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fabristunt

APU and air conditioning

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Hi,

I'd like some help in understanding the APU's "air heating power". To cut the long story short, I can't heat up the cabin using the APU.

 

I'm at the gate in a B737, on external power (no other services connected), packs off and the outside air temperature is +2°C. The aircraft loads with a cabin temp of about 22°C, which I think is saved with the panel state and matches the temperature I had when creating this panel state.

 

I now open door 1L and the 2 cargo doors. If there's no jetway, I'll open the aft left door too.

As I want the cabin temp to stay friendly while preparing for the flight and during a turnaround, I start the APU and put one pack on high, leaving the isolation valve on auto. The pressure gauge climbs to about 40PSI, while the duct temp drops to 0, as most of the hot bleed air goes straight into the manifold, bypassing the Air Cylce Machine.

Seems like the pack is doing its job. All clear so far. The engine bleed switches are on but the engines are off so it shouldn't matter.

 

However, after 30 or so minutes, after I'm done with the preflight, I check the cabin temp gauge only to find out that it's dropped to 2°C.

 

I've been experimenting on this, as I've read that the NG can actually use two packs on APU and may even burn less fuel then on one pack only.

Putting both packs on auto and leaving the isolation valve on auto as well gives me 40 psi on both packs, if I leave the engine bleed switches to on. Switching the eng bleeds off makes the pressure drop to about 15. (I'm still at the gate with engines off). Still no joy, the cabin temp drops to the outside air temp.

 

The introduction manual states the following on the air conditioning system:

 

1-Cabin temperature will stabilize at a normal rate depending upon air volume and outside temperature, and whether the airplane is subjected to sunlight heating.

 

2-If ground air conditioning/heating is selected from the GROUND CONNECTIONS menu, the conditioned air is pushed into the cabin via the cabin air mix manifold, just like the airplane.

 

3-Forward cabin zone temperature changes if forward doors are open and ambient OAT is significantly different than cabin temp.

 

All right, so I guess that that 1L door is letting out too much hot air and the APU just can't keep up. Really?

 

After starting the engines, in the ten minutes it takes me to taxy to the runway, the cabin temp goes up and stabilizes at around 24°C. Now theduct temp is twenty-ish, as the air is now going through the ACM because it needs to cool the cabin.

 

Am I doing something wrong? Is a single open door really too much to handle for the APU? I know +2°C is cold, but it's not that cold!

 

Edit: I forgot to mention: I'm running P3D V3.4, the P3D version of the NGX base pack and expansion pack and ASN for the weather.

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Good question.  I always rely on ground air, especially with extreme temperatures.  You got me recalling my USAF days, and I'd think nothing of cold Omaha winter days with aircraft on the ramp sucking air from ground units to be hospitable in the event of a need to launch quickly. I wouldn't expect the APU to handle such heating or cooling loads, be interesting to hear from one of the B737 line pilots that come by here.

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Hi,

 

I have not that issue with the APU bleed air and two packs on auto.

The only difference I can see is that I put the isolation valve to open but I don't think it could explain why it doesn't work for you.

 

Can you post a screenshot of your overhead panel?

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Can you post a screenshot of your overhead panel?

 

I'll take a couple of screens tomorrow evening. Leaving the isolation valve on auto closes it in the aforementioned situation so yes, keeping it on open is the reason you have a different behaviour. Still, this is normal ;)

 

I wouldn't expect the APU to handle such heating or cooling loads, be interesting to hear from one of the B737 line pilots that come by here.

 

I'd agree, but after all +2°C (in my case in the sim, nothing like an Omaha winter) isn't that cold after all! The bleed air from the APU is around 200°C!

 

Hearing from a real 737 pilot would be great. I may well be overestimating the APU's air warming power.

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I'll take a couple of screens tomorrow evening. Leaving the isolation valve on auto closes it in the aforementioned situation so yes, keeping it on open is the reason you have a different behaviour. Still, this is normal ;)

 

You are right.

In the SOP I'm following for the NGX, it is the normal procedure to have the isolation valve open when the bleed air comes from the APU. Maybe you could try to see if it fixes your temperature issue.

If you have both packs and the isolation valve in auto then only the left pack is supplied with the APU bleed air hence less warming power.

Also if it's not enough, you can still set both packs to high with the APU bleed.

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It works for the flightdeck and the rear of the cabin.. the closer to the door, the worse it gets. Also depending on wind.

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I've flown 737s to Fairbanks, Deadhorse, Barrow, etc. In the winter, you don't need to climb to altitude to get contrails - they are streaming away, right in front of you on the walk-around.

 

So, to answer your question - the APU should have no problem heating the plane. I'm pretty sure you're missing something on the panel that's giving you fits.

 

For cold WX ops, we close the outflow valve manual to about 90% closed. It stops the lav on the aft right from freezing. Also, we often turn the temp selectors to OFF so the packs put out a constant temp, rather than having them look at the cabin sensors for input.

 

Send a pic when you can.

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Putting both packs on auto and leaving the isolation valve on auto as well gives me 40 psi on both packs, if I leave the engine bleed switches to on. Switching the eng bleeds off makes the pressure drop to about 15.

 

 

As Romain said the isolation valve should be open. Leaving it on auto with both packs on and engine bleed on closes the valve which means only the left pack would be providing air. The reason the PSI dropped when you switched the eng bleeds off is because by doing that you made the isolation valve logic open the valve. The left pack sends a portion of its output directly to the flight deck bypassing the mix manifold. As Matt stated this can mess with the controller logic since the flight deck will heat up faster than the cabin. So at minimum turning the cont cab to off should help (off fixes the temperature output to 24C for the left pack and 18C for the right).  You didn’t state what variant you’re using but if you’re in an 800, or 900 make sure you have the trim air on too.

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Ok, so, Ive tried this again, but this time in a 738. With both L doors open, one pack on APU and about 5°C OAT it can keep the cabin temps in the mid 20s. There is a slight difference between fore and aft cabin temp, but all looks right.

 

Something could be up with the panel state then, I'll try again with the 737 later today. It doesn't have cabin temp indications for different zones, but the cabin temp still shouldn't drop to the OAT.

 

You didn’t state what variant you’re using but if you’re in an 800, or 900 make sure you have the trim air on too.

 

Actually, I did. 4th line, 1st post.

 

the APU should have no problem heating the plane. I'm pretty sure you're missing something on the panel that's giving you fits.

 

That's what I was thinking, the APU should struggle that much. I'm kinda sure I didn't miss something, could be a panel state issue maybe. I'll report back later today.

 

Thank you all for your answers!

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I had time to take some screens and here they are.

 

Just after loading the 737-700 at the gate with external power on. I opened the 1L door and took the photo

1%20off_zpsk2fvt937.jpeg

 

APU running and warmed up

2%20apu%20on_zpscwnpb5ia.jpeg

 

As soon as I turn the APU BLEED and PACK 1 on, the duct temp falls down (as it is supposed), but there is a strange behaviour in the AIR MIX VALVE. It instantly jumps from full HOTto less then half HOT

3%20bleed_zpsqg6p3u6n.jpeg

 

The cabin starts to cool down rapidly

4%20drop_zps0kx0ds8b.jpeg

 

With two packs on the situation doesn't change. The AIR MIX VALVE moves to cold as soon as the pack is turned on. The only way to change the AIR MIX VALVE towards HOT is to fully turn the temp selector clockwise. This is the only way I found to keep the cabin warm on the 737-700.

5%202%20packs_zpsdksku6sg.jpeg

 

If I try to switch pack 1 off and use pack to, the AIR MIX VALVE of the pack that is not working goes to full hot, and on the working pack drops to less then half HOT

6%201%20off_zpsvf1jifwi.jpeg

 

N.B. This doesn't happen on the 737-800. On that plane just the left pack on auto can keep the cabin temp over 20 even with all four doors open and +2°C OAT.

 

Am I doing something wrong or is there something really messed up here?

 

Regards,

Fabrizio

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i could not fully replicate your results,

although i see the same behavior with 1 pack, using 2 packs worked fine in the 700..

 

-------

entry fwd L, aft L open, cargo doors fwd and aft open

-------
oat -16c
---------
left pack auto, right pack off
iso valve closed
900 stabilizes 18c
800 stabilizes 18c
700 stabilizes 0c  
600 stabilizes 0c  
 
(note that the heating did seem to have some effect as it took a fairly long time to reach zero, like 6 minutes at 8x speed)
 
both packs auto
iso valve open
900 stabilizes 18c
800 stabilizes 18c
700 stabilizes 18c
600 stabilizes 18c
------
oat 32c ... lets compare cooling behavior to heating
-----
left pack auto
iso valve closed
900 stabilizes 25c
700 reaches 40c then very slowly oscillates between 24c and 40c
 
both packs auto
iso valve open
900 stabilizes 25c
700 stabilizes 25c
 
anyway i don't know if the apu or packs are less powerful in the 600/700 ..maybe that is expected behavior that one pack is not sufficient for these conditions..
 
not really sure why you are having problems while using both packs either
 
cheers
-andy crosby

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With 2 packs on auto, ISOL Valve open and both temp selectors manually placed on full warm, the APU can keep the cabin warm in 6°C with 1L, 1R and 2R open. Something I don't understand happens to the AIR MIX Valve if the temp selector is left on auto. It jumps to cold and cools down the cabin. Is this normal?

 

I have no problem in managing the temp selectors, just wanted to make sure there isn't something wrong on my NGX :)

 

Regards,

Fabrizio

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There is something wrong with the air mix valves. For a start, with the pack off and temperature control auto, the mix valve should drive full cold. In the NGX it runs to full hot. Secondly, if you close the isolation valve with the APU supplying air the left pack shuts down (mix valve should run full cold) but the left mix valve stays where it is. The right pack mix valve (which should now be controlling zone temp) runs full hot instead as if it was the right pack which has shut down. Finally these are motorised valves which move slowly and shouldn't jump suddenly as the packs turn on and off.

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Should I open a support ticket?

 

Please do.  It helps to ensure things like this end up in the bug tracking system and not forgotten when they start working on this product again.

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Done, thanks for help!

Maybe they'll fix it one day. No big deal, I'll manually move the temp selectors for now.

 

Regards,

Fabrizio

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