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APU start technique question

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APU start technique question

Old 4th Jul 2019, 04:00
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APU start technique question

So I fly a Boeing product long haul.
Cold soaked APU.
Some use the following technique: after landing check turn APU switch to ON but not start till just prior to parking.
ON position opens the APU door and allows the cold soaked APU to come up to ambient temperatures.
Thats their explanation anyway, seems reasonable.
Was corrected yesterday by a Captain who calls that an incorrect technique and not what Boeing recommends. Advocates an immediate start after landing.
Any guidance from the maintenance Gods?
Any difference between Honeywell or P&W APU’s?
Mechanical sympathy would dictate some ‘warming’ time prior to start.
Ideas? Opinions?
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Old 4th Jul 2019, 08:03
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Originally Posted by B2N2 View Post
So I fly a Boeing product long haul.
Cold soaked APU.
Some use the following technique: after landing check turn APU switch to ON but not start till just prior to parking.
ON position opens the APU door and allows the cold soaked APU to come up to ambient temperatures.
Thats their explanation anyway, seems reasonable.
Was corrected yesterday by a Captain who calls that an incorrect technique and not what Boeing recommends. Advocates an immediate start after landing.
Any guidance from the maintenance Gods?
Any difference between Honeywell or P&W APU’s?
Mechanical sympathy would dictate some ‘warming’ time prior to start.
Ideas? Opinions?
Turn the switch to start. Hold momentarily. Release.

Where does this nonsense of open the door to warm the APU up come from? Boeing documentation?

How many times have you started an APU after x hours of cold soak in cruise for maintenance? Did you warm it up first?
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Old 4th Jul 2019, 11:30
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I don't know which aircraft are you flying, but me thinks if APU cannot be used in a cold-soaked state, it shouldn't be installed or certified.

What are you going to do if a generator or engine fails in cruise? Divert towards the equator, fly at 1000ft with APU inlet door open to "warm it up"?

KISS principle very much applies here.
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Old 4th Jul 2019, 13:18
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Originally Posted by FlyingStone View Post

What are you going to do if a generator or engine fails in cruise? Divert towards the equator, fly at 1000ft with APU inlet door open to "warm it up"?
.
Then I’ve got 3 remaining

Where does this nonsense of open the door to warm the APU up come from?
About half the people I fly with seem to do it so I’m guessing it’s tribal law.

Last edited by B2N2; 4th Jul 2019 at 13:28.
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Old 4th Jul 2019, 14:23
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What do you do in winter? Suppose you land somewhere that's -25 Celsius.

Whole thing sounds like someone's home made SOP. Flown Boeing, Airbus and Douglas aircraft and never came across this until now.
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Old 4th Jul 2019, 15:03
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Originally Posted by FlyingStone View Post
I don't know which aircraft are you flying, but me thinks if APU cannot be used in a cold-soaked state, it shouldn't be installed or certified.

What are you going to do if a generator or engine fails in cruise? Divert towards the equator, fly at 1000ft with APU inlet door open to "warm it up"?

KISS principle very much applies here.
this! x1000
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Old 4th Jul 2019, 15:44
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Seem to recall this was a procedure on classic B747's but I only worked on the SP!
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Old 4th Jul 2019, 16:04
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I think this technique is more to allow the APU to be started more rapidly when you finally need it than anything to do with cold soak. If opening the door is part of the start sequence then if the door is already open, that sequence will take less time...
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Old 4th Jul 2019, 16:14
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A quick look in the FCOM will dispel any confusion, however this sounds very much like what we would call were I work, a “technique” a rather illogical one at that.
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Old 4th Jul 2019, 18:06
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Well, I’ve heard from maintenance, that their procedure was to start the APU 3 min after Master Switch pressed ON (Airbus). I’m not sure what is the technicality behind that. So I started delaying APU start after landing and since then never had an APU FAULT and second attempt.
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Old 4th Jul 2019, 20:32
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I suspect this is one of those technique-y things to minimize the thermal stress and stretch out the life of the APU. I agree that an immediate start provides a warm fuzzy that the APU would actually start when you needed it.
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Old 4th Jul 2019, 21:17
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Originally Posted by 763 jock View Post
What do you do in winter? Suppose you land somewhere that's -25 Celsius.

Whole thing sounds like someone's home made SOP. Flown Boeing, Airbus and Douglas aircraft and never came across this until now.
Yup, sounds like homegrown bulls**t. Seen a few such things in my career. Usually associated with new bosses.
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Old 4th Jul 2019, 21:21
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B2N2 I'd be asking the same questions as you. What parts of the APU assembly exactly, and by what temperature difference, get un-soaked in those 6-10 minutes? My first impression is it is one of those "look busy, Jesus is coming" ideas.

I did receive a grease guru advice in this direction, however. On one particular APU that had a history of 30% failed starts, after ECB change did not help. The suggestion was to open the inlet flap during descent and begin ventilating the whole compartment. Then attempt a start just before landing while still airborne, hoping for some ram effect help. (GND OAT 35-ish).

Though we did manage a string of successful lightups this way, the general agreement among the tech crews was, for all we knew, it might have been the goat sacrifice and curses that actually did the trick.

After two more component changes that did not help, that particular unit went u/s about 10 days later and ended up being replaced altogether.
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Old 5th Jul 2019, 01:39
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I was unfortunate enough to have flown classic B737-400 into places without ground power to support the aircraft for a decade some 20years ago. Coupled with ‘not-so-sharp’ grease monkeys at home base workshop, APU start success( on the ground) was a an issue. Low engine bleed pressure, sticky APU start valve were some excuses those grease monkeys mentioned. I know nothing because I am just a pilot. Carrying extra fuel for one engine running transit would put landing performance beyond the curve on the 5600ft runway.

Starting the APU on descend successfully put some worries to rest. No scientific proof but it seems APU has better start success in the descend compare to on the ground.

I would not recommend this practice to anyone in this era where everyone is waiting to shoot you bum-bum if you deviate from the books.
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Old 5th Jul 2019, 04:03
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We did airstart 90% of the time in the 73 because of shoddy equipment.

If it’s -25C then well I guess your warming it up from -52C cold soak to -25C.


I suspect this is one of those technique-y things to minimize the thermal stress and stretch out the life of the APU.
Nothing in the MX manual that I have access to and nothing in the FOM except “start APU”.

There are a lot of home grown techniques that make sense but I’m not sure this is one of them.

is it a feel good or does it actually make a difference.


Last edited by B2N2; 5th Jul 2019 at 05:41.
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Old 5th Jul 2019, 05:16
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The 737CL apu seems to be the more troubling of all the twin engine airplanes, I definitely see the failed starts on the ground with this type; it eventually gets cranking after a few tries but can understand why a pilot might start on descent, into a remote or poorly served port. But all other twins seem to be reliable enough, in my experience.
The mx requirement on A330 to select Master switch ON and wait approx 3mins before start, is to prime the APU fuel system, the fuel pump specifically if I recall correctly, to prevent cavitation. But that is a mx req.

if an operator/airplane is ETOPS certified, the apu is condition monitored and inflight starts may be required from time to time in addition to other apu reliability tests and checks as part of the normal mx program... these inflight test starts will occur specifically whilst the apu is cold soaked to ensure a successful start, if not, it gets looked at. This is what SHOULD be happening. but I recognise not everyone does the same thing and there are differences between some mx programs and mx staff competency.

Bottom line, I would recommend sticking to official guidance. If your apu doesn’t work the way it should whilst cold soaked, on the ground, then this is exactly where you want to find out about it! Log it, get it fixed! Don’t be part of the problem.
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Old 5th Jul 2019, 13:48
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My thought - Unless the airport is huge, you are only taxiing in for a few minutes.. How is opening a flap going to warm up cold soaked metal in a few minutes??
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Old 5th Jul 2019, 17:33
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Originally Posted by flightleader View Post
Coupled with ‘not-so-sharp’ grease monkeys at home base workshop,
works both ways... I've come across plenty of 'not so sharp' flight crew.
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Old 6th Jul 2019, 08:23
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Originally Posted by Cough View Post
My thought - Unless the airport is huge, you are only taxiing in for a few minutes.. How is opening a flap going to warm up cold soaked metal in a few minutes??
Well...metal conducts heat fairly well and the purpose is to reduce thermal stresses I suppose.
Last night we taxied 19 minutes....

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Old 6th Jul 2019, 08:36
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Originally Posted by B2N2 View Post


Well...metal conducts heat fairly well and the purpose is to reduce thermal stresses I suppose.
Last night we taxied 19 minutes....

How well does air conduct heat?

What does Boeing say about thou shalt open the door before starting said APU?
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