Wayside

It might not even be a NTSB reportable event.

mduell

If it's a structural failure they should.

toomer

Does anyone know if a report was ever logged for this event?

Sorry to bump an old thread back up - but since I was on this flight, I was curious to know if there was ever anything official noted in regards to what happened. So far I'm not finding anything on either faa.gov of ntsb.gov, but I'm not sure I know all of the right places to be looking...

Thanks!

n198ua

Here's the SDR, I didn't know how to link directly so I just threw the pdf in my dropbox.

https://www.dropbox.com/s/fjnfuu7pq0...itled.pdf?dl=0

I'll leave it up for a couple of days.

Cheers.

Wayside

Many transport airplanes have a ram air system that diverts outside air into the cabin when the air conditioning system is off in flight. The cold air could have been some of the ram air being introduced into the cabin, depending on what the cause of the loss of pressure was in the first place.

DenverBrian

Aft cargo door seal failure. There you go.

hayzel7773

Pretty sure she thought their was a rush of air because of the noise that is made when the masks drop. Very scary

LarryJ

The loss of pressure itself produces a significant cooling effect.

Lawyerneering

Couldn't multiple door seal failures be the cause of the reported inflow of air?

spin88

Air flows from higher pressure (inside the cabin) to lower pressure (outside the cabin). It would be cooler after depressurization as the same heat in the cabin would be dissipated over less air - since part of it went outside in a rush - this is how air-conditioning works. And you would feel that cooling air rush past you as it sought the lower pressure behind the leaking door.

The air outside would have been something like 10-20 degrees below zero. If that air rushed in after depressurization (e.g. with a hull rupture leaving a hole) you would know it, it would be colder than anything you had ever felt before.

Scary, but given what people report it was a leak causing loss of pressure, if it were really a rupture letting in cold air, the reports would be far more dramatic.

televisor

That definitely isn't how air conditioning works. Air conditioners generally use a heatpump to transfer heat out of air that is being recycled in a room.

And the "same heat in the cabin would be dissipated over less air" is wrong too. The air that remains isn't dissipating energy into the air that is leaving - the amount of energy stays the same. The actual reason for the temperature drop is more complex, but fortunately there's a wikipedia article that helps explain what's going on, if you want more detail just try to find your nearest fluids engineer.
https://en.wikipedia.org/wiki/Joule%...Thomson_effect

toomer

Shoot - I didn't grab it in time. Any way you could re-post it again? I promise, I'll check back more quickly this time ...

QBK

Point of order: actually, this is how some air conditioners work (so spin88 was correct about that). The linked Wikipedia article (https://en.wikipedia.org/wiki/Joule%...Thomson_effect) points out that the Joule-Thomson effect is used in heat pumps and air conditioners. televisor is also correct that A/C units use heat pumps, but "heat pump" is a fairly generic term for any device that moves heat, and the Joule-Thomson effect is one method.

I think the source of the confusion here is that an airliner decompression incident is a one-shot event -- air rushes out. It's not a cycle. A/C systems operate on a closed cycle. But that doesn't mean the underlying physics has to be different! In A/C systems based on the J-T effect, a compressed working fluid (not air) is placed in Chamber A and then squirted through a valve into Chamber B at lower pressure, which leaves cold fluid in Chamber A that can soak up heat. Then it gets compressed again in Chamber B, and heat is dumped into the environment of Chamber B before the compressed fluid is put back in Chamber A.

But, if you really wanted to, you could use depressurization as a cyclical heat pump to cool a regional jet!
1. Let the air leak out a small hole. Now you have an RJ full of cold, low-pressure air.
2. Use a pump (outside the jet) to pressurize a tank of air (outside the jet). It'll get hot because you squeezed it; let this heat dissipate into the atmosphere.
3. Release the high-pressure air from the tank into the jet.
4. GOTO 1.

Point to televisor; it's correct that in a Joule-Thomson expansion, no energy is transferred to the air that leaks out (or to anything else, for that matter). It's a pretty odd effect: work is done (i.e. "energy is transferred") but it's not done to anything external! It all goes into the internal energy of the air that's left in the cabin. Temperature is kinetic energy. If you transfer kinetic energy into potential energy, the temperature drops. It turns out that air molecules like to be close to each other (attractive "intermolecular forces"), and when you expand the air, some work has to be done in order to drag the air molecules apart. This energy comes out of their kinetic energy, so they slow down, and that makes the air colder.

What makes this extra confusing is that (1) this wouldn't happen for an ideal gas, so it's counterintuitive; and (2) in most expansion processes, the working fluid cools down precisely because it does do work on something external. Plenty of steam engines and air conditioners use that kind of effect, and if that was what was going on in an airliner decompression, it would be correct to say that the remaining air "dissipated" energy into the departing air.

That said, spin88's language is kinda backward -- if you had the same heat distributed over less air, that air would have to be hotter. And televisor's statement that "the amount of energy stays the same" is a bit misleading too; the energy pretty much follows the air, so when some of the air leaves, its energy leaves too. The energy density stays the same inside the jet, but some of that energy has been transferred from kinetic (temperature) into internal potential energy ("intermolecular forces").

QBK, keeping Flyertalk's science impeccable since 2010.

P.S. not a fluids engineer, but I hope a physicist will do...