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Haddock

Cabin pressurization alarm

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Hi, I  did a flight today, and was quite happily climbing through 20+000 feet, when the pressurization alarm started to sound; I had forgotten to switch the packs and bleed-air on. Should I interpret this alarm as "No point carrying on with your flight as all your passengers are dead", or is the situation recoverable? I mean,  I know that turning on the bleeds and the packs would silence the alarm after a few minutes in the sim, but would anyone still be alive back there?

 

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There wouldn't really be a point in sounding an alarm after everybody died, would there? ;-)

The altitude alarm sound to let you know that the cabin altitude is getting excessively high (around 14500 ft.). Once you turn on the packs, close the outflow valves, and start pressurisation, cabin altitude should drop.

Level off, monitor cabin altitude. If it starts to drop, nothing on the aircraft will stop your from continuing the flight.

 

In any case, tea without biscuits it is.

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Depending on how quick your reaction have been, there's a good chance oxygen masks didn't even drop and cabin pressure is still in survivable area. Anyway, I would declare emergency and land as soon as possible because of possible health problems of some passengers.

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Hi. Thanks for the replies, and the table LJ.

 

There wouldn't really be a point in sounding an alarm after everybody died, would there? ;-)

 

Exactly Thomas, my next question would have been "Why?"

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Depending on how quick your reaction have been, there's a good chance oxygen masks didn't even drop and cabin pressure is still in survivable area. Anyway, I would declare emergency and land as soon as possible because of possible health problems of some passengers.

No need to over react as symptoms of hypoxia are fully reversible if caught quickly enough. I would don my oxygen mask, descend to below 14000 feet, wait for the cabin to pressurise then remove the mask and continue. No harm done. If the cabin does not pressurise or if the passenger oxygen masks deployed then I would land immediately. However I wouldn't declare an emergency because it isn't one.

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No need to over react as symptoms of hypoxia are fully reversible if caught quickly enough. I would don my oxygen mask, descend to below 14000 feet, wait for the cabin to pressurise then remove the mask and continue. No harm done. If the cabin does not pressurise or if the passenger oxygen masks deployed then I would land immediately. However I wouldn't declare an emergency because it isn't one.

For average healthy man, that's true, but there are many people with different kind of health problems that wouldn't feel so good after hypoxia. Maybe 299 passengers are ok but 1 is feeling difficulties - enough for medical emergency.

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And  the animals  that are on board as well  don't forget  them :)

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Might I interest you in some Air Crash Investigation regarding a very similar incident.

 

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For average healthy man, that's true, but there are many people with different kind of health problems that wouldn't feel so good after hypoxia. Maybe 299 passengers are ok but 1 is feeling difficulties - enough for medical emergency.

10000 ft isn't remotely dangerous. If it was the warning level would be set lower.

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10000 ft isn't remotely dangerous. If it was the warning level would be set lower.

Agree, I flied unpressurized aircraft at fl120 for 2 hours - I'm physically in very good condition, and my O2 saturation is very good - felt very light symptoms of hypoxia. As I said, it depends how fast his reaction has been.

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Agree, I flied unpressurized aircraft at fl120 for 2 hours - I'm physically in very good condition, and my O2 saturation is very good - felt very light symptoms of hypoxia. As I said, it depends how fast his reaction has been.

Don't forget, in the days before pressurisation, 10,000 ft cabin altitudes were common and you didn't need to be 100% fit to fly as a passenger by any means.

 

The thing is, you wouldn't declare an emergency as a precaution in case one or two passengers might be affected. Clearly if someone was seen to be suffering problems that would be different.

 

The 737 QRH says if you can reestablish Auto pressurisation control then you can continue normally. However if the rubber jungle has deployed then that's not really an option. :wink: Apart from anything else that would show cabin altitude exceeded 14000 ft in which case there might be more chance of a medical problem for some pax.

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The 737 QRH says if you can reestablish Auto pressurisation control then you can continue normally. However if the rubber jungle has deployed then that's not really an option. :wink: Apart from anything else that would show cabin altitude exceeded 14000 ft in which case there might be more chance of a medical problem for some pax.

 

Not to mention the panicing passengers :P And all that unsightly tubing hanging out of the ceiling.

And of course, now that it has already deployed, the emergency back up oxygen has already 'gone off' and if you were to lose pressurization again, then the passengers would be SOL.

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And of course, now that it has already deployed, the emergency back up oxygen has already 'gone off' and if you were to lose pressurization again, then the passengers would be SOL.

 

To be more precise... The oxygen generators for passengers don't go off until someone actually pulls on the mask cord. There is one generator for every 3 masks or so (can't remember exactly), and pulling on any of those starts the generator.

 

Of course, these are just details. If the masks were to drop even on the runway (for argument's sake) I'm pretty sure all the passengers would pull on them.

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I would also suggest the use of a check list when you fly to avoid situations like this.

 

Dave

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Also keep in mind that under FAR Part 91 in the US pilots are allowed to operate up to 12,500 with no supplemental oxygen and from 12,500 to 14,000 for 30 minutes with no supplemental oxygen for the pilots so at the 10,000 foot cabin altitude when the alarm goes off there shouldn't be a problem. 

 

Something to keep in mind in the OPs situation is that you should reinstate bleeds slowly. We had a flight where they forgot to turn on the bleeds after engine start and ended up getting a cabin altitude warning. They had done the memory item which included fully closing the outflow valve. When the FO discovered the bleeds were off he immediately turned them both on. The pressure increase was so significant some people had ruptured ear drums. 

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Something to keep in mind in the OPs situation is that you should reinstate bleeds slowly. We had a flight where they forgot to turn on the bleeds after engine start and ended up getting a cabin altitude warning. They had done the memory item which included fully closing the outflow valve. When the FO discovered the bleeds were off he immediately turned them both on. The pressure increase was so significant some people had ruptured ear drums. 

 

IIRC if you leave the control in AUTO it should maintain normal-ish rates.

 

 

 


To be more precise... The oxygen generators for passengers don't go off until someone actually pulls on the mask cord. There is one generator for every 3 masks or so (can't remember exactly), and pulling on any of those starts the generator.

 

Any moment a single mask is out, you should consider emergency oxygen to be depleted. I could see how you could block maybe one or two rows off in case of malfunction of the latches or some such, but at the same moment the jungle goes off, for all intents and purposes, you are flying without reserve oxygen (pax) and you may not continue. If for no other reason, then there would be no indication for passengers to pull on the cords should a real loss of pressure event occur.

Not even considering the fact that you are virtually guaranteed that at least one person per row will pull the cord the instant they see a mask in front of them.

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I'm with those noting the air pressure up around 10,000 being just fine.  I've been up higher than 10,000 with no oxygen and was just fine.  This was both while flying, and various ski trips (Monte Cristallo's lodge is pretty close to the peak of 10,500ish, and to be honest, most of us were pretty loaded when we were up there, which as any good pilot should know, only enhances the effects of hypoxia).  I've been a little higher, unpressurized in a plane, too, for about 5 hours (CKM-BCB in a 207...woooooo!).  No issues there.

 

The FAA makes note that you start feeling the effects around 6,500, and occasional medical references speak to 8,000 and above for the possibility for altitude sickness.  It's a graduated thing.  It's not like you're 100% fine below some altitude, and as soon as you cross it *BANG* you're 100% unconscious.  That's the danger of it all, really.  You could be up at FL180, unpressurized (though not legally), and be "fine" for a while, and slowly fade out over time, not even realizing it because your body doesn't have enough oxygen to recognize the issue.

 

Interesting video of a hypoxia situation:

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I've been up higher than 10,000 with no oxygen and was just fine.  This was both while flying, and various ski trips (Monte Cristallo's lodge is pretty close to the peak of 10,500ish

Oh, that's nothing. I've driven up to 14,110 feet and made it just fine. :P  ^_^  :mellow:

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IIRC if you leave the control in AUTO it should maintain normal-ish rates.

True. In the case I mentioned the pressurization would have been in manual if they did the memory item correctly. That being said when we do bleeds off takeoffs there is a noticeable pressurization bump when reinstating the engine bleeds one at a time so even in auto turning both bleeds on simultaneously would likely be uncomfortable. 

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True. In the case I mentioned the pressurization would have been in manual if they did the memory item correctly. That being said when we do bleeds off takeoffs there is a noticeable pressurization bump when reinstating the engine bleeds one at a time so even in auto turning both bleeds on simultaneously would likely be uncomfortable. 

 

Even auto can only open the valve so fast :) but it should be better than to blast the air in with closed valve.

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