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B787 O2 supply

Old 7th Nov 2019, 21:10
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Originally Posted by Flight Alloy
No mask can work as a 'pressurized' mask, aka delivering more pressure to the lungs than ambient...
Right....
Google CPAP...
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Old 7th Nov 2019, 21:18
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Originally Posted by Flight Alloy
No mask can work as a 'pressurized' mask, aka delivering more pressure to the lungs than ambient, ..... pressurized breathing is only possible with suits that also pressurize the chest area to the same pressure as the breathed gas
TBH I'm struggling to reconcile that statement with the practical training in pressure breathing that I and I'm sure some others here with an ex-military flying background will have experienced in the past.

It certainly is an interesting sensation, but as long as the differential isn't too excessive a pressure suit is not required, certainly not if you are going to only be pressure breathing for a short period of time.....

Last edited by wiggy; 7th Nov 2019 at 21:45.
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Old 7th Nov 2019, 22:05
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Thanks Wiggy.

Flight Alloy - have you experienced decompressions and pressure breathing in a chamber?


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Old 7th Nov 2019, 23:01
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Flight Alloy, what are you on about. You have obviously never worn a flight crew mask with Emergency set on the dial. PPrune...
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Old 7th Nov 2019, 23:36
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Flight Alloy: It is true that limited pressure breathing is possible, but this is limited to just the slight muscular 'overcapacity' the diaphragm muscles are capable of, in the range of fractions of an atmosphere.

Capn Bloggs: Flight Alloy, what are you on about. You have obviously never worn a flight crew mask with Emergency set on the dial.

Trying to get a ball-park figure I found a 2003 Lancet article on the "Physiological hazards of flight at high altitude"
https://www.thelancet.com/journals/l...059-3/fulltext
"Which mentioned "positive pressure breathing oxygen masks that deliver up to 70 mm Hg", that's just under 10% of an atmosphere (760mm Hg).

Any idea what pressure modern flight crew masks actually deliver?
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Old 8th Nov 2019, 00:19
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Originally Posted by fab777
At VMO, gear extended and in a side slip, maybe. So definitely not realistic in the real life. 4000 is a correct figure.
I've demonstrated a 6500 FPM descent in a light A-319. You need to disconnect the autopilot, hand fly to get more flight spoiler extension. But it went down in a hurry. LA Centre had asked us to "expedite descent". My F/O looked a little concerned..
But I realize this thread is about the 787. I only flew the 767 plus the Airbus.
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Old 8th Nov 2019, 01:13
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Originally Posted by Peter
Any idea what pressure modern flight crew masks actually deliver?
Not numerically. I do know that when our masks are on Emergency, there is significant overpressure, to the point where it is difficult to talk normally. The Emerg setting is for smoke/fumes use, not for a "normal" depressurisation.
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Old 8th Nov 2019, 03:49
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Originally Posted by Retired DC9 driver
I've demonstrated a 6500 FPM descent in a light A-319. You need to disconnect the autopilot, hand fly to get more flight spoiler extension. But it went down in a hurry. LA Centre had asked us to "expedite descent". My F/O looked a little concerned..
But I realize this thread is about the 787. I only flew the 767 plus the Airbus.
The A319 (or A321) doesn't have any extra spoilers with AP OFF, only A320. Maybe your FO was concerned you didn't know that.
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Old 8th Nov 2019, 04:04
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Originally Posted by Peter H
Flight Alloy: It is true that limited pressure breathing is possible, but this is limited to just the slight muscular 'overcapacity' the diaphragm muscles are capable of, in the range of fractions of an atmosphere.

Capn Bloggs: Flight Alloy, what are you on about. You have obviously never worn a flight crew mask with Emergency set on the dial.

Trying to get a ball-park figure I found a 2003 Lancet article on the "Physiological hazards of flight at high altitude"
https://www.thelancet.com/journals/l...059-3/fulltext
"Which mentioned "positive pressure breathing oxygen masks that deliver up to 70 mm Hg", that's just under 10% of an atmosphere (760mm Hg).

Any idea what pressure modern flight crew masks actually deliver?
Peter, thanks for putting some numbers to our claims, it makes determining fact from fiction objective as it should be. Some documentation I found, Hypoxia an Invisible 'Enemy Cabin depressurization effects on human physiology' from 2006, states this:
Another additional means for hypoxia protection is positive pressure breathing, which is usually found in modern crew oxygen masks and means the delivery of pure oxygen under pressure into the respiratory tract. For civil applications positive pressure breathing is able to increase additionally the oxygen partial pressure by around 20 to 30mbar provided that the overpressure condition is limited to some minutes only.
At sea level, oxygen has a partial pressure of about 200 mbar, at 40,000 ft, ambient pressure is reduced to around 180mbar, so with 100% O2 setting providing 180mbar of oxygen and a further 30 mbar of pressure breathing, one is back up to 210mbar of breathing oxygen, about the sea as at sea level. In this configuration, the pressure breathing element only provides 14% of the total oxygen delivered.
As one can plainly see, the overpressure provided is minimal, compared to sea level, the overpressure is 3% of atmospheric, at altitude, it constitutes only 15% over ambient.
So as I had stated before, yes, limited pressure breathing is possible, but it only constitutes an overpressure of 3% of an MSL atmosphere, aka a very minimal overpressure fraction, again due to the fact that our lung muscles have very limited overcapacity to process that pressure and not suffer any rupture injuries. I am sure it may 'feel' significant to the pilots who have experienced it, but numbers show that the overpressure component is a minimal, almost negligible component of the breathing system at altitude in all aircraft without the provision of pressure suits.
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Old 8th Nov 2019, 04:18
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Originally Posted by The Bartender
Right....
Google CPAP...
Thanks for the tip.The medical journal Thorax,
gives a CPAP average treatment pressure of 7.8mbar above atmospheric, which comes out to around 0.8% above ambient pressure. I wouldn't exactly call that 'overpressure breathing', a stiff wind should cause higher pressure differential. Before doing the whole LMGTFY game, perhaps you would consider informing yourself a little bit too before spouting unfounded statements?
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Old 8th Nov 2019, 05:02
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Gentlemen, I regret you are mixing many different issues and talking physiological nonsense. CPAP has nothing to do with oxyenation in OSA but may improve arterial oxygenation in the abnormal lung. Positive pressures cant simply be added to partial pressures and the phrase
lung muscles have very limited overcapacity to process that pressure
is just a mismash of words.

Bottom line is that sudden depressurisation at 35 to 40,000 feet will kill at least some without oxygen regardless of how fast you can expedite descent. A non venturi non pressurised mask providing 100% oxygen will prevent cerebral apoptosis. A pressurised mask will prevent fumes being inhaled and a demand valve pressurised system can allow normal function

I remain staggered that everyone isnt up in arms that passengers are in effect being put at risk in a seat lottery over a safety provision that has been mandatory for decades

I will now withdraw to prevent my repetition
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Old 8th Nov 2019, 05:13
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Originally Posted by Capn Bloggs
Not numerically. I do know that when our masks are on Emergency, there is significant overpressure, to the point where it is difficult to talk normally. The Emerg setting is for smoke/fumes use, not for a "normal" depressurisation.
Looking at 14.5 psi at 0 vs 3-5 psia at altitude. To make a significant amount of change is 3-8 psia. A mask will cover a triangle 6 inches tall and 6 inches wide. The area of that is 18 square inches, so you would need between 54 pounds and 144 pounds of strap tension to hold the mask on the person's face; not including the preload to keep from leaking out past the rubbery flesh of the face.

I expect the pressure delivered is on the order of .1 to .4 psig, which is a slight positive pressure that works because the actual need is to provide a partial pressure of oxygen equivalent to 10,000 ft and the 3-5 psi at altitude on pure oxygen accomplishes that; there's also a need to exclude smoke/toxic gases. Yes, even that low gauge pressure makes it tough to talk. Considering that lungs have a section area close to 100 square inches, even a 0.1 psi load is like having a 10 pound weight on your chest to resist.

From the scuba people:

"The important number isn't the total pressure in the lung but rather the difference in perssure between inside the lung and outside the lung. If the pressure in the lung is 95-110 cm H2O higher than the pressure outside the lung, the lung can rupture.

(100cmH2O= 0.097ATA= 0.098bar= 1.42PSI= 73.55mmHg approx.)"
Search scubaboard with the above text for more, sometimes grisly, details. Mind that this is the rupture pressure; it will be very uncomfortable before reaching this level.
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Old 8th Nov 2019, 08:05
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Cabin O2 masks are certified up to 25.000 ft.
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Old 8th Nov 2019, 08:46
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Originally Posted by Radgirl
Bottom line is that sudden depressurisation at 35 to 40,000 feet will kill at least some without oxygen regardless of how fast you can expedite descent.
By "sudden", do you mean near-instantaneous ?

That would imply some kind of catastrophic structural failure where lack of oxygen might well be the least of your problems.

In a more likely scenario, does anyone have ballpark figures for the likely cabin pressure decay vs time resulting from, say, a window blowout, assuming that all the packs are still running and (by then) the outflow valve has closed ?
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Old 8th Nov 2019, 09:15
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MechEngr

..so you would need between 54 pounds and 144 pounds of strap tension to hold the mask on the person's face; not including the preload to keep from leaking out past the rubbery flesh of the face.
Just to try and tie up any remaining scepticism over pressure breathing and mask pressures from last night.. I've had time to check logbooks and we used to train under medical supervision for pressure breathing at 45K, using mil kit, individually fitted ( FWIW bone dome, attached mask with a rigid outer shell but no pressure jerkins or similar).

However that was then, this is now and these days I certainly wouldn't want to risk the same using a generic quick don one size "fits all" ( or doesn't ) Eros type mask as found in many flight decks..

Bottom line (for me) is that whilst you can protect yourself from a decompression quickly and reasonably adequately on the flight deck anybody in the cabin is very exposed, as Radgirl has pointed out..

Last edited by wiggy; 8th Nov 2019 at 11:18.
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Old 8th Nov 2019, 09:43
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Originally Posted by DaveReidUK

In a more likely scenario, does anyone have ballpark figures for the likely cabin pressure decay vs time resulting from, say, a window blowout, assuming that all the packs are still running and (by then) the outflow valve has closed ?
First of all, I do not have an answer to your question. But I'd say it is very dependant on the aircraft type and the damaged that occurs. Pressure decay is also dependant on the volume of the aircraft vs the size of the hole on it. Then you'll add the air conditioning, outflow valve position etc to the equation. I would imagine that situation gets very bad much quicker in a F16 than in an A380.

[QUOTE=Radgirl;10613667]
MaŽam, I am very hesitant to say that there would be fatalities due to hypoxia even when starting the emergency descent from high levels. If we stick to "high level", the National Airlines 27 encountered an uncontained engine failure at 39000ft with hull breach and one passenger sucked out. All but that one survived. With the Aloha 243 incident there isn't much to debate: the pressure was gone that instant (at 24000ft), yet all but one survived. With the Southwest 1380 the depressurization at 32000ft was described as "rapid", yet again all but one survived. The fatalities obviously weren't due to hypoxia.

In all these cases the cause of death was anything but hypoxia. I would say that there are far more serious consequences with hull breach than risk of hypoxia, if the plane is brought down below 10000ft as soon as possible.
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Old 8th Nov 2019, 09:53
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Originally Posted by Flight Alloy
Thanks for the tip.The medical journal Thorax,

gives a CPAP average treatment pressure of 7.8mbar above atmospheric, which comes out to around 0.8% above ambient pressure. I wouldn't exactly call that 'overpressure breathing', a stiff wind should cause higher pressure differential. Before doing the whole LMGTFY game, perhaps you would consider informing yourself a little bit too before spouting unfounded statements?
My CPAP machine can supply at up to 20 cmH2O. Not sure how that translates but when I first started using it, it woke me in the middle of the night with the feeling of drowning in air. A very disconcerting feeling I can assure you. Obviously this was at normal atmospheric level, not FL45 or so.
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Old 8th Nov 2019, 10:45
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Originally Posted by Retired DC9 driver
I've demonstrated a 6500 FPM descent in a light A-319. You need to disconnect the autopilot, hand fly to get more flight spoiler extension. But it went down in a hurry. LA Centre had asked us to "expedite descent". My F/O looked a little concerned..
But I realize this thread is about the 787. I only flew the 767 plus the Airbus.
The speed brakes on the 787 are phenomenally powerful, and using full speed brake coupled with a loss of cabin pressure triggers a further extension. I’ve seen upwards of 10,000fpm in the sim, and easily 4000fpm with the speed held back at about 250kts.
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Old 8th Nov 2019, 10:45
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Originally Posted by bucoops
My CPAP machine can supply at up to 20 cmH2O. Not sure how that translates but when I first started using it, it woke me in the middle of the night with the feeling of drowning in air. A very disconcerting feeling I can assure you. Obviously this was at normal atmospheric level, not FL45 or so.
That comes out to about 19mbar, a fairly high setting from what I have read, but still only amounts to 2% over ambient pressure. Just goes to show how 'sensitive' ones lungs are and how accustomed we are to 'normal' breathing.
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Old 8th Nov 2019, 11:45
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In the 744 sim both outflow valves open and with all packs running results in a 4000fpm cabin ROC. One outflow valve is about the size of four windows... I would infer from this that the packs can probably keep pressurisation at level with two or three busted windows, hence no explosive decompression would ensue from that. Peel off some skin or blow out a cargo door of course and that's a different story...
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