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APU start technique question

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APU start technique question

Old 8th Jul 2019, 17:12
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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...
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Old 8th Jul 2019, 18:59
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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.

Last edited by Uplinker; 8th Jul 2019 at 19:10.
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Old 8th Jul 2019, 19:22
  #43 (permalink)  
 
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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
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Old 9th Jul 2019, 03:17
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Originally Posted by Check Airman


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.
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Old 9th Jul 2019, 09:35
  #45 (permalink)  
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Originally Posted by Exup
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.
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Old 9th Jul 2019, 14:16
  #46 (permalink)  
 
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Originally Posted by Exup
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.
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Old 12th Jul 2019, 19:12
  #47 (permalink)  
 
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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!
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Old 14th Jul 2019, 20:09
  #48 (permalink)  
 
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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
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Old 3rd Mar 2021, 15:33
  #49 (permalink)  
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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.
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Old 7th Mar 2021, 12:07
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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 !
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Old 7th Mar 2021, 15:03
  #51 (permalink)  
 
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Originally Posted by FlyingStone
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.
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Old 7th Mar 2021, 16:06
  #52 (permalink)  
 
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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".
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