PDA

View Full Version : APU start technique question


B2N2
4th Jul 2019, 04:00
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?

extreme P
4th Jul 2019, 08:03
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?

FlyingStone
4th Jul 2019, 11:30
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.

B2N2
4th Jul 2019, 13:18
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 :8

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.

763 jock
4th Jul 2019, 14:23
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.

QuarterInchSocket
4th Jul 2019, 15:03
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

aeromech3
4th Jul 2019, 15:44
Seem to recall this was a procedure on classic B747's but I only worked on the SP!

FullWings
4th Jul 2019, 16:04
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...

haughtney1
4th Jul 2019, 16:14
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.

Romasik
4th Jul 2019, 18:06
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.

yoko1
4th Jul 2019, 20:32
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.

oceancrosser
4th Jul 2019, 21:17
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.

FlightDetent
4th Jul 2019, 21:21
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.

flightleader
5th Jul 2019, 01:39
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.

B2N2
5th Jul 2019, 04:03
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.

QuarterInchSocket
5th Jul 2019, 05:16
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.

Cough
5th Jul 2019, 13:48
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??

Golden Rivet
5th Jul 2019, 17:33
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.

B2N2
6th Jul 2019, 08:23
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....:}

extreme P
6th Jul 2019, 08:36
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?

B2N2
6th Jul 2019, 09:13
The book says START APU

Like with everything we do in life there are better and there are worse techniques.
We all know “that driver” that does everything we all do yet it’s unpleasant to drive with him/her.
Start, shift, accelerate, brake, corner, they do all that.
Yet their techniques are what makes it uncomfortable.

Flying single engine piston the POH says “advance throttle” it doesn’t state not to slam it open yet that’s a bad technique. A better technique is a 3 second count from idle to full power and the same back to idle.
Short crankshaft, swinging 60-70lbs weight aka prop and so on.

Your mom says to close the door behind you.
Implied is a non verbal warning not to slam it.

So the book says “START”.
Insight and technical understanding would warrant a particular technique....
So we just go braindead and turn the switch regardless of conditions or do we apply a light coating of mechanical empathy?

Golden Rivet
6th Jul 2019, 13:56
A cool down cycle is normally written into the APU ECU software. If the manufacturer thought that a warm up cycle was also required, it would be included.

Just hit the start button and let the APU take care of itself.

bill fly
6th Jul 2019, 18:26
The book says START APU...
So we just go braindead and turn the switch regardless of conditions or do we apply a light coating of mechanical empathy?



B2N2, When you started this thread you asked for opinions. You got them.
Don’t now round on your contributors and call it brain dead to do the job as taught.

PENKO
6th Jul 2019, 18:53
BN2, some advocate not lowering the gear whilst the flaps are still running.
Others won't switch on the bleeds immediately after the APU comes on line.
Some say don't cool the brakes with the fan, arguing they're more efficient when warm.
And other do the controls checks reaaaaaallllyyyyy slllllooooooooooowwwwwwww in order to prevent...I have no idea exactly what.

And now you tell us to have some mechanical empathy by warming up an APU that's designed to be started at -56 C.
Noted.

QuarterInchSocket
7th Jul 2019, 14:59
So the book says “START”.
Insight and technical understanding would warrant a particular technique....
So we just go braindead and turn the switch regardless of conditions or do we apply a light coating of mechanical empathy?

you assume the need for a technique without considering that perhaps there has been an analysis conducted by a team of professional engineers in an office working for the apu oem, the aircraft manufacturer and the airline whose sole job description might be to consider the effects of temperature on apu/jet engine performance, BEFORE apu certification and IN-SERVICE, on a worldwide fleet, with experience in a range of operating conditions.
An assumption is also made that somehow all pilots around the world have been applying the suggested ‘technique’, leading to the apu reliability rates that underpin etops certification.
No - I just don’t think so. I think pilots have been cranking straight away relying on inbuilt protections (as designed and certified) to auto-shutdown if the apu is unhappy.

There is nothing wrong with developing technique, but some measure of reason needs to be applied and tested, and I don’t think delaying an apu start for cold soaked apu because it is thought to be healthy for it, is reasonable. There is no evidence that supports the technique unlike the other examples you gave, which can be sensed immediately and reacted to.

all that said, I think the suggested technique is one of little risk with fod ingestion being the biggest issue but even then, apu’s generally have a screen that captures the crap.

B2N2
7th Jul 2019, 15:14
you assume the need for a technique without considering that perhaps there has been an analysis conducted by a team of professional engineers in an office working for the apu oem, the aircraft manufacturer and the airline whose sole job description might be to consider the effects of temperature on apu/jet engine performance, BEFORE apu certification and IN-SERVICE, on a worldwide fleet, with experience in a range of operating conditions.
An assumption is also made that somehow all pilots around the world have been applying the suggested ‘technique’, leading to the apu reliability rates that underpin etops certification.
No - I just don’t think so. I think pilots have been cranking straight away relying on inbuilt protections (as designed and certified) to auto-shutdown if the apu is unhappy.

There is nothing wrong with developing technique, but some measure of reason needs to be applied and tested, and I don’t think delaying an apu start for cold soaked apu because it is thought to be healthy for it, is reasonable. There is no evidence that supports the ‘technique’ unlike the other examples you gave, which can be sensed immediately.

Thank you, enlightened response.
Im a very simple carbon based life form and mostly water at that. Easily confused.
So getting told off 180 degrees apart confuses me lol.
I think I can put this to bed now.
:ok:


As far as the brain dead comment, we have all flown with brain dead people.

Check Airman
7th Jul 2019, 15:19
I don't have a dog in this fight, but interesting discussion anyway. Let's say you land and go to the ramp within 5 minutes. How much warm air actually gets to the APU? Wouldn't you have to be moving pretty fast to get enough ram air to actually warm up the APU?

On the 320, some of our airplanes display the APU temp even when it's off in flight. Opening the APU flap at cruise in -50 degree air doesn't change the indicated temperature (was just around freezing if I recall), but that could simply be the lower limit of the sensor.

CV880
7th Jul 2019, 17:20
B2N2 is clearly referring to 747 operations. Opening the 747 Classic APU inlet door immediately after landing and in some cases on approach was a common practice especially after long haul flights to improve APU starting. The 747 Classic APU is not used for any emergency or back up functions so much of the chatter above is irrelevant. In fact the 747 Classic APU is not certified for in flight starting if the ram scoop on the inlet door is deleted. Most operators deleted the ram scoop per Boeing SB to eliminate a small drag penalty. There is a placard on aircraft with the scoop deleted that reads "APU IN FLIGHT STARTING PROHIBITED. In Flight Operation After Ground Start Allowed".

FlightDetent
7th Jul 2019, 17:30
No issue with warming up the lubricant and various movable parts to pamper the thing. Point being made multiple times already is that opening the flap AFTER landing and delaying the start by 4-12 minutes for such un-soak achieves precisely nothing towards that direction.

Mileage, timezone and jetlag may vary.

QuarterInchSocket
7th Jul 2019, 18:15
B2N2 is clearly referring to 747 operations.

I did consider the 747CL along with the 737CL both of which share similar characteristics with respect to issues relating to starting reliability. My rubbish talk is geared toward the modern machines and despite etops apu controls not applying to the heavies as is the case with some of the lighter twins, the learnings relating to apu performance from the more stringent controls of an etops program I would reasonably assume to apply to apu’s more broadly. Airlines, apu manufacturers and airplane manufacturers do monitor many metrics including MTBF of components on an apu both on-wing and as a result of overhaul etc. They’re watching! If there was a trend, they’d react... or at least, that is the expectation from historical occurrences and experience.

to op - apologies if my prev post has caused offence. I see many people try to reinvent the wheel, some of the suggested shapes that get churned out aren’t very well thought out. The latest rediculous one came from a chap who saw nothing wrong with leaving the apu bleed on with packs switched off on the ground (737NG). I can’t recall if there is mechanical risk to apu health in this config, but I do know that it results in approx 40% loss of efficiency, half that (20%) if only one pack is used for the 131-9b on 737ng, (off the top of my head); I can’t remember where I read it so I wouldn’t be able to produce evidence even if I tried! Hah.

PENKO
7th Jul 2019, 22:56
B2N2 is clearly referring to 747 operations. Opening the 747 Classic APU inlet door immediately after landing and in some cases on approach was a common practice especially after long haul flights to improve APU starting. The 747 Classic APU is not used for any emergency or back up functions so much of the chatter above is irrelevant. In fact the 747 Classic APU is not certified for in flight starting if the ram scoop on the inlet door is deleted. Most operators deleted the ram scoop per Boeing SB to eliminate a small drag penalty. There is a placard on aircraft with the scoop deleted that reads "APU IN FLIGHT STARTING PROHIBITED. In Flight Operation After Ground Start Allowed".
But the issue is the same on all aircraft, from 747 to Cessnas: the urge of some pilots to invent procedures based on unverified, unofficial conjecture, sometimes called mechanical sympathy our even airmanship...


Maybe he should ask the captains exactly which parts of the APU need warming up. The core of a cold soaked APU will never reach anywhere near ambient temperature in the ten minutes it takes to taxi to the gate IMHO.

172_driver
7th Jul 2019, 23:33
The latest rediculous one came from a chap who saw nothing wrong with leaving the apu bleed on with packs switched off on the ground (737NG). I can’t recall if there is mechanical risk to apu health in this config, but I do know that it results in approx 40% loss of efficiency, half that (20%) if only one pack is used for the 131-9b on 737ng, (off the top of my head); I can’t remember where I read it so I wouldn’t be able to produce evidence even if I tried! Hah.

Sorry for hijacking...

What about having packs off and APU bleed on? With or without APU running? 8 years now on the 737NG and I thought I had pinned down the essentials.
​​​​​​​

B2N2
8th Jul 2019, 01:13
But the issue is the same on all aircraft, from 747 to Cessnas: the urge of some pilots to invent procedures based on unverified, unofficial conjecture, sometimes called mechanical sympathy our even airmanship...


Maybe he should ask the captains exactly which parts of the APU need warming up. The core of a cold soaked APU will never reach anywhere near ambient temperature in the ten minutes it takes to taxi to the gate IMHO.


Amen.......

misd-agin
8th Jul 2019, 03:04
The technique of turning the APU to ON for a bit of time before turning it to START comes from decades ago and has nothing to do with ‘heating’ the APU.

It’s to let the APU start sequencing happen. Mx decades ago said quick application to START could generate the FAULT light we’d often get after the first attempt. “Second start normal.”

The FAULT rate decreased significantly with a pause at the ON position. Over time guys started putting it to ON but not starting it until later, sometimes as long as the entire taxi-in. I’ve done that for decades and haven’t had an APU FAULT during start for years. I’ve seen new FO’s get the FAULT light. I tell them to cycle OFF then ON and leave it for awhile. The next attempt is normal. “Want me to write that up?” “No. It works with patience.”

stilton
8th Jul 2019, 08:03
Never done this or seen a reason to


This is the same sort of reasoning as delaying APU start after landing until the last possible moment to save a few pounds of fuel


Had a few FO’s do this, most of them forgot to crank it altogether, I’d be busy parking then we’d realize we had to keep an engine running until it was up and running


Didn’t save much fuel with that technique..

PENKO
8th Jul 2019, 08:20
Maybe you should write it up, force Boeing to come out with a new SOP.

Or maybe things have improved over 'decades'.
Or maybe Boeing decided in their wisdom that an occasional fault does not warrant the added complexity and forgotten APU starts.
Or...

I appreciate what you're saying, but the byproduct, as you clearly see in this thread, is that you leave your FO's confused. And when they become captains, they will pass that confusion on to the next batch of FO's, citing warming up times and mechanical sympathy.

I occasionally see a clear example of this when young FO's on my Airbus refuse to select flaps directly after lowering the gear, parroting the 'mechanical sympathy' line, based on an SOP from an earlier version of the Airbus they never flew. The SOP changed before they were born, but some captains decided to stick with it, and voila......

CaptainProp
8th Jul 2019, 08:49
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.


Airbus (320) FCOM states “Wait at least 3 s before selecting APU START pb-sw.” and as stated by someone else here that’s probably just for the ECB to perform it’s power-up test before crew initiates the start sequence. Never heard or read about 3 min.

CP

Occy
8th Jul 2019, 10:05
Only pilots could complicate the simple act of using a switch that has 3 positions - on, off and start.

Check Airman
8th Jul 2019, 10:11
Only pilots could complicate the simple act of using a switch that has 3 positions - on, off and start.

:D

Some people think too hard. I wait a few seconds before starting the APU, as recommended (A320). I've seen more than one person push the start switch immediately after the master though.

172_driver
8th Jul 2019, 11:28
Only pilots could complicate the simple act of using a switch that has 3 positions - on, off and start.

Crying with laughter :ok: :ok:

jmbrito
8th Jul 2019, 18:12
My two cents. On my company (B767 mainly), we used to have issues with APU starting.What we've understood was if after you put the switch to ON, allow a few seconds, when advancing the spring loaded to Start, you do it slowly and gently until you see the RUN RUN and then gently "walk" the button back to the ON position. We don't release it. The fingers are allways in the button.

As we must comply with ETOPS regulations, monthly and on each aircraft we need to do a inflight start and cruise level.

Using the method above, we've been able to start APU on a single attempt. At cruise level you cannot get more cold and soaked...

Uplinker
8th Jul 2019, 19:59
For what it’s worth; at one time we used to have frequent APU starting faults on A330 during a short taxi in at a hot location.

I don’t remember who came up with the fix, which was to push the master pb at about 8000’ while being vectored for the approach. The APU would then start normally and reliably first try on taxi in.

We supposed that venting the APU compartment helped in some way, but I like the fuel pump theory proposed earlier in the thread. Anyway, our fix worked very well.


PS, there is nothing wrong with mechanical sympathy. You wouldn’t select TOGA as soon as a main engine “Avail” light comes on or accelerate at full speed down the road as soon as your cold car engine starts, but I have seen many do the same thing to their APUs. I don’t have the photo to share, but once saw the result of people repeatedly restarting failed APUs without applying the recommended starter cycle cool down periods and also putting APU bleed and both packs on the second the “avail” light came on. The poor thing was a mess of blackened and knackered parts.
I start the APU, and when it is “available” I do something else for 30s or so. Then APU bleed on, do something else for 30s or so, then a pack on. Same in reverse when shutting down.

Exup
8th Jul 2019, 20:22
747 classic had issues with cold soak but it wasn’t the APU but the APU Battery that caused the issue, usually due to the fact tha the battery heater mate was not working

misd-agin
9th Jul 2019, 04:17
:D

Some people think too hard. I wait a few seconds before starting the APU, as recommended (A320). I've seen more than one person push the start switch immediately after the master though.

That's exactly what maintenance recommended - slow down and give it a second. AB has it in writing. Guys on the Boeing's came up with other ways to slow the process down and allow the steps to occur. Either leaving it in the ON position for some time or using the 'RUN, RUN' technique. Both seemed to work.

I haven't seen the 'RUN, RUN' technique used in years. I'm guessing holding the selector in START and waiting for the 'RUN, RUN' flash was another way to gain additional time for the start process to work.

B2N2
9th Jul 2019, 10:35
747 classic had issues with cold soak but it wasn’t the APU but the APU Battery that caused the issue, usually due to the fact tha the battery heater mate was not working

Ah....now we’re getting somewhere.
That might be exactly where that tribal technique comes from.

QuarterInchSocket
9th Jul 2019, 15:16
747 classic had issues with cold soak but it wasn’t the APU but the APU Battery that caused the issue, usually due to the fact tha the battery heater mate was not working


No, was definitely the APU. Boeing in conjunction with I think it was Garrett tried to track down the cause. They had a bunch of recommendations which are still relevant today; nominally oil <viscosity> and fuel <flow>, but that doesn’t exclude other things that are still part of the equation such as battery health. Note that it was these types of enquiries (as a result of operator reports) that helped lead to the improved apu of today.

i do agree that the airbii need a little more finesse than the flying pigs, it’s because they are a little more precious... haha

To the other chap asking about the off topic 737ng apu f/f - your mx staff would be able to validate by checking input monitoring and/or their books, but yes, the numbers are in that vicinity. Turn the apu bleed switch off on gnd if not using packs (excluding starting ops, obviously); save a tree or two.

UAV689
12th Jul 2019, 20:12
The cold soaked thing does sound like some rumour thats spread about your crews.

what I have seen on the 737ng, is put it on, door opens, runs through i presume a self test etc, then if you hit start a few mins later, the start is almost instant. I have seen a few guys do this.

ironically it was to save on their apu use stats that we get monitored by, but I think the computer will just count from when the switch is in on position not actually up to speed and burning fuel!

Again all speculation on my part!

sb_sfo
14th Jul 2019, 21:09
Exup
On all the 747 Classics I worked on, the APU battery was inside the pressurized cabin, aft cabin L/H side, behind the closets near the rear lavs. Hard to see how it could get cold-soaked in that location.
edited to address

B2N2
3rd Mar 2021, 16:33
Did I ever provide closure?
As a result of this thread I ended up calling the tech support line of Garrett, the APU manufacturers.
His short answer? The software takes care of that during the self test, it won’t start if it’s outside its parameters.
OFF-ON-RUN release to ON.

Uplinker
7th Mar 2021, 13:07
We definitely had a cold soak issue on one of our A330s. There was a very short taxi in from the runway at our destination and after an ~ 8 hour flight, the APU would not start.

If we turned the master switch on at around 8,000' during approach, the APU would start normally on first attempt when we taxiied in.

My thoughts are that something in the APU or the APU bay was freezing up, preventing it working and this might have been due to a missing APU door seal or similar, which allowed cold air to circulate the APU bay during the cruise.

By opening the APU inlet during approach, the warm Caribbean air was able to defrost whatever it was that was affected, and the APU fired right up just after we landed.

Never got to the bottom of it, but the 'fix' worked !

BraceBrace
7th Mar 2021, 16:03
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.

1 tiny detail: APU does windmill a little with forward speed which is why we preferred to start them on short final. Makes a gigantic difference for flights longer than 2-3 hours. On the classic 737 that was. Had many failed starts on the apron, none on short final. And we had some APU fires at higher attitudes in the company, so they’re tricky to say the least.

CV880
7th Mar 2021, 17:06
Re SB-SFO in post #48 the classic 747 APU battery was in the unpressurised tailcone area just in front of the APU firewall. It sat on a heating pad to stop it from freezing and, inside a box with a fan to cool it in high ambient temperatures. The 744 had the APU battery in the closet as you state.
Copy of classic AMM 24-30-00--"The main battery and the APU battery are nickel cadmium and are identical and interchangeable. The main battery is located on the floor behind the flight engineer's instrument panel. It receives power from a charger located on the E10 rack (Fig. 1). The charger input is from the ground service transfer bus which is powered from ac bus No. 1, APU gen 1, or external power No. 1. The APU battery is located in the tail section
between the aft pressurized bulkhead and the APU firewall. The APU battery receives its power from a charger located in the E13 rack".