APU operation

[B737 lover]

I'm currently flying a B732 in Africa, according to our procedures(Sops), we have to switch off the APU before takeoff. Just wondering incase of a single or both generators failure after takeoff especially at night what will happen? Do some of you operate in another way and why? please help!

[Check Airman]

You seem worried for some reason. The majority of carriers shut down the APU as part of the after start checklist.

If a single generator fails, the screens "blink".

If both fail, your night vision is enhanced.

[boeingdream787]

I thought he said 732..........!! No screens here bro. Just dials and instruments. Am of the opinion that the APU should positively be left on till AT LEAST 10,000 ft. No harm done.Only thing is the gain that one might get if an engine pops in due to a malfunction or a birdie hit......
Kinda no brainer if u ask me.....:uhoh:

[Cough]

Our lot do the same. As I remember for 732, a few lights for fuel pumps, elec, and pitot static illuminate but that's about it. Rest taken care of by the transfer bus. All sorted after 1 minute once the APU has come alive.

10,000 ft then APU off:- Could argue about it for ages (ie. do you restart it over high terrain etc, do you accept an aircraft with an inop APU...) What I am sure of is this...my lot wouldn't pay the fuel bill to run it across all our fleets for the extra 30mins of running per sector.

[boeingdream787]

Kudos........kinda agree to that.......!!!:D

[Rainboe]

Let me get this right. You think you should perhaps leave the APU on up to 10,000' every single flight, burning say 50 kgs of fuel, every single flight, just in case of the once in a career (approximately) possibility of an engine failure and loss of a generator, in which case the remaining generator will have load shedding take place to reduce load, and besides which you will still be able to light up the APU if needed to provide you with an additional electrical source? Am I seeing it right? Why?

How frequently are you expecting engine or generator failures down there Bro?

I flew 732s for 6 years, 734s for 2 years, and now 737NGs for 18months, interspersed with various model 747s. The last engine failure shutdown I had was roundabout 25 years ago, on a 747-200. Never a peep out of any model 737 engine. I have to keep the APU going on ETOPs flights. I have had a generator failure recently on a 737NG at 41,000'. The APU, much to our surprise, started and handled with no problem. Loss of a generator is not a serious problem. It is really no great help having the APU running all the time, and is a positive fuel waster.

Look on it as an insurance policy. You are paying the premiums of all that APU fuel every flight. What is the benefit? A once in a blue moon benefit that is not really all that useful? Worth it? Nah.

[mavrik1]

Classic 737's apu's will only run to 18,000ft then they cough! New gens with their variable inlet guide vanes will keep going higher, shut down before take off is still recommended. duel gen failure on take-off? You must be cursed! unless both hair dryers stop, eg volcanic ash or something, then you worse things to worry about. Call it a bad day at the office.

[BOAC]

Classic 737's apu's will only run to 18,000ft then they cough! - not in my experience. They will run far higher than that.

As an 'aside', in the 'old days' with DanAir on the LGW-CDG hops (737 3/400, and cruise normally at 250) we found that leaving it running (on the manufacturer's recommendation) drastically increased the MTBF and maintenance costs due to the reduction in the short-cycle start-up/shut-down thermal shocks and more than paid for the minimal fuel burn.

[A37575]

If the flight path after take off or during descent is through moderate to heavy rain, hail or sleet, Boeing FCOM Supplementary Procedures suggests that consideration should be made to starting the APU and use a slower speed to improve engine tolerance to heavy precipitation intake.

This is where good airmanship comes into the equation and having the APU immediately available is a wise precaution.

[jettison valve]

I rather wonder what would happen if this...

"EAD 2007 - 0188R1: Two A330 operators have reported uncontained APU generator failures on ground. (...)"

... occurs in flight. :confused:

Restricting APU use in flight will not only save fuel but also minimize the risk of a 24000rpm rotor and hot oil roaming through the rear end of your plane in flight...:\

Regards,
J.V.

[Rainboe]

It's a personal thing, but I feel mildly uncomfortable with leaving an APU on all the time on an ETOPs flight. We've been doing transatlantics with 737s, so a roundtrip will have the thing running for about 13 hours continuously. I think it's a safety thing- a jet engine with attached gearboxes driving compressors and generator constant speed drives all buried within a fuselage..... obviously Boeing are totally happy with it though. There has been a history- remember the BAC 1-11 (was it Frontier?) that had the tail burn off inflight back in about 1967 because of APU uncontained fire or exhaust leak. I have started a 737NG APU first time at 41,000', so for the very remote possibility of engine/generator failure, I think it is an unnecessary precaution.
But as for running it in the air in normal ops- there is absolutely no point. You have time to get it started if needed. Only if you despatch with a generator out is there any excuse.

[Cough]

Well if we are talking that sort of stuff, check out this AAIB report (http://www.aaib.dft.gov.uk/publications/bulletins/january_2007/boeing_737_528__g_gffe.cfm) and form your own mind...

Cough.

[stator vane]

i have flown with airlines that had us turn it on when doing cat 3A approaches--

also, on the 3/400's it often took two starts to get the thing going.

the beauty is--at least in the left seat--you can leave on if you wish and turn it off when you wish. weigh the options and make your own decision.

the fact that some APU's have gone bad, should be measured the same way--how many times in a career would that happen.

i've never left it on up to 10,000 ft---intentionally that is! ha.