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Switching from APU power to external


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Hey all,

 

I was wondering if anyone could help with the proper procedure in switching from APU power to external power, mainly when arriving at a gate (without momentarily losing power in the aircraft). 

It seems that when the APU is running and external power is connected, the AC Power push button always denotes ‘AVAIL’. Now I’m not sure if this a bug or proper operation but this seems to make it a bit of a lottery to know whether power will be transferred to external power when the APU is powered down or not.
 

I wonder if I’m missing something here but I’ve not managed to find any other way to know whether external power will be automatically switched to or whether there will be a momentary loss of AC power whilst I toggle this button on after powering down the APU. 
 

I’ve noticed this buttons state is still changing in the background despite the light on it not changing when another power source is supplying the aircraft. So for example if I leave a gate after transferring power to the APU without first turning off external AC power, when arriving at the gate, after the flight, provided this button is not pressed again, external power will be automatically switched to. However, if this button is pressed again in the interim it will not transfer power. 
 

It feels like to me that the logical indication on this button should be displaying ‘IN USE’ or not depending on the state of the button [provided external power is connected] regardless of whether the APU is running/the gen is or or not. 

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  • BobbyFuzzy changed the title to Switching from APU power to external

Nope that’s how it works. The way the logic works for the electric system is that it will seek power from a source in this order. Onside generator, inside (apu), offside generator, outside (external power). You can just remember it with this. Onside inside offside outside. As a result as soon as the apu gen is available with the engines off it will always try to automatically switch to it and stay on it after a flight until shutting down the apu. Now sometimes the ground power will fail as soon as it’s needed. To help prevent a total loss of power I normally turn the APU gen off to make sure the external power will hold then shut the apu down and immediately turn the apu gen back to auto. (Note this is not company procedure just something I personally do to prevent something stupid from happening)

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There was a thread somewhere else a while back.  One of the CRJ Pilots commented that this is exactly how the behaviour works....

 

He Also Gave the Pro Tip about using the ELEC page to confirm before shutting down the APU....

 

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There is no way of knowing if the AC will be active before deactivating the APU or the APU generator. The AC button will show "AVAIL" all the time as long as the APU is running. The ELEC page also dosn't help here. If the AC button is not in the right state, the aircraft will loose power and wipe the FMC. 

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If you wanted to, you could turn the APU generator off for a moment and see if the external power picks up. If it doesn't, you can turn the APU generator back on (i.e. it's only off for a second or so) and all will be well. If it does pick up, then you can just turn the APU off, then position the switch back to AUTO.

I'm not sure how practical that is in the sim with how quick the buttons react and how long it takes to look at the EICAS to make sure the power transferred correctly, but it is an option.

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I think a few of the comments here have perhaps missed the point—through no fault of their own.

 

I flew the CRJ for years so allow me to pipe in.

 

Your concern is valid. There should never be a guessing game. If done properly you should never lose power when switching from the APU gen (or engine gens) to the GPU. Not even momentarily. It's not a lottery at all. If you find you're losing power then you're most likely doing something wrong. It's most likely not your fault though. The Aerosoft CRJ has a minor issue with the AC EXT button that I'll explain.

 

First things first. If the AVAIL light is lit on the AC EXT button then it means the GPU is ready to give power. In order for a smooth transition of power from the APU to the GPU, the AC EXT button must be pressed in BEFORE turning off the APU gen. The problem (that I alluded to above) is that in the sim it's really hard to tell if the button is pushed in or not. In the real plane it's very easy. You can tell just by looking at it. Unfortunately though the sim doesn't replicate this very well. The visual difference between the button being out or in is almost imperceptible. You have to zoom in quite a bit. I usually have to press the button a few times before I'm sure that it's pressed in. If you see a 2mm space between the button and the panel then it's out. If you see a 3mm difference then it's in. That's how minute the difference is. Again, it's not like that in the real plane.

 

Once it's pressed in you can then go ahead and disconnect the APU gen. When you do this you should immediately see the AC EXT light change from AVAIL to IN USE and the power should switch automatically with absolutely no loss or interruption. If you turn off the APU gen and the AC EXT button still shows AVAIL then the button was not pushed in. As long as the button says IN USE then you're good to go. As someone else mentioned you can always verify this by going to the AC ELEC synoptic page. That's the standard protocol on the line. Any time the source of power is changed we check the AC ELEC page.

 

Also the proper procedure to transfer power from the GPU to the APU at the gate is to turn the APU gen on and then unpress the AC EXT button BEFORE disconnecting the GPU. It's not a big deal in the sim but in the real world not doing this can apparently cause arcing when the ground crew remove the GPU. So follow this pattern and you'll be fine: 

 

GPU  APU:

1. APU gen ON (AC EXT button will switch from IN USE to AVAIL, this is normal)

2. AC EXT button OUT 

 

APU (or engine gens) → GPU

1. AC EXT button IN (the light will still show AVAIL, this is normal)

2. APU (or engine gens) OFF (AC EXT light will now switch to IN USE)

 

Also, if I may add, in standard operations we don't use the APU after landing unless we need it for air conditioning. If we don't need it we will typically leave it shut down until we're about to start engines prior to the next flight. With this method we taxi into the gate with the AC power being provided by the engine gens. Once we get to the gate we shut down the right engine and wait for the GPU to be connected. Once the GPU is connected we use the same method as I mentioned above (verify AC EXT button is pressed in prior to turning off the engine gen). 

 

I only mention that for realism's sake. There's absolutely nothing wrong with firing up the APU after landing—or on approach if you wish. It's just not standard.

 

 

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Thank you all for getting back to me! Such great responses!

 

On 5/2/2021 at 6:47 PM, LesOReilly said:

He Also Gave the Pro Tip about using the ELEC page to confirm before shutting down the APU....

 

 

This is what I've been doing but this only seems to be showing if the GPU is connected (which is obvious as you will have already selected it using Dave) and doesn't show whether or not the load will be transferred or not.

 

On 5/2/2021 at 10:20 PM, Chaxterium said:

the AC EXT button must be pressed in BEFORE turning off the APU gen

 

This exactly seems to explain what I was wondering about. I thought, surely it can't be a lottery like this in the real aircraft! 😅 So this explains it. Having had another (close) look I can see the button is depressed. Very difficult to see though like you say! Would be great to see Aerosoft take a look at this for future updates.

 

Appreciate the extra insight into a typical procedure also. Always trying to achieve some extra realism in my flights so that's very useful to me.

 

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I guess maybe since I have mine connected to a Switch on a panel that is an On/Off Switch this makes it easier for me.... since well It has a physical On/Off position...

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  • Deputy Sheriffs
21 hours ago, BobbyFuzzy said:

This exactly seems to explain what I was wondering about. I thought, surely it can't be a lottery like this in the real aircraft! 😅 So this explains it. Having had another (close) look I can see the button is depressed. Very difficult to see though like you say! Would be great to see Aerosoft take a look at this for future updates.

 

Yeah, exactly. This was actually a question that I was wondering too when I flew the CRJ for the first time in sim. It's easier in the real aircraft to at least feel the tactile switch being pushed in and make sure that way that it's ON before shutting down the APU. In sim I have just taken the habit of never touching that switch and always leaving it to ON position just in case. However I am sure that our real world pilot advisors would crucify me for saying that because of the possible safety hazard it can cause IRL if the ground crew tries to plug or unplug the GPU while the GPU is connected on bus. 😅 But at least that way I never lose power from the aircraft.

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32 minutes ago, Secondator said:

 

Yeah, exactly. This was actually a question that I was wondering too when I flew the CRJ for the first time in sim. It's easier in the real aircraft to at least feel the tactile switch being pushed in and make sure that way that it's ON before shutting down the APU. In sim I have just taken the habit of never touching that switch and always leaving it to ON position just in case. However I am sure that our real world pilot advisors would crucify me for saying that because of the possible safety hazard it can cause IRL if the ground crew tries to plug or unplug the GPU while the GPU is connected on bus. 😅 But at least that way I never lose power from the aircraft.

We leave it in. Or at least >80% of us do. It’s just easier if it’s left in. Then again we aren’t actually on ground power alone all that much. For the next 5 months from the first power up at 4 or 5 in the morning until the last shutdown between 10pm-midnight either the APU or the engine/engines are on to control the cabin temperature.

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On 5/4/2021 at 5:31 PM, Matthew2312 said:

We leave it in. Or at least >80% of us do. It’s just easier if it’s left in. Then again we aren’t actually on ground power alone all that much. For the next 5 months from the first power up at 4 or 5 in the morning until the last shutdown between 10pm-midnight either the APU or the engine/engines are on to control the cabin temperature.

 

Interesting. I flew the CRJ for two airlines in Canada and both were very big on making sure the AC EXT was out before calling for the GPU to be disconnected. 

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4 minutes ago, Chaxterium said:

 

Interesting. I flew the CRJ for two airlines in Canada and both were very big on making sure the AC EXT was out before calling for the GPU to be disconnected. 

Yeah, no one cares here. Even searched our manual and not a single word about it.

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3 minutes ago, Matthew2312 said:

Yeah, no one cares here. Even searched our manual and not a single word about it.

Funny how different airlines harp on different things. One airline I flew for never touched the 14th stage bleed switches. Always left them open. My other company was anal about closing them at the end of the day.

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47 minutes ago, Chaxterium said:

Funny how different airlines harp on different things. One airline I flew for never touched the 14th stage bleed switches. Always left them open. My other company was anal about closing them at the end of the day.

 

Indeed. With every project we have to decide what airline to see as the one to replicate. Even things like flap settings can be done different between airlines.

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5 hours ago, Chaxterium said:

Funny how different airlines harp on different things. One airline I flew for never touched the 14th stage bleed switches. Always left them open. My other company was anal about closing them at the end of the day.

Thank god I've never had to actually fly a 200. Only once a year in the sims. 

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  • 4 weeks later...
On 5/2/2021 at 5:20 PM, Chaxterium said:

Also, if I may add, in standard operations we don't use the APU after landing unless we need it for air conditioning. If we don't need it we will typically leave it shut down until we're about to start engines prior to the next flight. With this method we taxi into the gate with the AC power being provided by the engine gens. Once we get to the gate we shut down the right engine and wait for the GPU to be connected. Once the GPU is connected we use the same method as I mentioned above (verify AC EXT button is pressed in prior to turning off the engine gen). 

 

I only mention that for realism's sake. There's absolutely nothing wrong with firing up the APU after landing—or on approach if you wish. It's just not standard.

Why would you shut the right engine down? You'll lose half your brakes if you do that. At my airline we shut down #1 2 minutes after landing and proceeded to the gate with #2 running until the jetway was connected the external AC power was established.

 

You're right about the transfer to GPU power. This was never a problem on the 200 because you took the GEN off line prior to shutting down an engine or APU. On the 700/900 you leave the GEN's in AUTO, as you know. My company refused to recommend we check the external AC power by turning the GEN off to make sure it switched but we all did it anyway. Some would turn the GEN off, make sure the external AC power picked up the load, then turn the GEN back on and shut the engine down. That always seemed super silly to me so I preferred to turn the GEN off, shut the engine or APU down and turn the GEN switch back to AUTO as part of the shutdown flow.

 

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13 hours ago, ductmonfault said:

Why would you shut the right engine down?

 

We shut down the right engine after we got to the gate. Once the parking brake was set. Our SOPs prohibited us from doing single engine taxi (honestly I have no idea why) but if we did do single engine taxi I'm sure we'd have taxied on the left engine for the reason you mentioned. Or just turn on the 2B pump.

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