40-degree Celsius air in supply ducts?

[johnnybroom 0]

Sorry if this has been already posted (I did my best to search the forum). I have been encountering the following problem: a quick rise to 40+ C. air in certain supply ducts (usually AFT and/or CONT CAB) shortly after I either switch on the APU bleed or start the engines thus using engine bleeds for bleed air. In turn, my cabin temp rises substantially -- if I don't notice it right away, sometimes cabin temps get into the mid 40s C. and I get an amber zone temp alert. All temperature control switches are set to AUTO, the problem happens regardless of outside air temperature, and my RAM doors are fully open during this problem. PACK temperatures are normal, with right pack temp registering around 19 C., and the left slightly higher at around 23 C. If I move the appropriate temperature control switch(es) to a cooler setting (one or two notches into the "cold" range), the respective supply duct temperature quickly decreases to an acceptable range. My question is twofold: 1. The problem doesn't seem to be with the PACKs, since their temps are normal, and my problem involves temps jumping extremely only in certain zones (indicating mix manifold temp is normal). With mix manifold temp normal, the air must be getting heated somewhere between the manifold and the supply duct -- the only thing I can imagine is the trim air going nuts and thinking it needs to modulate (i.e. heat) the individual supply duct zones way (way) more than it does. I could shut off the trim air system entirely, or the individual trim air modulating valves by selecting OFF on the respective temperature control switch, but that would seem an unnecessary (and perhaps dangerous) step to having a comfortable cabin when AUTO should keep temps normal. Furthermore, when the amber zone temp light illuminates as it does when I fail to notice the temp rise, the associated trim air modulating valve should close (perhaps it does not in the NGX simulation), yet even so, the supply duct temp does not cool until I move the temp selector into the "cold" range. Does anyone know if trim air does this sometimes, and what I can do to remedy it? Do I have the wrong idea assuming this is a trim air malfunction? 2. Am I doing something procedurally wrong that could somehow be subjecting my virtual passengers to an oppressive, tropical climate? My pneumatic system initiation procedure with APU bleed is standard, as I understand it, with APU ignition, trim air defaulted on, recirculation fans switched to auto, PACKs switched to auto, isolation valve switched to open, and all bleeds turned on. If this is just a real-life NG idiosyncrasy, I'll continue to deal with it manually as I mentioned, but it seems hardly likely most pilots would be alright with such a quirk, 40-degree C air blowing on their passengers with temp controls set to AUTO. Or, perhaps, is this a fix that PMDG needs to institute? Thanks guys, Bryan Yunis

[VLJ510 16]

Had same problem made a thread not long ago, what fixed for me was turn them off AUTO to about the 9/10 O'clock position and turn the trim air off. The trim air off helps as that feed warm manifold air I think witch just makes things hotter. So off help heaps. After I did that the temps were normal and manageable. No more melting passengers! Also full names are required in the PMDG forum. Just a heads up as mods can delete the post.

[Cristi_Neagu 134]

I remember seeing somewhere that there is no AUTO setting. The AUTO means that those are the automatic pack controls, and the AUTO label just happens to be there. Maybe i read it somewhere in Simon's post, but i can't be sure.

[Chock 17,769]

Whatever you do, don't go into the air ducts to check, unless you have a motion sensor and a flamethrower, that's where Xenomorphs go. Although they mostly come out at night, mostly. Al

[jbassue 181]

Whatever you do, don't go into the air ducts to check, unless you have a motion sensor and a flamethrower, that's where Xenomorphs go. Although they mostly come out at night, mostly. Al

So that's what the noise was, oh.

[Chock 17,769]

It's only a problem if you take on cargo at LV-426, so I'd avoid landing there. Al

[johnnybroom 0]

what fixed for me was turn them off AUTO to about the 9/10 O'clock position and turn the trim air off.

Thanks, VLJ. I'll try that. Are you aware of any repercussions of turning trim air off, other than the inability to modulate specific zones? I suppose that's kind of the "default" setting, like Cristi said, so I'd just be getting relatively comfortable PACK air. Do you turn the trim air back on at any point in the flight? Also, thanks for the tip re: my name. I changed the original post. Bryan Yunis

[VLJ510 16]

No repercussions that I know of for Trim Air off setting. Like I said I think that just gets warm air from the manifold and puts in the ducts. Also in no phase of flight have I had to turn it on yet to keep a nice temp. I think it's more for cold/winter operation. I just know I had same problem, melting temps with trim air on and temps set to auto, after some help here and some reading on this system the temps became normal and now I just manually set the temp. 9/10 O'clock seams nice 21 -23c or so. I've done almost 40 flight so far, the first 10 I had temps problems and trying to resolve myself, then came here to resolve the issue, sadly I don't think may people monitor the passenger cabin temps. Glad to see someone else is!

[Spin737 102]

No repercussions that I know of for Trim Air off setting. Like I said I think that just gets warm air from the manifold and puts in the ducts. Also in no phase of flight have I had to turn it on yet to keep a nice temp. I think it's more for cold/winter operation. I just know I had same problem, melting temps with trim air on and temps set to auto, after some help here and some reading on this system the temps became normal and now I just manually set the temp. 9/10 O'clock seams nice 21 -23c or so. I've done almost 40 flight so far, the first 10 I had temps problems and trying to resolve myself, then came here to resolve the issue, sadly I don't think may people monitor the passenger cabin temps. Glad to see someone else is!

The 9 o'clock position is still AUTO mode. The selectable range is about 65-85F. C is 65, course and W is 85. The 12 o'clock position is where the lable is that says AUTO, but that's just where they put the lable. You can select AUTO or OFF. I've never seen anyone move trIm air to OFF unless it was in association with QRH. In flight, it seems a duct temp or around 18C results in a cabin temp in the low to mid 20s. That seems to keep the FAs happy.

[Flamin_Squirrel 71]

Not sure I realy understand those pictures. First one, well one's set to full cold, so why are freezing passengers a suprise?

[VLJ510 16]

The 9 o'clock position is still AUTO mode. The selectable range is about 65-85F. C is 65, course and W is 85. The 12 o'clock position is where the lable is that says AUTO, but that's just where they put the lable. You can select AUTO or OFF. I've never seen anyone move trIm air to OFF unless it was in association with QRH. In flight, it seems a duct temp or around 18C results in a cabin temp in the low to mid 20s. That seems to keep the FAs happy.

I understand that auto even when moved is still auto but for me if left there (default 12 O'clock) temps are unacceptable and WAY higher the the max of selector range of 85, hitting 40c and I have similar duct temps. Also a 737 pilot told me to turn the Trim Air off for a bit ( http://forum.avsim.n...65#entry2081665 ) I just leave off 24/7 and the problem fixed so... plus I have the right temps I want. maybe after SP1 "auto" won't melt people? Also seams not everyone plane is acting the same so while 'X' may work on your plane it dose not with might as far temp regulation. Like the ones with attitude landing issues etc I don't have that problem while others do.

[Spin737 102]

Isn't it fun trying to figure out if it's really screwy or not programmed well or if there's are bugs?If you turn off the trim air, you're basically running the left pack with the cockpit controller and the right pack takes an average of the other two zones' desired temps. If you were getting ZONE TEMP lights, you'd probably end up turning the TRIM AIR off, like you did. What are your duct temps saying?

Not sure I realy understand those pictures. First one, well one's set to full cold, so why are freezing passengers a suprise?

I don't think it's a surprise - the point of those slides is that the packs will cool air to the coldest requested air and the trim air heats the air back up for the zones that have a higher request. If you've never been forced to sit through the Boeing computer based training, consider yourself lucky.

[spesimen 135]

sadly I don't think may people monitor the passenger cabin temps. Glad to see someone else is!

hahaha!!! you're not the only one !. i dunno why, but i am a total junkie for fsx planes that actually simulate the a/c systems and cabin temps. it's a waste of time but i love loading cold and dark in places where it's super hot or cold, opening the doors and watching the temps stabilize to oat...and then once i start preflighting making sure to get the cabin to a civilized temp before boarding starts. i wandered into this thread wondering about the same issue you described. a couple times i've completely frozen my pax, and a few others i've boiled them alive. still unsure what i had set wrong but i've always had trim air on i think, it also seemed like just changing the position of the temp controls to 9oclock would usually just stabilizie things assuming the ducts and bleeds were all set correctly....these varying results came when i thought i pretty much used the same procedure each time... but anything is possible when you mix beer and simming.. i'll definitely keep my eyes open cheers-andy crosby

[johnnybroom 0]

The 9 o'clock position is still AUTO mode. The selectable range is about 65-85F. C is 65, course and W is 85. The 12 o'clock position is where the lable is that says AUTO, but that's just where they put the lable. You can select AUTO or OFF.

Ok, that makes sense. The whole temperature arc is technically AUTO so moving the selector to 9 o'clock, for instance, is just saying "cooler," not "cooler than auto." It *still* bothers me, though, that the 12 o'clock position, presumably half-way between the warmest and coldest, offers such varying temperatures, and the main concern here, occasionally such hostile temperatures. As Spin noted (in agreement with FCOM) the selector range is 18ºC to 30ºC, so how could I be getting 40ºC air in the supply ducts with the selector in the middle (default)? Isn't that just indication that the automation of trim air is going nuts, trimming the already-comfortable PACK air a good 20ºC more, out of the normal temp range? Are there really conditions (even cold weather ops, e.g.) in which you'd need that much hot bleed air trimming the ~20ºC PACK air to provide a warm cabin? Bryan Yunis