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747 Classic cold soaked APU

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747 Classic cold soaked APU

Old 7th Nov 2006, 03:34
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747 Classic cold soaked APU

I heard you can open the APU door inflight on the Classic to get the thing spinning to warm its vital bodily fluid's. Our engineers have been having some trouble firing it up on the ground after a long flight. If so is it written in the BOM. I dont have the good book with me at the moment.
Any help is much appreciated.
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Old 7th Nov 2006, 03:53
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I'm no expert on this type..........

I would think the procedure would involve switching APU on then pulling the APU Battery bus - APU Door P83 circuit breaker after a bit which feeds the door motor, then APU switch off. May have to pull other CBs to properly accomplish this. Not sure of how much air you need for that - the Air/Ground relay prevents the door driving full open while airborne.

A FE or old-timer wrenchie ought to know the right answer to this one.
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Old 7th Nov 2006, 05:20
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There are some aircraft certified for inflight apu use. They have a different door with a scoop on it. You can see it while the door is closed its pretty obvious. We do have an occasional cold soak problem, but I have not heard of spinning it in flight.
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Old 7th Nov 2006, 07:27
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IIRC, there is a CB panel in the overhead right by door 5L. There is an APU battery charger CB as well as a APU battery warmer CB. If you pull and reset both of those, that seems to do the trick.

In addition, placing the APU switch to ON will open the door. It seems to work very well below 10.000'.
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Old 7th Nov 2006, 07:42
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The APU Master switch has three positions; START-ON-STOP. ON- opens APU door,Opens Apu fuel valve,Starts AC or DC fuel pump. Opening APU door in flight by selecting Master Switch to on was a common practice in our operation to facilitate APU start after landing.
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Old 7th Nov 2006, 10:35
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Cool

Door opens to 50% in flt compared to on the ground, which is why you have to be careful going into flt mode when on the ground as the APU can surge/stall when the door goes to 50% open/closed.

I wouldn't recommend pulling cb's in flt, one of them is tied into pressurisation on our a/c!
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Old 7th Nov 2006, 16:54
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Originally Posted by HotDog
The APU Master switch has three positions; START-ON-STOP. ON- opens APU door,Opens Apu fuel valve,Starts AC or DC fuel pump. Opening APU door in flight by selecting Master Switch to on was a common practice in our operation to facilitate APU start after landing.
Exactly but two more things. We do it on final appraoch to get the turbine spinning, and only on acft. that have the scoop (Inflight Start certified).
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Old 7th Nov 2006, 22:28
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We do it on final appraoch to get the turbine spinning, and only on acft. that have the scoop (Inflight Start certified).
Yes, of course it is done on approach, that goes without saying. However if you carry out this procedure on aircraft that still have the inlet door scoop fitted (they were removed on our fleet to lessen aerodynamic drag), the APU will airstart when it reaches a windmilling speed of 1400RPM when the 7% multispeed switch actuates, energizing the APU ignitor and actuating the fuel control unit. On airplanes without air scoop, opening the door in flight will not result in sufficient windmilling speed to actuate the 7% multispeed switch but nevertheless, is enough to reduce the effects of cold soak. After landing, the APU is started normally by the momentary START position of the APU Master switch.
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Old 8th Nov 2006, 11:56
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I was always taught that cold soak is a term used during certification trials and if an APU won't start at the end of a sector it has a problem. We weren't allowed to sign based on 'suspect due cold soak'.
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Old 9th Nov 2006, 00:05
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Quite a few had this problem as they were getting older.
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Old 9th Nov 2006, 03:14
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Open the door as soon as possible after landing, and try to start it as late as possible before block-in. Airborne door openings seldom necessary.
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Old 9th Nov 2006, 05:32
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Depends on your taxi time, only used to open the door on approach with APUs that had a history of slow starts after a long haul sector.
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Old 9th Nov 2006, 17:51
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More often than not the real problem is a malfunctioning blanket or weak battery. Write it up so mx can find the problem.
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Old 9th Nov 2006, 17:56
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Cool

Write it up so mx can find the problem.
A = suspect cold soaked, APU started and ops nml on ground.

There fixed it!
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Old 11th Nov 2006, 15:04
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Ccold soaked Whale APU

Originally Posted by inept
I heard you can open the APU door inflight on the Classic to get the thing spinning to warm its vital bodily fluid's. Our engineers have been having some trouble firing it up on the ground after a long flight. If so is it written in the BOM. I dont have the good book with me at the moment.
Any help is much appreciated.

Very common with prior employer to have FE open APU inlet door on final approach so it would spool up when started on taxi in. Was very effective after a crossing with very cold temps at ALT.
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Old 11th Nov 2006, 17:41
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Originally Posted by Zoner
More often than not the real problem is a malfunctioning blanket or weak battery. Write it up so mx can find the problem.
That was the reason the batter charger CB and the Blanket CB were pulled back by door 5L. The loss of power and re-application of power puts the chager in the high mode, same with the blanket..
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