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Guest BeaverDriver

APU Question

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Something I'm not doing right here. On startup from a Cold and Dark or Long Turn, I can start the APU with no problems - simply click to "START" and release and away it goes. However, after the flight I can't seem to get it started, so there's something I'm missing. I do the same procedure (usually after I'm on the ground again) where I click the APU switch to START, but no go. I will sometimes hold it in the START position for a minute or so, but it's still not going. Obviously the engines are still running so the fuel pumps are on (at least some), so that shouldn't be the issue. I've checked in the FCOMM Manual and Section 7 lists the amplified procedures for Engine/APU ops, but I'm not seeing anything specific on the APU itself. Is there another chapter I should be looking at to find what I'm doing wrong?

 

Many thanks.

 

 

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Glenn,

 

The APU must be started on the ground (in the case of the 744, at least - though I'm not sure of any operator that would encourage starting an APU in the air unless it's necessary: APU time is money, both in fuel and in maintenance). "Usually on the ground" may be the source of your issue, in that case.

 

Additionally, might you have any FUEL IMBALANCE messages on the EICAS?

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The APU must be started on the ground (in the case of the 744, at least - though I'm not sure of any operator that would encourage starting an APU in the air unless it's necessary: APU time is money, both in fuel and in maintenance). "Usually on the ground" may be the source of your issue, in that case.

 

I thought the APU was supposed to be available in the air -- does this only apply to ETOPS twins?  The other day I was simulating engine failures on takeoff on the 777 and it seemed to me proper to turn on the APU to relieve load on the working engine and provide backup hydraulic pressure and bleed air (for packs).  Also in the unlikely event of a second engine failure wouldn't it be useful to have the APU on? 

 

Mike

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Hi Kyle,

Thanks for this. That does help as I was initially attempting to start it in the air (on 75/67's using ETOP's one of the tests is to be able to start the APU in the air, but that's probably a whole different animal). I didn't notice any fuel imbalance or any EICAS caution messages but I'll check that more carefully. Yours is the only logical explanation because it I can start it initially, it should start at the end of the flight as well. I did wonder if there was a specific fuel tank/pump I had to have running (similar to the 737 which requires the inner right pump to be on if I remember correctly) but I couldn't find anything in the FCOMM that describes that. I'm absolutely certain it's finger trouble in the cockpit so those aren't always documented in the manuals, real or otherwise.

 

Thanks again Kyle, we'll see how it goes tonight with your suggestions in mind.

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Mike,

 

Applying the right logic to the wrong airplane. You can lose half of your engines and still have the redundancy of a twin, while the APU being restricted eliminates a potential extra fire hazard when you're out in the areas of the world where fire protection might not be enough to prevent your fail from melting off. The 747 doesn't even have a RAT (note that it's deactivated in the failures menus - that's a 748 item only). Windmilling turbines will provide enough for what you need in the unlikely event that you lose all the engines (in theory). If you still have one or more engines, you'll have the GEN(s) of the operative engines and windmilling from the other engines to supplement that.

 

----------

 

Glenn,

 

You're welcome. Start the APU on the ground and you're good to go (also: shameless plug for the Intro Manual, as it mentions this and more items that may trip you up).

 

My comment for the imbalance was due to the fact that if you'd left the OVRD pumps on, you could've exhausted MAIN 2 and 3, which the APU draws from.

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As Kyle said, the APU can only be started when on ground. However, it can be kept running up to 20 000 ft after takeoff. It's typically used in case of an "APU-to-pack take-off", if more performance is needed. The APU bleed can then feed one pack up to 15 000ft. The APU generators however are not available in flight. 

 

The fuel source for the APU is either main pump aft 2 or main pump aft 2+3 depending on aircraft configuration. You can verify this by observing that the amber "PRESS"-light(s) on the relevant pump switch(es) extinguishes when the APU is running. There is no need to switch the pumps on manually for APU use. If there is no fuel in the tanks, you should get the "APU FUEL" EICAS message. 

 

If you still have problems, please report back with what EICAS messages you have and also if you have any status messages*.

 

*Status messages are shown in white text on the STAT-page (secondry EICAS). If there are many messages you may have to cycle through the pages of status messages with subsequent pushes on the STAT-button.

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some 747 classics had the possibility to be started in the air. pretty sure some -400 had that option too.

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For those who like to hear it "straight from the horse's mouth", good discussion about this in the FCOM starting on page 895 of the PDF (section 7.30.1).

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I think Kyle called it (and others). I was trying to start it in the air, which wasn't working. This flight I waited until I was taxiing in and it started right away. There may be some mechanism that if you try to start it in the air, it causes it to fail even after you're on the ground. I think that is what was happening to me. All is good now and I do seem to be able to get it working as long as I'm on the ground as stated.

 

Kyle, I did go through the Intro manual a couple of times and of course, you're right about starting it on the ground. For some reason I thought it was possible to start it in the air as well, but I'm not sure where I got that from, except possibly from a friend who flies twins on ETOP's missions. To be fair though, he stated straight up that it's quite possible this does not apply to anything with more than 2 engines, so he wasn't misleading me. The leap was all mine :) .

 

Thanks very much for your inputs, and especially for that FCOM reference. That's what I was looking for (but on my iPad which isn't the best for searching for specific items). I'll have a good read of that tonight.

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Interestingly, the Boeing engineers thought the odds of all 4 engines failing were so remote that they saw no need for the option to start the APU in the air. Good discussion, brothers.

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i tried it in the air, too.

 

when on the ground i noticed the APU was not running, so i went to START again. would not work. if you turn it back to OFF, and then to START, it will actually start.

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when on the ground i noticed the APU was not running, so i went to START again. would not work. if you turn it back to OFF, and then to START, it will actually start.

 

 

So you're saying you left the APU in ON after a failed start in the AIR... and it wouldn't start when you went to START on the ground?

 

Interestingly, the Boeing engineers thought the odds of all 4 engines failing were so remote that they saw no need for the option to start the APU in the air.

 

 

 

Obviously they had a re-think with the 747-8

 

Cheers

John H Watson

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The APU can't be started in the air because the start logic needs to see an open APU inlet door (And the door won't open in the air... if it wasn't already open before takeoff). It should open, however, when the aircraft lands. I don't know why you have to go back to OFF.

 

Cheers

John H Watson

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So you're saying you left the APU in ON after a failed start in the AIR... and it wouldn't start when you went to START on the ground?

 

 

 

 

Obviously they had a re-think with the 747-8

 

Cheers

John H Watson

Not sure what you mean by the -8. The RAT is for Hydrualic System 3 only. As far as the APU in the -8 FCOM says is usable from ground up to 20,000 just as the -400. This is for Pneumatics only up to 15k no electrical power.

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