Maintaining cruise altitude while depressurised
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We now know that the quick don masks in the cockpit are rated up to 40,000 ft as per placard.
What is the rated maximum altitude of the re-breather (bag type) masks in the cabin?
How much time during that 7 min descent down through 25,000 ft will the passengers' and F/As' brains be oxygen deprived to the point that brain damage may be probable?
What is the rated maximum altitude of the re-breather (bag type) masks in the cabin?
How much time during that 7 min descent down through 25,000 ft will the passengers' and F/As' brains be oxygen deprived to the point that brain damage may be probable?
Let's look at the likely event leading to a decompression. In the event of window failure, B747 outflow valve (one) breaking off - yes it does happen - or a fairly big hole (QF30) you would have the worst survivable case IMHO. The cabin alt will NOT immediately be at outside alt, nothing like it. You will meet the cabin on the way down according to reports from those who have "been there and done that". Much greater structural damage and it becomes academic interest only, you're screwed and the aircraft is unflyable. If you have a total failure of the air ducts into the packs somehow, the cabin climbs quite slowly.
In reality you have more than enough time to get your sweep on mask properly fitted and commence descent without rushing things. A turning entry is to be recommended because you really don't want meal or bar carts floating around so about 25* AoB will give a little positive G to keep the carts on the floor, initiate the descent gently as the nose drops - on Boeings anyway - and get you off the airway. Don't forget to turn back parallel around five miles offset. Unless you are ABSOLUTELY certain that there is no structural damage, do not increase speed. It is reported that Boeing stated that had QF30 sped up much it would have been likely catastrophic. Also be sure that the A/P is responding correctly - the electrical wires were broken on QF30.
You will, of course, have loaded the escape procedures into the FMC at the end of your active route and also have a route copy in RTE 2 to allow for the turn back case. Having briefed your crew and yourself about the way to go and being aware of the wind - jet streams? - it is a matter of putting into practice what you have briefed. You needed to be very thorough too with the escape tracks changing frequently in the critical area. Not a time for a crew meal!
In an airline from the Great Southland, we had quite complex escape tracks for the L888 Silk Road route and the tracks over China for the LHR-HKG-LHR legs. The escape procedures for Afghanistan are simple.
L888 was a horrible concept in the worst case deco, over 20 minutes at F20.2, a few descents and still F140 when you got to your diversion port after almost three hours.
In reality you have more than enough time to get your sweep on mask properly fitted and commence descent without rushing things. A turning entry is to be recommended because you really don't want meal or bar carts floating around so about 25* AoB will give a little positive G to keep the carts on the floor, initiate the descent gently as the nose drops - on Boeings anyway - and get you off the airway. Don't forget to turn back parallel around five miles offset. Unless you are ABSOLUTELY certain that there is no structural damage, do not increase speed. It is reported that Boeing stated that had QF30 sped up much it would have been likely catastrophic. Also be sure that the A/P is responding correctly - the electrical wires were broken on QF30.
You will, of course, have loaded the escape procedures into the FMC at the end of your active route and also have a route copy in RTE 2 to allow for the turn back case. Having briefed your crew and yourself about the way to go and being aware of the wind - jet streams? - it is a matter of putting into practice what you have briefed. You needed to be very thorough too with the escape tracks changing frequently in the critical area. Not a time for a crew meal!
In an airline from the Great Southland, we had quite complex escape tracks for the L888 Silk Road route and the tracks over China for the LHR-HKG-LHR legs. The escape procedures for Afghanistan are simple.
L888 was a horrible concept in the worst case deco, over 20 minutes at F20.2, a few descents and still F140 when you got to your diversion port after almost three hours.
wanabee777, the pax masks are certified for 25000 ft. I guess it is a level where sustained reasonable oxygen supply by the mask is assured.
A slow emergency descend would probably have passengers pass out during the descend (hopefully after donning the mask). They would then wake up again at lower levels, and the oxygen provided would suffice to prevent death or brain damage.
A slow emergency descend would probably have passengers pass out during the descend (hopefully after donning the mask). They would then wake up again at lower levels, and the oxygen provided would suffice to prevent death or brain damage.
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Originally Posted by safelife
A slow emergency descend would probably have passengers pass out during the descend (hopefully after donning the mask). They would then wake up again at lower levels, and the oxygen provided would suffice to prevent death or brain damage.
So, in regard to this thread's title, how long can an airliner maintain cruise altitude (in this case FL410) while depressurized?
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The airplane can maintain FL410 until it runs out of fuel.
The limitation is the people on board. With a pressure-breathing oxygen mask, a typical pilot will likely remain conscious for 15-20 minutes or more. However, if said pilot has never experienced a rapid depressurization, and doesn't know what to expect, he may well hyperventilate and/or panic. The pressure changes on the body are NOT trivial, and each person reacts differently, both physically and psychologically.
OTOH, with only the dixie-cup O2 masks in the back, any passengers and cabin crew would be unconscious in a minute or less, and dead within a few minutes. There is not enough ambient oxygen above 35,000' to sustain life.
The limitation is the people on board. With a pressure-breathing oxygen mask, a typical pilot will likely remain conscious for 15-20 minutes or more. However, if said pilot has never experienced a rapid depressurization, and doesn't know what to expect, he may well hyperventilate and/or panic. The pressure changes on the body are NOT trivial, and each person reacts differently, both physically and psychologically.
OTOH, with only the dixie-cup O2 masks in the back, any passengers and cabin crew would be unconscious in a minute or less, and dead within a few minutes. There is not enough ambient oxygen above 35,000' to sustain life.
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As mentioned earlier it is the partial pressure of oxygen that matters.
At the surface we breathe air at roughly 1 bar ambient pressure, so 21% oxygen multiplied by 1 bar gives a partial pressure of 0.21 bar.
At a pressure altitude of 38,662 feet the ambient pressure is 0.2 bar, so 21% oxygen multiplied by 0.2 gives a partial pressure of just 0.04 bar.
Most of us would be incapable of thought or meaningful action, and most likely we would be unconscious with a partial pressure of <0.1, so no wonder the TUC is measured is seconds with a partial pressure of 0.04.
Breathing 100% oxygen at ambient pressure at 38,662ft gives a partial pressure of 0.2 bar, almost the same as at sea level - you really don't need pressurised breathing systems at this height.
By 44,647ft the same 100% oxygen at ambient pressure delivers a partial pressure of 0.15 bar, about the same as flying an unpressurised aircraft at 10,000ft without oxygen - probably not ideal for dealing with anything that needs a high degree of concentration, decision making, or fine motor skills.
There is a big difference in performance between the oxygen masks supplied to pilots versus the passengers. In medical use, rubber masks with a reservoir bag only deliver oxygen up to about the mid 80% mark. These masks are nearly identical to passenger masks in how they work. Pilot masks are a better fit and deliver oxygen via a high flow regulator and not a reservoir bag so should deliver much closer to 100%.
So, at 38,662 feet the passenger is only getting 0.17 bar partial pressure which may not be enough for anyone with pre existing health problems. The idea that even a small amount of oxygen will help stave of brain damage even if unconscious only holds true if you have an open airway to actually breathe through, and I suspect that strapping unconscious people upright in seats is a great way to ensure that their heads tip forward, closing the airway.
At the surface we breathe air at roughly 1 bar ambient pressure, so 21% oxygen multiplied by 1 bar gives a partial pressure of 0.21 bar.
At a pressure altitude of 38,662 feet the ambient pressure is 0.2 bar, so 21% oxygen multiplied by 0.2 gives a partial pressure of just 0.04 bar.
Most of us would be incapable of thought or meaningful action, and most likely we would be unconscious with a partial pressure of <0.1, so no wonder the TUC is measured is seconds with a partial pressure of 0.04.
Breathing 100% oxygen at ambient pressure at 38,662ft gives a partial pressure of 0.2 bar, almost the same as at sea level - you really don't need pressurised breathing systems at this height.
By 44,647ft the same 100% oxygen at ambient pressure delivers a partial pressure of 0.15 bar, about the same as flying an unpressurised aircraft at 10,000ft without oxygen - probably not ideal for dealing with anything that needs a high degree of concentration, decision making, or fine motor skills.
There is a big difference in performance between the oxygen masks supplied to pilots versus the passengers. In medical use, rubber masks with a reservoir bag only deliver oxygen up to about the mid 80% mark. These masks are nearly identical to passenger masks in how they work. Pilot masks are a better fit and deliver oxygen via a high flow regulator and not a reservoir bag so should deliver much closer to 100%.
So, at 38,662 feet the passenger is only getting 0.17 bar partial pressure which may not be enough for anyone with pre existing health problems. The idea that even a small amount of oxygen will help stave of brain damage even if unconscious only holds true if you have an open airway to actually breathe through, and I suspect that strapping unconscious people upright in seats is a great way to ensure that their heads tip forward, closing the airway.
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Originally Posted by Intruder
The airplane can maintain FL410 until it runs out of fuel.
The limitation is the people on board. With a pressure-breathing oxygen mask, a typical pilot will likely remain conscious for 15-20 minutes or more.
The limitation is the people on board. With a pressure-breathing oxygen mask, a typical pilot will likely remain conscious for 15-20 minutes or more.
http://www.boldmethod.com/learn-to-f...xygen-systems/
Last edited by wanabee777; 14th Mar 2016 at 18:00.
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From a breathing point of view the pilot will remain conscious using any of the above systems provided they deliver a gas mix with a sufficient partial pressure of oxygen. The quantity of oxygen available should be the only time limiting factor if using the right emergency oxygen system within it's operational limits.
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So, if what you say is correct, the pilots should be able to use a "re-breather" (bag) type mask indefinitely at 40,000 ft.
No offense, but somehow, I don't believe it.
No offense, but somehow, I don't believe it.
Last edited by wanabee777; 16th Mar 2016 at 11:41.
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I find this whole thread a bit confusing unless I missed something.
As I recall, whether the oxygen source is gaseous or chemical, the cabin masks will deploy either automatically above a given cabin altitude or by pilot command. When they deploy the passengers who pull on the mask will initiate the oxygen supply, whatever the aircraft external altitude. The oxygen will then be available until it runs out. The intent is to provide oxygen until a safe altitude can be reached which I think is 14000ft but maybe 10000ft.
As I recall, whether the oxygen source is gaseous or chemical, the cabin masks will deploy either automatically above a given cabin altitude or by pilot command. When they deploy the passengers who pull on the mask will initiate the oxygen supply, whatever the aircraft external altitude. The oxygen will then be available until it runs out. The intent is to provide oxygen until a safe altitude can be reached which I think is 14000ft but maybe 10000ft.
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Wanabee, the chart that you yourself posted shows the useful ceiling of a rebreather type mask is 25,000 feet, so no - a pilot couldn't use a rebreather mask at 41,000 feet.
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25,000 ft is the FAA's regulatory limit.
I'm saying that the re-breather mask is ineffective for anyone above a certain altitude. From what I remember during hypobaric chamber training, that altitude for a healthy, non-smoking, adult is somewhere between 30,000 and 35,000 ft.
Above 35,000 ft, using a re-breather device, like the drop down masks in the cabin, will result in blood O2 saturation levels below 70% with brain damage and death within a short period of time.
I'm saying that the re-breather mask is ineffective for anyone above a certain altitude. From what I remember during hypobaric chamber training, that altitude for a healthy, non-smoking, adult is somewhere between 30,000 and 35,000 ft.
Above 35,000 ft, using a re-breather device, like the drop down masks in the cabin, will result in blood O2 saturation levels below 70% with brain damage and death within a short period of time.
Last edited by wanabee777; 14th Mar 2016 at 20:39.
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Let's look at the likely event leading to a decompression. In the event of window failure...
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I agree, have done all along, just tried to show the physiology and why some mask types work and some don't as altitude increases.
I think that we are both saying the same thing.
I think that we are both saying the same thing.
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My latest comment related to Lonewolf_50 not believing the opening statement of this thread.
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Sod's law would dictate that your pressurisation failure hit you as you were over the New York/ Boston area - 30 degrees left or right ain't gonna help! As you drop your descent will likely trigger TCAS TAs and RAs so now there will be several other aircraft roller-coastering up and down, doubtless triggering further TCAS advisories in yet more aircraft. Would the TCAS systems in the various aircraft be able to coordinate their advisories with multiple conflicts?
Would it not be better to try to get a clearance before you plummet? Or at least announce what you are about to do and wait a few seconds for ATC to acknowledge? Mild hypoxia amongst a few pax is surely a price worth paying to avoid a you-know-what?
Would it not be better to try to get a clearance before you plummet? Or at least announce what you are about to do and wait a few seconds for ATC to acknowledge? Mild hypoxia amongst a few pax is surely a price worth paying to avoid a you-know-what?
Ideally, when you have time, you also add to that squawk a Mayday or Pan call telling your current controller what's happening. You are not requesting a clearance for descent, you are telling the controllers what you are descending and what you will be doing next. I would expect that any avoidance/deconfliction that a controller would give you would be a heading change you would not be expected to maintain level.
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If the theory is that a big enough hole to depressurize the plane instantly will cause a crash and render the oxygen situation moot, then what if Aloha Airlines Flight 243 happened at a higher altitude?
Just my 2 cents as an ATCO:
The Squawk is far, far more important than the RT call in an emergency descent depending on airspace. We use filters to hide aircraft that aren't within a few thousand feet of our sector. Sectors are split vertically and laterally. If I'm controlling a sector from FL345 and above for example, I don't want to see turboprops on my screen, so I can filter them out.
I'll give you an example from my airspace. You're departing Europe for North America. You're passing through BAGSO to the NAT tracks At FL380 on a busy day. You need emergency descent. There's my "super" sector, an upper sector below me, Dublin upper, Dublin lower and then Dublin approach. All have various filters in place to hide nuisance flights. Your squawking 77 overrides all those filters and sets off audio alarms in both Shannon and Dublin. Everyone in every sector sees your mode c rolling like a slot machine and starts moving traffic in their sectors out of your way.
Without the squawk it takes an extra few seconds for everyone to call each other and discuss.
If you're coming off the NAT tracks eastbound with us, doing a 180 and descending is the worst bloody thing you can do. IF you can, tell us what you're doing, we'll give essential traffic if you need it, leave you alone if not. But if we see and hear that 77 alarm there's an awful lot of pairs of eyes on you and we'll get everything out of your way pronto
The Squawk is far, far more important than the RT call in an emergency descent depending on airspace. We use filters to hide aircraft that aren't within a few thousand feet of our sector. Sectors are split vertically and laterally. If I'm controlling a sector from FL345 and above for example, I don't want to see turboprops on my screen, so I can filter them out.
I'll give you an example from my airspace. You're departing Europe for North America. You're passing through BAGSO to the NAT tracks At FL380 on a busy day. You need emergency descent. There's my "super" sector, an upper sector below me, Dublin upper, Dublin lower and then Dublin approach. All have various filters in place to hide nuisance flights. Your squawking 77 overrides all those filters and sets off audio alarms in both Shannon and Dublin. Everyone in every sector sees your mode c rolling like a slot machine and starts moving traffic in their sectors out of your way.
Without the squawk it takes an extra few seconds for everyone to call each other and discuss.
If you're coming off the NAT tracks eastbound with us, doing a 180 and descending is the worst bloody thing you can do. IF you can, tell us what you're doing, we'll give essential traffic if you need it, leave you alone if not. But if we see and hear that 77 alarm there's an awful lot of pairs of eyes on you and we'll get everything out of your way pronto
Last edited by Una Due Tfc; 15th Mar 2016 at 10:09.