Maintaining cruise altitude while depressurised
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There is no way that the partial pressure of 100% oxygen is enough at jet cruise levels without pressure breathing. Any jet company mandating the retention of high cruise levels following a decompression of a should be shut down; they clearly have no idea of what they're doing. Turbo-prop cruise levels on the other hand would generally be viable from a pilots' hypoxia stance.
With Mt. Everest at approx. 29,100', safe to come down to FL310-320 anywhere in the world, I assume it's a B787/777 discussed here, in which the crew oxy masks would need to be certified up to FL430 (777).... a pity for those poor pax making their way to the washroom when disaster struck, sure they wouldn't be feeling too well after lying in the aisle for 4 minutes without a mask...
There is the added complication of congested airspace in regions such as western Europe and the northeast US - immediate plummeting might set off a chain reaction of TCAS alerts in the area and - worse case scenario - a mid air collision.
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.... a pity for those poor pax making their way to the washroom when disaster struck, sure they wouldn't be feeling too well after lying in the aisle for 4 minutes without a mask...
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Breathing O2 using the passenger masks or "walk around" bottles would have no effect at cabin altitudes above 30,000 ft. JMHO
On the NG 100% oxygen under positive pressure is supplied when cabin altitude is above a preset value. I think it's around FL 290. Why would this happen if positive pressure is only used to remove smoke/vapors?
Breathing O2 using the passenger masks or "walk around" bottles would have no effect at cabin altitudes above 30,000 ft. JMHO
As for the 777 flight crew masks - must admit I always thought it was odd myself that the placard was lower than the aircraft's ceiling .....
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It has something to do with the partial pressure of O2 vs CO2 on either side of the lung's aveoli. Better off left to a physiologist. Hemoglobin normally binds oxygen at high partial pressures of oxygen.
Pressure
https://www.khanacademy.org/science/...rtial-pressure
Pressure
https://www.khanacademy.org/science/...rtial-pressure
Last edited by wanabee777; 12th Mar 2016 at 12:17.
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pressure breathing
Air is a mixture of gasses, one being oxygen. Each component may be said to have a partial pressure in accordance with the percentage of the whole
As air pressure reduces the partial pressure exerted by the oxygen component reduces and at about 10,000 ft the oxygen content becomes marginal to sustain normal activity and so extra oxygen is required, As altitude further increases an increased proportion of oxygen in the overall gas mixture is required to the point that even 100 % oxygen does not reach the level of oxygen partial pressure at 10,00 ft. This is at about 25,000 ft. At this point the need is for an increased pressure of oxygen to sustain activity, so called pressure breathing. The military practice this in special chambers and also practice sudden cabin pressure failures with the associated physiological effects.
The normal flight crew masks will supply 100 % oxygen and will prevent extraneous gas ingress by using a small excess pressure but they are by means "pressure breathing" and cannot sustain activity for long above 25,000 ft
The physiological effects of sudden loss of cabin pressure above 25,000 ft include but are not limited to;Gut gas expansion causing pain and evacuation downwards with part of other gut contents. If there is excessive gut gas the gut may become twisted with associated extreme pain.
Partial blockages of sinuses and middle ears with associated extreme pain and possible eardrum puncture. Pain in teethe fillings
As air pressure reduces the partial pressure exerted by the oxygen component reduces and at about 10,000 ft the oxygen content becomes marginal to sustain normal activity and so extra oxygen is required, As altitude further increases an increased proportion of oxygen in the overall gas mixture is required to the point that even 100 % oxygen does not reach the level of oxygen partial pressure at 10,00 ft. This is at about 25,000 ft. At this point the need is for an increased pressure of oxygen to sustain activity, so called pressure breathing. The military practice this in special chambers and also practice sudden cabin pressure failures with the associated physiological effects.
The normal flight crew masks will supply 100 % oxygen and will prevent extraneous gas ingress by using a small excess pressure but they are by means "pressure breathing" and cannot sustain activity for long above 25,000 ft
The physiological effects of sudden loss of cabin pressure above 25,000 ft include but are not limited to;Gut gas expansion causing pain and evacuation downwards with part of other gut contents. If there is excessive gut gas the gut may become twisted with associated extreme pain.
Partial blockages of sinuses and middle ears with associated extreme pain and possible eardrum puncture. Pain in teethe fillings
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Well, there has been some misinformation in this thread! The relevant parts of FAR25 and AC25-20 require than in the event of a worst case rapid depressurisation, the cabin alt must never exceed 40,000ft and must not be above 25,000ft for more than 2 minutes. At 40,000ft on 100% O2 (not pressure breathing) the level of hypoxia is roughly that of flying unpressurised at 10,000ft without oxygen, i.e. perfectly survivable but you would not want to go much higher. The 25,000ft rule is to cover cases where pax don't get their masks on; more than two minutes above this without supplemental O2 risks permanent neurological injury.
In view of this it seems the airline concerned had poor understanding of both certification requirements and high-altitude physiology. Not good.
In view of this it seems the airline concerned had poor understanding of both certification requirements and high-altitude physiology. Not good.
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The relevant parts of FAR25 and AC25-20 require than in the event of a worst case rapid depressurisation, the cabin alt must never exceed 40,000ft and must not be above 25,000ft for more than 2 minutes.
The max operating altitude of the 777 is 43,100 ft.
Atmospheric PPO vs Alveolar PPO
Another thing working against us, as this paper points out, is that, because the alveolar PP of water vapour and carbon dioxide are relatively constant, the partial pressure of alveolar oxygen decreases faster than the partial pressure of O2 in air.
p.s. I've skied at 12,000 feet and did notice a little shortness of breath but didn't stay there to experiment
p.s. I've skied at 12,000 feet and did notice a little shortness of breath but didn't stay there to experiment
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just a guess - Power point ranger figured it would be ' safer' for plane to fly back along route to avoid possible 'oncoming' traffic at lower altitude, etc. Probably did this while playing with his home pc and flight simulator and did not want a ' head on ' collision ??
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It's been a long time, but if I remember my B-52 boldprint correctly it called for a descent to 42,000 feet or below in the event of pressurization loss, or 34,000 feet if pressure breathing wasn't available. I may be wrong, but 42,000/34,000 sticks in my mind.
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And, even with an O2 mask, extended flight at high cabin altitude can cause deadly decompression sickness.
Back with one of the earlier iterations of Connie Kallita's cargo airline a DC-8 captain made a very bad decision to continue climb to FL330 after a failure to pressurize on takeoff.
From the NTSB narrative:
There were some definite CRM issues with this crew:
https://www.fss.aero/accident-report...report_key=975
Back with one of the earlier iterations of Connie Kallita's cargo airline a DC-8 captain made a very bad decision to continue climb to FL330 after a failure to pressurize on takeoff.
From the NTSB narrative:
Witnesses reported seeing the airplane takeoff and they thought the left forward overwing exit was not in place as the airplane became airborne.
According to flight crew interviews and written statements, the F/E was unable to maintain cabin pressurization after takeoff. The first officer (F/O) and F/E, reported there was no cockpit discussion about staying at a lower altitude until the cause of the lack of pressurization could be determined. The captain made a decision to continue the climb. The flight crew donned their oxygen masks and the flight continued. In the vicinity of FL290 both the F/O and F/E reported the captain was not responding to radio calls, however, the captain indicated via hand signals that he wanted the flight to continue the climb. The flight continued to FL330. While at FL330, the captain's condition continued to deteriorate and the F/O took command of the airplane, and requested a descent.
Once the airplane was level at 8000 feet, the flight crew was queried by air traffic if they wanted to declare an emergency. The flight crew declined to declare an emergency and requested to continue the flight to Atlanta, its planned destination. The flight crew then requested to divert to Charlotte. After being informed of the location of the Greater Cincinnati Regional Airport, the flight crew elected to divert there for landing and medical assistance for the captain. The flight landed without incident and the captain was removed and taken to a local hospital.
According to flight crew interviews and written statements, the F/E was unable to maintain cabin pressurization after takeoff. The first officer (F/O) and F/E, reported there was no cockpit discussion about staying at a lower altitude until the cause of the lack of pressurization could be determined. The captain made a decision to continue the climb. The flight crew donned their oxygen masks and the flight continued. In the vicinity of FL290 both the F/O and F/E reported the captain was not responding to radio calls, however, the captain indicated via hand signals that he wanted the flight to continue the climb. The flight continued to FL330. While at FL330, the captain's condition continued to deteriorate and the F/O took command of the airplane, and requested a descent.
Once the airplane was level at 8000 feet, the flight crew was queried by air traffic if they wanted to declare an emergency. The flight crew declined to declare an emergency and requested to continue the flight to Atlanta, its planned destination. The flight crew then requested to divert to Charlotte. After being informed of the location of the Greater Cincinnati Regional Airport, the flight crew elected to divert there for landing and medical assistance for the captain. The flight landed without incident and the captain was removed and taken to a local hospital.
In a written statement made after the accident, the first officer stated:
...I took command when I felt that it was necessary without creating a feeling of mutiny. Since Captain XXXX is a strong willed person and when he is in command he is not to be questioned, I followed his orders....
In a written statement made after the accident, the flight engineer stated:
...It is my opinion that the only way to get the airplane to a lower altitude would be to override the captain by use of physical force. When the captain requested higher altitudes, the first officer and I strongly protested, both verbally and by hand signals. These protests were repeated at least three times during the climb...All protests were disregarded by the captain....
...I took command when I felt that it was necessary without creating a feeling of mutiny. Since Captain XXXX is a strong willed person and when he is in command he is not to be questioned, I followed his orders....
In a written statement made after the accident, the flight engineer stated:
...It is my opinion that the only way to get the airplane to a lower altitude would be to override the captain by use of physical force. When the captain requested higher altitudes, the first officer and I strongly protested, both verbally and by hand signals. These protests were repeated at least three times during the climb...All protests were disregarded by the captain....
