When do you use the APU?

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jltope
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When do you use the APU?

Post by jltope » 13 Mar 2009, 20:38

When are you supposed to use the APU? I assume while I am at the gate that I do not use the APU, but what about when you are pushing back from the gate before the engines are started?

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Re: When do you use the APU?

Post by DMM200 » 13 Mar 2009, 22:59

Not to speak for the moderators, but in the future, please see if your question has already been answered before posting. There is already a topic on this, and it has been posted quite recently.

On to the APU. If there is ground power available, you can use that prior to pushback. However, once the ground crew (or yourself if you don't have AES) tell you "Okay, hold on as we prepare the aircraft for pushback," it is a good time to start it up, as you need it for all electrical sources, air conditioning, etc. when the engines are not started. Plus, you need the APU to start the engines :P .

After landing, you usually turn it on well before entering the ramp area. You need this until you are parked and the Ground Power is connected. DO NOT CONNECT GROUND POWER WHILE THE ENGINES ARE RUNNING.

Of course if you don't have payware, what I just said above is absolutely meaningless.
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Re: When do you use the APU?

Post by Aaron Robinson » 13 Mar 2009, 23:28

DMM200 wrote:Not to speak for the moderators, but in the future, please see if your question has already been answered before posting. There is already a topic on this, and it has been posted quite recently.
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Re: When do you use the APU?

Post by jltope » 14 Mar 2009, 03:10

DMM200 wrote:Not to speak for the moderators, but in the future, please see if your question has already been answered before posting. There is already a topic on this, and it has been posted quite recently.

On to the APU. If there is ground power available, you can use that prior to pushback. However, once the ground crew (or yourself if you don't have AES) tell you "Okay, hold on as we prepare the aircraft for pushback," it is a good time to start it up, as you need it for all electrical sources, air conditioning, etc. when the engines are not started. Plus, you need the APU to start the engines :P .

After landing, you usually turn it on well before entering the ramp area. You need this until you are parked and the Ground Power is connected. DO NOT CONNECT GROUND POWER WHILE THE ENGINES ARE RUNNING.

Of course if you don't have payware, what I just said above is absolutely meaningless.
Thank you very much for your detailed response. I didn't think you could start the engines without APU in real life. I did not see the post from Ulsterjohn (John Hanna) because it was posted in a different section of the message board according to FJ flyer.

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Re: When do you use the APU?

Post by Aaron Robinson » 14 Mar 2009, 03:44

jltope wrote:I didn't think you could start the engines without APU in real life.
You can, using an airstart cart.
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Re: When do you use the APU?

Post by FJ flyer » 14 Mar 2009, 06:39

DMM200 wrote:After landing, you usually turn it on well before entering the ramp area. You need this until you are parked and the Ground Power is connected. DO NOT CONNECT GROUND POWER WHILE THE ENGINES ARE RUNNING.
Just to clarfiy, a pilot at Air NZ said they usually turn the APU on while taxiing in.
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Re: When do you use the APU?

Post by seahawks7757 » 14 Mar 2009, 08:50

FJ flyer wrote:
DMM200 wrote:After landing, you usually turn it on well before entering the ramp area. You need this until you are parked and the Ground Power is connected. DO NOT CONNECT GROUND POWER WHILE THE ENGINES ARE RUNNING.
Just to clarfiy, a pilot at Air NZ said they usually turn the APU on while taxiing in.

And I was about to say that is inacurate. on Davids Part. At Horizon we connect the ground power on the 400's when they park after we chock them and they feather the engines. We never use the APU on the 400's of possible. But sometimes we must. Also we do start at the gate so the engines are still started then too ;)
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Re: When do you use the APU?

Post by FJ flyer » 14 Mar 2009, 09:28

seahawks7757 wrote:
FJ flyer wrote:
DMM200 wrote:After landing, you usually turn it on well before entering the ramp area. You need this until you are parked and the Ground Power is connected. DO NOT CONNECT GROUND POWER WHILE THE ENGINES ARE RUNNING.
Just to clarfiy, a pilot at Air NZ said they usually turn the APU on while taxiing in.

And I was about to say that is inacurate. on Davids Part. At Horizon we connect the ground power on the 400's when they park after we chock them and they feather the engines. We never use the APU on the 400's of possible. But sometimes we must. Also we do start at the gate so the engines are still started then too ;)
I don't know what happens on props, but at Air NZ on 737/A320 that's what they do. Sorry, I should've clarified more.
Also, to add to what I said, he also said that you turn the APU off while they are taxiing out and the engines are on (obviously).


You can't say truthfully that there is one procedure that everyone uses, it varies from airline to airline and from aircraft to aircraft and from props to jets.
The only thing you can say is the same is that they don't use the APU while you've got external power connected.
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Re: When do you use the APU?

Post by seahawks7757 » 14 Mar 2009, 09:42

And to actually add on the Jets. On our CRJ's the captain will sht down #1 but will keep #2 running while we plug them in. So they do typically keep one running to keep from having to turn on the APU.
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Re: When do you use the APU?

Post by Carlo » 14 Mar 2009, 10:05

But doesn't that defeat the point of the APU in the first place, using an engine for power?

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Re: When do you use the APU?

Post by seahawks7757 » 14 Mar 2009, 12:16

Carlo wrote:But doesn't that defeat the point of the APU in the first place, using an engine for power?
No because it only takes about 30 seconds to hook them up to ground power.
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Re: When do you use the APU?

Post by Aaron Robinson » 14 Mar 2009, 15:04

Let's keep this topic to real life, not the technical side of FS. Let the other topic handle that. Thanks.
FJ flyer wrote:Just to clarfiy, a pilot at Air NZ said they usually turn the APU on while taxiing in.
FJ flyer wrote:You can't say truthfully that there is one procedure that everyone uses, it varies from airline to airline and from aircraft to aircraft and from props to jets. The only thing you can say is the same is that they don't use the APU while you've got external power connected.
This is a very complicated area regarding what is the right thing to do. As you may have figured out, this exact area is one thing I work on for Northwest/Delta. There are good reasons for every kind of variation, but many are airline-specific or depend on non-flight factors.

Originally APUs were run in the air in case all engines failed, power would still be available. Engine reliability has advanced to a point where this is typically no longer the case.

One could choose to turn on the APU during taxi for several reasons. If taxiing with one engine and crossing an active runway, what if that one engine dies? You can then quickly start the other and taxi clear. What if you won't be hooked up to external power immediately upon arrival? Would you keep the engines running for a minute until you do, or transition from engines to APU to external power?

Finally, you may need to run the APU in additional to external power. For example, what if you need to cool/heat the cabin but no ground air is available (or it's extremely cold/hot)? Then you would want to use the APU for that purpose. However, external power would still reduce the load on the APU.
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