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Maintaining cruise altitude while depressurised

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Maintaining cruise altitude while depressurised

Old 12th Mar 2016, 16:57
  #41 (permalink)  

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Dredging the old memory cells from military times. My understanding is that at about 35,000' 100% oxygen will be equivalent to the partial pressure of oxygen in the air at 10,000'. That is liveable, but you wouldn't want to start doing any vigorous exercise. Above that level, pressure breathing is required, and that is satisfactory up to about 50,000' ( I never flew that high, so stand to be corrected). After that, a pressure suit is required. I think draglift's figures are about correct. In the pressure chamber at 40,000+, the removal of the mask caused almost instantaneous loss of critical faculties.
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Old 12th Mar 2016, 17:18
  #42 (permalink)  
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The requirement was due to terrain but with no work done on escape routes, this was the fix.
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Old 12th Mar 2016, 21:46
  #43 (permalink)  
 
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Time of Useful Consciousness

Wikipedia has some good info on TUC.

Note that TUC is reduced by up to 50% in the event of a rapid decompression.



https://en.wikipedia.org/wiki/Time_o..._consciousness
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Old 12th Mar 2016, 22:39
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I doubt the veracity of the opening statement of this thread.
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Old 12th Mar 2016, 23:18
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I cannot imagine why anyone would want to carry out a level 180' turn in the event of decompression at FL410.
Decompression at that level is potentially fatal (in terms of crew l.o.c.) within half a minute or so. The risk of a descent on track is tiny compared to the risk of remaining level and losing consciousness.

Sounds very odd to me.
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Old 13th Mar 2016, 00:05
  #46 (permalink)  
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Originally Posted by jack11111 View Post
I doubt the veracity of the opening statement of this thread.
I can assure you Jack11111 it is true. If it wasn't for the fear of repercussions I would happily give you further details including manual and SUPP references. I assume you don't believe it because it sounds so absurd?
Well, I fully understand that and it's why I put it out there for comment. The obvious thought initially was "is it just Me?" I'm glad you don't believe it. It's the response I was hoping for.
Please realize that this procedure has now been changed but was in play over the last 3 months.
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Old 13th Mar 2016, 03:57
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What sort of "terrain" is around 41,000 feet?

Or am I missing something?




"There is an airline over the last 3 months that has maintained a procedure on certain routes at cruise altitudes up to 41000ft,"
"The requirement was due to terrain but with no work done on escape routes, this was the fix."


Mount Everest is called the world's highest mountain because it has the "highest elevation above sea level". We could also say that it has the "highest altitude". The peak of Mount Everest is 8,850 meters (29,035 feet) above sea level. No other mountain on Earth has a higher altitude
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Old 13th Mar 2016, 11:47
  #48 (permalink)  
 
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Drift down

Band a lot
It is not quite as simple as being above the terrain. If you have an engine failure or lose pressure at high level you must descend in accordance with a plan. The drift down must end above the terrain so where the terrain is higher than 10.00 you need to put some thought into it ahead of the problem, preferably before flight but in the cruise before reaching the higher terrain will do. Being above the highest terrain in the world sounds great but doesn't solve the problem
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Old 13th Mar 2016, 11:54
  #49 (permalink)  


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The risk of a descent on track is tiny compared to the risk of remaining level and losing consciousness.
And both would carry a similar risk of collision with an aircraft below - but with a "push the yoke forward quick" response, at least the crew of the troubled aircraft would have a chance of seeing and avoiding a potential collision
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Old 13th Mar 2016, 17:26
  #50 (permalink)  

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And both would carry a similar risk of collision with an aircraft below - but with a "push the yoke forward quick" response, at least the crew of the troubled aircraft would have a chance of seeing and avoiding a potential collision
I seem to recall in the good old days, a turn of 30 degrees off track would solve that problem. Simples, no?
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Old 13th Mar 2016, 18:40
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in the good old days, a turn of 30 degrees off track would solve that problem.
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?
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Old 14th Mar 2016, 00:35
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Originally Posted by Band a Lot View Post
What sort of "terrain" is around 41,000 feet?

Or am I missing something?

The peak of Mount Everest is 8,850 meters (29,035 feet) above sea level. No other mountain on Earth has a higher altitude
And people go there without supplemental O2 all the time.
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Old 14th Mar 2016, 00:44
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How much time would it take for a 777 to descend from 41,000 ft down through 25,000 ft?

Now add 4 minutes for the 180 turn.
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Old 14th Mar 2016, 01:01
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I have logged two previous notes relevant to this subject:


1. Air Astana flight BKK - ALA travels well west of Almaty before turning 170 (not quite 180) to return to Almaty
If emergency descent over terrain is not a problem then why do they do this?


2. Garuda have still not re-fitted O2 in lavatories on 737-800's.
What is the prospect for someone in toilet getting back to O2 supply before a) passing out 2) dying - especially if modesty lures them into trying to get their pants back on first

Last edited by WingNut60; 14th Mar 2016 at 01:47.
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Old 14th Mar 2016, 02:03
  #55 (permalink)  
 
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Knee-jerk Responses

Whomever/whatever persons/groups decided to take the O2 masks out of the lavs were very myopic; the masks were there for very good reasons. Hopefully this will be universally rectified ASAP, before someone is killed. The cockpit doors being "secured beyond recall" has already resulted in predictable loss of an aircraft; this should also be rectified before another such event occurs. Knee-jerk responses to 9/11 have really hurt aviation; the biggest farce is the existence of the TSA. Sam
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Old 14th Mar 2016, 02:29
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The peak of Mount Everest is 8,850 meters (29,035 feet) above sea level. No other mountain on Earth has a higher altitude
And people go there without supplemental O2 all the time
But it takes longer than 30 seconds to climb.!!

Many days of slowly ascending allows your body to get acclamated. Plus being in good physical condition.

None of which applies to 99.99 % of a few hundred passengers
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Old 14th Mar 2016, 04:43
  #57 (permalink)  
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Originally Posted by wanabee777 View Post
How much time would it take for a 777 to descend from 41,000 ft down through 25,000 ft?

Now add 4 minutes for the 180 turn.
Tip over time and settle at 6000ft per min (say) Add 3 mins.

Over 7 mins above FL250
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Old 14th Mar 2016, 06:26
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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?
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Old 14th Mar 2016, 09:26
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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.
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Old 14th Mar 2016, 10:51
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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.
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