Crew/Pax Oxy Q's
 

Your in you Boeing/Airbus fitted with a liquid [edit: should read GASEOUS] OXY system at FL300+ then one of the following happens, what would you do?

1. The Crew OXY drops to 0 psi with suitable ERAs.

2. Crew OXY drops to 0 psi with NO suitable ERAs.

3. PAX OXY drops to 0 psi with suitable ERAs.

4. PAX OXY drops to 0 psi with NO suitable ERAs.

I know it's a fairly broad question, I hope this will lead to further discussion.

Regards CCA

CCA,
I'm intrigued by your questions linked to a LIQUID oxygen system. The Boeings and Airbusses I've worked on have GASEOUS flt crew oxygen systems, oxygen generators for the cabin and supplemented by gaseous therapeutic oxygen bottles in the emergency equipment stowages.
Your questions are still up for debate but I'm interested to know if you've come across a civil airliner with LOX?

Liquid oxygen? That'll give you some serious sinus pains :-)

I think, in all fairness to CCA, that he/she made a 'slip of the keyboard', and was referring to GASEOUS Oxygen systems, i.e. bottled Oxygen.

It was always my reasoning, and my course of action on the one occasion that it did happen to me, to descend immediately to 25,000 feet. Range will suffer, and diversion to an ERA may be necessary, but operating anywhere above 25,000 feet without oxygen availability is unacceptable.

If MEA or Grid MORA was significantly above 10,000 feet, then an immediate route change would have to be added to the descent to 25,000 feet.

At 25,000 feet, time of useful consciousness (TUC) is 4 minutes, and, if operating an aircraft with an emergency descent capability of 6,000 to 7,000 feet per minute, one could be safely assured of descent to 10,000 feet in less than 3 of those 4 minutes.

Having descended to 25,000 feet, and made a route change if necessary, the fuel situation can then be re-evaluated to ascertain whether an En-Route or Off-Route Diversion is justified or required.

What does intrigue me in reading CCA's post is the consideration for operation where no ERA was available. Doesn't your Regulatory Authority and/or your Company's Operating Policy require that the aircraft must carry sufficient fuel to divert to at least an adequate airport following decompression at ANY phase of flight? (Not an ETOPS consideration, it applies to all aircraft).

Regards,

Old Smokey

Appologies all, "brain fart" I did mean GASEOUS.

Also I should have said ERA not immediately available, ie your over the atlantic/pacific with either no crew or no pax oxy, would you do the same thing in both cases?

Regards CCA

Either way (pax or crew 02) I think you're pooched. Descent is required.

Just a thought though - I've never seen it in a Boeing or an Airbus procedure but what about getting the portable bottles from the cabin.

would you do the same thing in both cases?

Absolutely YES. The passenger's and cabin crew's lives are just as important as your own, and oxygen deprivation at higher levels around 40,000 feet could be fatal.

Depressurisation at 25,000 feet would, at the worst, lead to the passengers being slightly hypoxic, with negligible risk of death. On a daily basis, turbo-prop aircraft take passengers to 25,000 feet without emergency oxygen for ALL passengers.

Remember that the Time of Useful Consciousness at 25,000 feet is 4 minutes, and, for an aircraft able to accomplish descent to 10,000 feet well inside those 4 minutes, not even the TUC has been exceeded, before we begin to consider more serious consequences which follow on after the TUC period has been exceeded.

Regards,

Old Smokey

Thanks Old Smokey, also as nnc0 asked how useful are the pax bottles? I'd imagine they are only useful below whatever altitude 100% oxygen is not required.

Would a PBE be any better?

Cheers CCA

CCA,

There's a number of advantages and disadvantages to be considered when comparing gaseous (bottled) oxygen to that available from chemical generators, here's a few of them -

(1) Oxygen available from a chemical generator is not subject to the risk of pressure loss / system rupture, as was the original point considered when you began this thread,

(2) Once started, a chemical generator cannot be turned OFF, whereas the gaseous supply can, leaving a 'mains' supply in reserve for any passengers requiring supplemental oxygen. (on some aircraft, supplemental oxygen is available full-time from the main gaseous supply, not possible with oxygen supplied from a chemical generator).

(3) Oxygen available from a chemical generator can not be supplied 'under pressure', thus it would have no flight deck use for the COMPLETE prevention of hypoxia if exposure to low pressure at high altitude was sustained or delayed,

(4) The duration of oxygen supply from chemical generators is quite short (10 to 15 minutes at the most), whereas the duration of supply of gaseous oxygen is limited only by the amount that the operator wishes to install and carry. This point is important, we operate 2 versions of the B777, one with, and one without gaseous 'bottled' oxygen. The much longer availability of oxygen from the gaseous system enables operations over regions of extended high terrain (e.g. Afghanistan) where the MEA is up to 18,000 feet, and passengers must remain on oxygen for some time after the initial emergency descent, until descent to 10,000 feet can be accomplished at a much later stage.

Your comments regarding the availability of PBE equipment to passengers is interesting. I EXPECT (but don't know for sure) that the in-built oxygen system would suffice to prevent hypoxia for the duration of an emergency descent, but what would kill such an idea is that the donning time, particularly for a passenger who has never used one before, would well exceed the Time of Useful Consciousness at high Altitude.

Whether PBE should be made available to passengers for use during smoke and fire emergencies is an entirely different issue. I can envisage that one day it may become a requirement to have PBE equipment available for all passengers, but not to cover the oxygen requirement for high altitude depressurisation and emergency descent.

Regards,

Old Smokey

Above what altitude is forced oxygen required (Ive found from 28,000'-47,000') and if the pax O2 bottles are considered as diluter masks what altitude are they good for?

Are the drop down maskes considered as 100% oxy, which would mean they're good for about 35,000' (according to a search I did)

I see the 777 quick donning mask provides 2.5mb of positive pressure @ FL400.

Further info on useful conciousness.

50,000' 6-9 sec
45,000' 9-15 sec
40,000' 15-20 sec
35,000' 30-60 sec
30,000' 1-3 min
25,000' 3-5 min
22,000' 5-10 min
18,000' 20-30 min
15,000' 30 or more mins

Cheers CCA

If Flt crew O2 fails, my (not B/AI) abnormal checklist calls for me to descend to FL100/MEA.

However, the prospect of 'unrelated' smoke in the flight deck following the failure and the fact that I'm a coward who breaths air and flies a multi pilot aircraft where the crew require to communicate (hence portable units have little use without a microphone) mean that a diversion is a very distinct possibility and more likely a certainty.

There was a mention of Afghanistan in an earlier post, I refer to my earlier statement of cowardice and at this point I consider fires to be quite cozy so I'll continue and hold my breath.:)

Taken directly from RAF "Human factors" Book:
"
Oxygen requirements and Cabin Pressurization"
The addition of an increasing proportion of oxygen into a pilot's breathing gas (airmix) can be used to avoid hypoxia until the point is reached at which he is breathing 100% oxygen to maintain an alveolar partial presure equivalent to that breathing at sea level. The point (100% oxy) is reached at an altiture of 33700'. However, as is noted above, it is permissable to allow alveolar partial pressure of oxygen to fall to 55 mm HG. With 100% oxygen as the breathing gas, this permits an altitude of 40,000' to be achieved. To go any higher than this it is required to deliver oxygen under pressure, ie Positive PREssure Breathing.
as a summary
0-10000 air only required, some higher function inpairment (night vision above 8k I believe, but may be wrong)
10000 - 33700 increasing percentage of oxy breathing in gas.
33700 - 40000 100% oxygen
above 40k 100% oxygen under pressure. "
:ok:
HOWEVER, if you were transatlantic, would oxygen be the limiting factor? I often cruise around 26K on my fleet, and the OAT at this level is around -25C..... if you had to carry out any significant diversion, say 1.5 hours, would this not neccesarily become a limiting factor? Apparently, the body reqires significantly more oxy as well when cold (to generate heat) than warm, thereby compounding the problem. I was led to believe, and I am not sure how truthful this is, that due to differing oxy reqirements (of the older, infirm and very young) that if an ETOPS aircraft had to div as a result of pressurization loss, and decend to a suitable level, it may not be possible to save all pax as a direct result of hypoxia and or hypothermia. Then again I am probably talking crap, but interesting none the less!

Either way, Cabin depress whilst on ETOPS or far away from the nearest diversion sends quite a shiver down my spine.... Very much as Cabin fire absolutely terrifies me as well. As a quick question to the cibvvies, do any of your drills for cabin fire include reducing speed and popping open doors or even jettisoning doors to assist in smoke elimination? I am led to believe having two doors open ( both at the rear) doesn half clear smoke quickly due to the "Chimney" effect of the airflow.

What does "useful consciousness" mean?

People live at 15 000 feet and above - carry babies to term, drive cars on curved mountain roads, etc. etc., day after day for their whole lives.

Useful Conciousness is the time you would have from depressurization until you would loose the ability (or reasoning) to put your oxy mask on. Having done the depress training myself (Up to 40k) the big problem is not physically putting the mask on, but actually coming to the conclusion that you need to put it one. one of the lads was in fits of laughter, others become almost sedated. People live at this altitudfe because they are aclimatised - te human body learns to deal with the lack of oxygen - it becomes more "efficient". also, individuals who live in these areas are not your average 40-aday types - physically they have much larger lungs, thus lung surface area, and can deal with the lack of oxgen. Climbers going up the himalayas are on oxygen WELL before the poor sherpas who are lugging all their luggage up for them - they have had the benefit of living there all their life! :ok:

I understand a 744 full of punters would probably generate more internal heat than a 737.

Is there any data out there to suggest what is the lowest OAT an A/C could fly, depressurised with a full load of punters, and not freeze? and for how long?

SMOC

Oxygen under pressure...
 

With regard to being 'force-fed' oxygen and high cabin altitudes, it is very dangerous to wait until 40,000 feet to breathe oxygen under pressure. The USAF figures that you'll begin to lose ground starting at about FL250, with things gradually becoming more critical as altitude increases. At a point about FL350, without being force fed, you'll be in trouble. Fortunately, the emergency descent will bring you down to reasonable levels quickly, and it's not a problem.

With reference to FCOM 1.35.20 Page 7, the 320 series Airbus I fly has a mask that automatically takes care of this problem. "Note: Overpressure supply is automatically started, when the cabin altitude exceeds 30,000 feet."


Tom