Delta 737-900 returns to SLC after pressurization fault caused ear and nose injuries

[flashcrash]

Aircraft stopped climb at 9000 ft and returned 16 minutes later. 10 out of 140 passengers injured.

FAA investigating cabin pressure problem that injured Delta passengers : NPR

Pressurization Issues Force Delta Air Lines Boeing 737-900ER To Return To Salt Lake City (simpleflying.com)

[Evan]

Puzzling. The report says they leveled off at 9000. Delta says they experienced the problem at 10000. But these are normal cabin pressurization altitudes. Why the injuries? t first, I thought it might be an over-pressurization event due to stuck outflow valves or something. Maybe we can just say it's just Boeing's fault and that'll be the end of it.

[3WE]

Maybe we can just say it's just Boeing's fault and that'll be the end of it.

Indeed. And demerits for Flashcrash for not highlighting it.

[Gabriel]

Puzzling. The report says they leveled off at 9000. Delta says they experienced the problem at 10000. But these are normal cabin pressurization altitudes. Why the injuries? t first, I thought it might be an over-pressurization event due to stuck outflow valves or something. Maybe we can just say it's just Boeing's fault and that'll be the end of it.

The absolute pressure it's not as much of a factor (in fact it is no factor) as much the magnitude of the CHANGE in pressure and the RATE of such change.

[Evan]

The absolute pressure it's not as much of a factor (in fact it is no factor) as much the magnitude of the CHANGE in pressure and the RATE of such change.

Okay, so perhaps the plane had been pressurizing and then suddenly lost pressure around 10,000ft, but it wouldn't be pressurizing much below 10,000ft cabin altitude, so the RATE of that change really isn't a factor either, right?

[3WE]

You never cease to amaze me. The plane is generally pressurized before takeoff and depressurized just before touchdown. I know, no cryptic acronyms…boring.

It is largely automated, that should excite you a little.

https://youtu.be/r2lT4oE4DxE?si=Q5KiQlaedHOu0STo

[Evan]

You never cease to amaze me. The plane is generally pressurized before takeoff and depressurized just before touchdown. I know, no cryptic acronyms…boring.

It is largely automated, that should excite you a little.

https://youtu.be/r2lT4oE4DxE?si=Q5KiQlaedHOu0STo

It never ceases to amaze me what I can lern on this forum. So... before takeoff... the engines are run in reverse, to vacuum the cabin pressure down to 8000' cabin altitude, somehow, despite the Negative Pressure Relief Valves (cryptic acronym: NPRV)? Fascinating! I heretofore believed the myths about cabin rates being selected at around 500-750fpm. Now, where is that bicycle of mine...

[Gabriel]

(Disclaimer: I don't know le logic of pressurization systems very well)

After lift off a pax plane may be doing over 3000 fpm, and these are 3000 ft where the outside pressure changes quite a bit (about 11% of 1 atm). Even if the plane kept doing 3000 fpm al the way to cruise, which it doesn't, in the last 3000 ft the change in outside pressure would not be as much (about 3.5% of 1 atm). Combine that with the fact that the climb rate when reaching cruise my be less than 1000fpm or less.

The airplane "knows" this and instead does a math like "ok, it will take us like 20 minutes from lift off to cruise, we need to take the cabin altitude from 0 to 8000 ft, so let's keep a cabin rate of climb of 400fpm no matter what". (Sidenote, to keep the rate of change in pressure constant you should increase the cabin climb rate as the cabin altitude climbs, because the pressure difference for each additional 1000ft is smaller the higher you are, but below 8000ft it is more or less linear, for example I said that from 0 to 3000 ft the change is about 11% of 1 atm, and from 5000ft to 8000ft it is 8.4%, and they don't go over 8000 ft of cabin altitude, so I believe that manufacturers don't bother with keeping a constant rate of change of pressure and instead just do a constant cabin altitude climb)

It takes some 5 minutes for the plane to reach 10000 ft (because in the middle the plane reduces the climb to accelerate, retract the flaps and accelerate to 250kts which is a sub-optimal climb speed but it has to be kept at 250 or less below 10000ft). So the cabin altitude would have climbed some 2000ft by then.

If there is a sudden depressurization at this point, the cabin altitude would suddenly go from 2000 ft to 10000 ft, a change of 24% of 1 atm. That's about exactly half of what you would get with a sudden depressurization at cruise, where the cabin altitude would go from 8000 ft to say 33000 ft, a pressure drop of 48% of 1 atm. So while not as bad, it is still quite a bit.

Of course the alternative could be that the plane was not pressurizing at all due to pilot error or system failure and the pilots only realized of that at 1000ft when the cabin altitude warning sounded. And maybe the injuries happened when the plane was climbing like 3000 fpm. But while 3000fpm will be a big discomfort for most, and possible a lot of pain for some, I am not sure if it would cause bleeding. Then again, less than 10% of the pax were severely affected.

[Evan]

Of course the alternative could be that the plane was not pressurizing at all due to pilot error or system failure and the pilots only realized of that at 1000ft when the cabin altitude warning sounded. And maybe the injuries happened when the plane was climbing like 3000 fpm. But while 3000fpm will be a big discomfort for most, and possible a lot of pain for some, I am not sure if it would cause bleeding. Then again, less than 10% of the pax were severely affected.

Part of what puzzles me here is that the report doesn’t mention decompression, but only a pressurization issue made apparent by a cabin altitude warning, so I assume there was no rapid change in pressure, but rather a gradual drop in pressure during the climb that could go unnoticed until the alarm caused them to recognize it. Sort of like Helios if the alarm had worked.

[Gabriel]

Sort of like Helios if the alarm had worked.

The alarm worked. The pilots didn’t.

[Evan]

The alarm worked. The pilots didn’t.

Ah, right. So, like Helios if the pilots had worked.

[Gabriel]

Part of what puzzles me here is that the report doesn't mention decompression, but only a pressurization issue made apparent by a cabin altitude warning, so I assume there was no rapid change in pressure, but rather a gradual drop in pressure during the climb that could go unnoticed until the alarm caused them to recognize it. Sort of like Helios if the alarm had worked.

Seems you were right (except for the Helios alarm thing)

When these "cabin did not pressurize" issues happen, I always wonder a couple of things:
1- You have most pax in pain, some of them even bleeding, but the crew (both flight and cabin) didn't realize? None of them felt at least a highly abnormal sensation in their ears?
2- How can we have an earlier identification of these events, instead of waiting for the cabin altitude warning to warn? Perhaps some alarm if the cabin rate of climb more or less matches the airplane's rate of climb, or exceeds a certain value? Perhaps an alarm if the differential pressure remains at almost zero? Perhaps add a an item in the after take-off checklist to check for cabin rate of climb?

https://avherald.com/h?article=51dc28f5&opt=0

[Evan]

Seems you were right (except for the Helios alarm thing)

When these "cabin did not pressurize" issues happen, I always wonder a couple of things:
1- You have most pax in pain, some of them even bleeding, but the crew (both flight and cabin) didn't realize? None of them felt at least a highly abnormal sensation in their ears?
2- How can we have an earlier identification of these events, instead of waiting for the cabin altitude warning to warn? Perhaps some alarm if the cabin rate of climb more or less matches the airplane's rate of climb, or exceeds a certain value? Perhaps an alarm if the differential pressure remains at almost zero? Perhaps add a an item in the after take-off checklist to check for cabin rate of climb?

https://avherald.com/h?article=51dc28f5&opt=0

Seems logical to me. Just a 'pressure anomaly' warning if cabin pressurization is doing something significantly 'strange', out of the ordinary. A master caution. A yellow CHECK PRESS message on ECAM/EICAS.

But that alarm should be built in to the pilot's eardrums. Why it doesn't warn them is a good question.

[Gabriel]

A yellow CHECK PRESS message on ECAM/EICAS.

What’s that? (hint check type)

[Evan]

What’s that? (hint check type)

I can't edit the post! I get this "Error information: " 403 error " message

Nothing works. Good thing whoever makes this crappy forum software doesn't make airplanes. I hope.

Anyway, not sure what you're getting at but I suppose the message would be something like CHECK CAB PR as a level-2 (amber) message.