View Full Version : apu on a 767

31st May 2007, 06:53
By no means an expert on the 767 or any big iron for that matter (a mere PPL), still a quick question regarding the APU if I may...

initially you'll turn it on, start up the engines then turn it off.
What I'm not clear about is when should one turn it on again? (somehow 10K comes to mind, but I'm not sure where I got that number from)


extreme P
31st May 2007, 08:46

Generally these days it will be after landing as you taxi in. Unless its CAT III; then 10 000'.

31st May 2007, 09:12
That's to serve as a backup in case of a R/L bus failure, I assume

31st May 2007, 10:18
usually...I try to have the run light illuminate.......just as you set the park brake:E

31st May 2007, 10:23
Generally these days it will be after landing as you taxi in. Unless its CAT III; then 10 000'.APU v CAT 3 must be airline specific - never done it in BA... Some now start APU ASAP after Landing to allow 1 engine to be shutdown... assuming of course the APU works :rolleyes:

31st May 2007, 17:23
Any reason for not turning it on in the air (with the CAT 3 exception)?

There are possible advantages - so why not?

extreme P
31st May 2007, 17:31
Boeing says the easiest way to save gas is to turn the APU off.

What are the advantages of having the APU on while airborne (assuming of course a "normal" airplane)?

31st May 2007, 17:35
Any reason for not turning it on in the air (with the CAT 3 exception)?
There are possible advantages - so why not?Errrr ... wastes fuel, wears out, might catch fire... I am not sure what the "advantages" are?
On a CAT 3, or other approach, I presume it is running in case of a generator failure? If so, I would be surprised if the transfer to APU would be "seamless", and might be enough to trip off enough systems to cause you to GA anyway?
I flew one type, albeit not airliner, where the APU had successfully been used for years on the ground. The moment is started being used airborne, or rather during landings, it experienced a high failure rate:{

31st May 2007, 17:46
err..extra redundancy in case of a Gen/bus failure, or is it a bad tradeoff?

extreme P
31st May 2007, 17:55
Since a generator failure is a non event why run the APU and burn fuel at near record prices for a "just in case"?

1st Jun 2007, 04:27
With the loss of a generator on a CAT 3 approach on the 75/6 you will lose your fail operational capability (unless it happens below AH) and may have to go around unless it happens early enough in the approach to rebrief for a DA versus an AH.

(Decision altitude versus alert height)

Having the APU up and running will normally provide for a no break seamless transition of electrical power and will allow you to continue using the same minima.(You still have 2 generators)

A big plus on low vis approaches and it is standard procedure at my airline to start the APU on the approach checklist. 10000 feet seems premature.
This is only on low vis approaches though, otherwise we just start it up after landing.

1st Jun 2007, 07:31
Some permissible unserviceabilities (MELs) require the APU to be running if operating an ETOPS sector (again rules are country specific).

SOPs also vary from airline to airline as far as when to start it if conducting Cat II/III ILS approaches (varies from 5000' to 20000' in my experience).

Boeing state that APU in-flight starts may not be successful if the aircraft is above 35000'.

Shore Guy
1st Jun 2007, 09:23
Our airline just recently instituted a procedure on both 757 and 767 whereby we are required to let the APU run for 10 minutes prior to introducing a pneumatic load (pack or packs). This on an APU design that has been around for a long as some of our junior pilots. If temps are at either extreme (warm or cold), most crews are not complying (our airline will not connect “conditioned” air).

Any other carrier establish this procedure?....Background?

1st Jun 2007, 22:01
Ten minutes does seem excessive, I think one minute is adequate.

We actually have no guidance on the subject.

Used to be one minute for electrical load and three for pneumatic on the 727!

World of Tweed
2nd Jun 2007, 00:14
Shore Guy: No such limits at my company.

Only the stipulation that when you've had the packs running before selecting them off for the engine start one must close the APU bleed valve before selecting packs off - then select the APU bleed back on.

We've had a number of uncommanded shutdowns using the "normal" just packs-off routine.

Regards EGT: In our latest "Best practice" APU usage guidance whilst on the ground on stand without Ground power available we are to run the APU with the Bleed valve off as this results in the lowest EGT.