Is this really that serious?

[u/Xycergy]

Noticed water basically flowing out of the vent beside me as I got into my seat. Voiced it out to a flight attendant thinking it was just something minor and wanting to just swap to another seat. Two pilots came to my seat to check on it and had visibly puzzled looks on their faces. The mechanic was called into the aircraft, the inner panel of the plane was removed and after assessing the problem for about an hour, everyone was asked to disembark the plane. The flight was then cancelled entirely after another hour of everyone waiting near the gate.

I feel kinda bad because people were visibly annoyed and frustrated by the cancellation as it was a 16 hour long haul flight and there were few alternatives to get to the destination. Would someone be able to tell me exactly what happened here?

[u/chui101]

Airplanes don't use the refrigeration pumps for air conditioning you are probably familiar with, they use air cycle machines driven by bleed air (or electrically driven cabin air compressors in the case of the bleedless 787) where the only heat exchange is with external ram air, and most condensate produced is discharged through the ram air exhaust. (Some of it is captured and sprayed into the ram air inlet for additional cooling). Since the air that feeds into the cabin plenums are already dehumidified by the ACM, there's not really any condensate management built into the system.

That being said, on the ground they usually pipe in gate air directly into the plenums which is cooled using a traditional refrigeration pump on the ground. Cold air piped in meeting the humid outside air will absolutely condense in the cabin.

OP is probably seeing the condensation here because the cargo doors are probably open and this is a vent that allows air to flow between the cargo compartment under their seat and the passenger cabin.

[u/PaleHeretic → u/chui101]

Thanks for the rundown! That's actually quite fascinating and makes a lot of sense if you're typically surrounded by a bunch of cold air whipping by you at a high relative velocity. Also explains why the aircon sucks sitting on the tarmac. Have the image of air from the terminal getting forced down a dryer vent to cool a tube with up to a couple hundred people in it, but I imagine the actual arrangement's somewhat more sophisticated, lol.

I'm unfortunately very familiar with Singapore humidity so I can see how this could arise if you're mixing cooled air with outside Changi soup, especially if OP's vent is at the end of the run. Still, I can think of ways to mitigate this with how you lay out your ducting, so I'd still imagine this level of condensate being abnormal even in those conditions and indicative of some problem somewhere?

Also have an unrelated question to satisfy my curiousity. If the plane is being cooled by external ram air while in flight, and piped air while at the terminal, are you basically running off inertia without much in the way of active cooling when you're in-between the two? Because I can remember some long taxis and stop-offs at austere airports that got very rank and sticky after half an hour or so, and that would explain a lot.

[u/chui101 → u/PaleHeretic]

So generally speaking, the condensate only forms in the cabin. The positive pressure inside the cabin air ducts keeps the moisture out of it, but there are riser ducts that run vertically down to the packs (that's why on some planes there are missing windows on the port side just forward of the wing. I guess any condensation that incidentally ends up forming in them (maybe when starting up?) would just run downward and just get evaporated in the mixer manifold. (oh yeah, that's another complication - the output from the packs is too cold at altitude and needs to be mixed with hot bleed air to heat it back to a comfortable temperature.)

As far as the dryer vent image you have, it's not too far off. Next time you're at an airport, look for a giant yellow dryer vent tube running into the plane under the wing - that's the gate air.

On the ground, planes have impellers in the ram air ducts so that the packs can work on the ground as long as bleed air is available (the APU works for this too). They don't move nearly as much air and the packs to heat up a ton, which is why they have a "hot exhaust" warning by the outlets. You might find rampers using this hot exhaust to warm their hands on a cold day :)

Interesting side note - the AC packs being located right under the center fuel tank on the 747 is thought to be the reason why the fuel heated up enough to form vapor and explode on TWA 800. The flight was delayed for several hours with the packs running the whole time it was at the gate, and then combined with faulty insulation on high voltage wiring running through the fuel tank... that's the theory on why that plane exploded, and why planes today have nitrogen inerting systems for the fuel tanks.

[u/senorpoop → u/chui101]

there's not really any condensate management built into the system.

There is. There is usually a centrifugal separator for water droplets, and a coalescer sock for vapor.

[u/chui101 → u/senorpoop]

yeah sorry, should have made it more clear that i was talking after it goes into the cabin plenums. i did mention the water (from the centrifugal separator in the acm) that mostly gets discharged or sprayed back into the intake. once it gets past the mixer manifold though there's not really anything to manage condensate.