumatic power up to the cruise altitude (as electric power)? I am not talking about full power but reduced pneumatic power with no electric power (load): (at sea level, 60 f) 160 ppm at 60 PSIA.

You may say that air density at high altitudes is low (which is true). But the recent APU's are provided with a dedicated compressor for APU bleed air (load compressor). The old APU's are not fitted with a load compressor if I am not mistaken.

Last question which is not related to the subject (I aplogize):
If the APU is operating in flight (for some reasons) and the APU catchs a fire (in a twin,tri, quad), is this considered as emergency and necessitate an rapid descent or land ASAP (let's say the fire was extinguished).

Any toughts, comments, and feedback are appreciated.
Thank you.

[cvrurass]

In the case of the 747-400 why rely on the APU air when you have 4 other good sources of air at cruise? Air does supply power to hyd for most flight controls but they also have electrics for back up.

[AeroTech]

Hi,

Thank you for your post.
If I am not mistaken the APU B747-400 is not certified for to be restarted in flight (same for the classics I guess). It is started on the ground and it will keep operating at a certain altitude, then it will shutdown (that's what I read in certain forum). I think there were 2 cases (vulcano ash encounter) where all engines flame-out occured and the restart was not eazy thing. In such case it seems (may be) even the autostart is not able to restart all engines In addition the B 747 is not fitted with the RAT (I guess we are little bit out of the subject of post which the reliability of the APU).

I hope I will receive more comments and opinions about the reliability of the APU's (old and the recent).
Thank you.
Best regards

[cvrurass]

As far as I know there is no altitude auto shut down for the 747-400 APU. When you start it on the ground and go into air mode apu door moves closed from 45 deg to 15 deg open and continues to operate unless of course you have a fault to cause auto shutdown ie loss of control speed (N1), N1 overspeed, high egt, compartment fire etc.

[whiskeyflyer]

I know its not nice to say but glad to see its not just me and the old APUs having problems.
We got the GTCP36-4A and we alleviated a lot of problems by changing oil filter element regularly (expensive but worth it) and keeping a close eye on the surge valve.
But I have yet to get the APU to meet its overtime limit of 3000 hours operation. We run them on condition and when they go U/S we change the APU. We have had APUs about 200 hours out of third party workshop, deciding to throw their impellers all over the place.
For us the APU is the weak link in our operation.
I believe reearch is well advanced on fuel cell powered APUs, but we will wait and see.

[CV880]

Just about any APU made by Airesearch/Garret (now owned by Honeywell) was overcomplicated and had little performance margin with the possible exception of their early efforts in the 727/737/DC9.
The 747 classic GTCP660 APU was terrible. The best thing Boeing ever did was install the PW901 in the 747-400. What a difference to ground cooling capability in hot weather an APU change made plus the reliability and time between removal increases.
The L1011 Hamilton Standard APU was a disaster. The best thing about it was it could be replaced in a very short time if required.
The 747 APU (all models) was available for pneumatics only if running on TO but it had to be manually shutdown by a certain altitude. This enabled a Packs Off take off in hot weather as far as the main engines were concerned with the APU providing air to one aircon pack for some cabin cooling. With some modifications to the APU control circuits and a ram scoop on the inlet door some 747 classic APU's were able to be started in flight below a set altitude but many long haul operators disabled this function and removed the ram scoop for drag reduction per a Boeing service bulletin. As far as I can recall the 747-400 APU was never available for inflight starting.

[AeroTech]

Hi,

Thank you for your post.
Sorry guys but I think I misled you regarding the reliability of the APU's. I didn't mean the reliabilty in general like better mean time between failures or overhaul.... I meant the failures that can occur in flight and cause automatic (protective) shutdown if the APU was operating in flight. For example an old APU (like in 737-200) is protected by 4 automatic shutdowns:low oil pressure, high oil temperature, overspeed, and fire. The APU in the 737- NG is protected by 21 automatic shutdowns, among them: APU inlet overheat, inlet air temperature sensor, loss of both EGT signals,...
So I tought that the recent APU's are more prone (may be) to shutdown while trying to save from probable damage during detected APU malfunctions, because they have more chance to fail: anomaly like inlet air temperature sensor failure or loss of both EGT signals,...etc are not critical or important than high oil tempearture or low oil pressure but yet they can cause automatic shutdown. Such anomalies (like APU inlet overheat, sensors failure,..) can be fixed at the gate or in hangar (does not necessitate the remove of the APU and repair on a shop). So my point it is good to have many automatic shutdowns (protective), but this may affect the utility and the use of the APU if it fails when it was needed.

So now after this long introduction :) , do you think that the recent APU's are more prone to AUTOMATIC (PROTECTIVE) SHUTDOWNS in flight than the old APU's?

Feedback appreciated.
Thank you.