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737-800 Flight crew O2 mask

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737-800 Flight crew O2 mask

Old 7th Oct 2014, 17:46
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737-800 Flight crew O2 mask

Evening

Was wondering why the Flight Crew O2 mask on the NG has a sticker with Max Cabin Altitude 40,000' on it, when the certified ceiling is 41,000'?
My colleague today said there was a similar thing on the 767, but couldn't remember why!

Thanks
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Old 7th Oct 2014, 18:37
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Agreed

However if flying along fat, dumb and happy at 41,000 and suffer a sudden depressurisation, useful consciousness circa 10 secs, would happily be eaten up by memory items and "oh sh#t factor". Start emergency descent -with structural uncertainty, thus not increasing speed. It would be handy to have faith in a system that could work at an altitude of greater than 40,000?!
As I say, I believe there is a logical reason-there has to be.
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Old 7th Oct 2014, 20:35
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I think most pilots would do memory items during the descent, not initiate descent after them. The survival instinct would have you well below 40,000 ft before memory items were complete. Unfortunately sim instructors may not teach this.
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Old 7th Oct 2014, 20:57
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As bubbers says, if you hang around doing memory items @ 41000' you shortly will not have any memory left.

I believe the 40000' is for sustained survival. I recall that over 40000' you would need pressure breathing.
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Old 8th Oct 2014, 08:03
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BOAC- but the first memory item for Cabin Altitude warning is put your mask on! A mask it seems doesn't work at 41,000'?!

Are you guys saying you would just descend, then worry about memory items? That's scares me
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Old 8th Oct 2014, 08:17
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I think most of us, with a big bang and the roof gone on the cabriolet, would be starting down pdq and stuffing the mask on on the way. Don't forget we are talking '2 crew' here. Not a lot of point in the other memory items either, is there - eg motor the outlfow closed etc. If you are really worried about this "40000ft" thing, think how long it is going to take you to be 'safe' below 40?

With a slow rise in cabin altitude etc etc you could be a little more relaxed, I feel?

Conversely, the thought of you sitting there level at 41000, putting on the mask, establishing crew comms, selecting manual, motoring the valve while looking at the stars through the roof...................... 'horses for courses', as they say?
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Old 8th Oct 2014, 09:37
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BOAC: The memory is fading somewhat but is it not the case that crew O2 systems give "pressure breathing" mode when the ambient cabin altitude is greater than 30,000ft?

An event which renders an instantaneous increase in cabin altitude from 8,000ft. to 40,000ft. would have to be quite catastrophic. For the other 99% of occasions surely a few seconds spent correctly establishing the crew on O2 and prolonging their time of useful consciousness would be time well spent?
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Old 8th Oct 2014, 09:44
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Not strictly 'pressure breathing' which is quite another event, but yes, there is an over-pressure provided at altitude to improve the oxy content.
For the other 99% of occasions surely a few seconds spent correctly establishing the crew on O2 and prolonging their time of useful consciousness would be time well spent?
- did I not say that?

We were discussing
and suffer a sudden depressurisation, useful consciousness circa 10 secs,
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Old 8th Oct 2014, 09:49
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I suspect its because its not authorised for sustained operation above 40K. Some wooly mindedness about talking to ATC etc in the event of a pressurisation event. Forget that just get going down, then 7700. Av-Nav-Com

PS don't forget that the mask and goggles will be covered in sh!te and dirt so you'll probably choke. No one checks them or cleans them
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Old 8th Oct 2014, 11:19
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BOAC: Yes, but you also suggested that "most people would start down and be putting their masks on on the way down". That was never a SOP with any of the five airlines I worked for.
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Old 8th Oct 2014, 12:16
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We did not say don oxygen on the way down. Survival mode as I mentioned would require immediate use of quick donning oxygen masks, the rest can be accomplished once descent is initiated. Passengers are along for the ride too and they have zero training so some will not get their masks on for what ever good they do at 41,000 ft.
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Old 8th Oct 2014, 12:21
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If your cabin altitude is 40,000 the masks are the least of your worries.
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Old 8th Oct 2014, 13:22
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- indeed, just think of the smell
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Old 9th Oct 2014, 01:16
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I have maybe the dumbest of dumbest of questions.... At 40.000ft you'll loose consciousness after ca. 10 seconds....

But what if you're holding your breath? Will your lungs explode, like an Alien-movie, or something like that
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Old 9th Oct 2014, 05:32
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I think BOAC is right here. And certification of the mask has nothing to do with your SOP's.
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Old 9th Oct 2014, 07:19
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But what if you're holding your breath
- if you ever experience a 'rapid' decompression from, say 8000' to 41000' you will find it very difficult to 'hold your breath'. Quite a lot of what is inside you gets outside, hence the 'smell' particularly after a night on Ruddles at RAF North Luffenham - ONE of the reasons the attending medics wear masks
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Old 9th Oct 2014, 08:19
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You know just for info, a former colleague of mine just got an A350 type rating, and IIRC what he said, the latest gizmo is a 'one-touch' button that automatically configures for and performs the optimum emergency descent.

Ain't FBW a wonderful thing?
Does it consider structural integrity before increasing speed to Vmo/Mmo?
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Old 9th Oct 2014, 14:11
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Substitute 'hope'?
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Old 9th Oct 2014, 19:00
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3 items of recall actions in your NG @ 41,000ft:

1) Stay alive, by swiftly and correctly donning O2 mask.

2) Ensure your colleague(s) done the same, by establishing comms with he/she/it/them.

3) FLY the aircraft to a safer mammal-friendly environment.

Why all the discussion?
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Old 10th Oct 2014, 07:03
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I don't know about the A350 emergency descent “button”, but some newer bizjets will descend automatically if an unsafe cabin altitude is detected. Autothrottle will engage immediately, idle, 90 degree turn, lvl off at 15000 feet, cruise at 250 kt, without pilot intervention. No button to push. This has been around a few years, in different flavors (some still require pilots to handle throttles or engage a/t, and they all engage only above 30000 feet methinks). if I recall correctly these systems were required by some countries to certify bizjets for up to 51000 feet. There is some sense to this, obviously the same size hole in an a380 at somewhat theoretical 41000 feet would leave a Gulfstream crew above a lot less time to react (and I actually see them up there, overpowered, with large wings, looking for a direct route)

... so even if potential risks may outweigh benefits in a large transport a/c, Im not surprised if avionics manufacturers, the usual suspects like Honeywell and Collins, who already have these tricks up their sleeve will tout them to Mr A, Mr B, and to airlines. Fully Automatic tcas ra execution is next...no buttons to push. “what's it doing now?”. Sorry for the massive thread drift...

Last edited by deptrai; 10th Oct 2014 at 08:39.
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