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Maintaining cruise altitude while depressurised

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Maintaining cruise altitude while depressurised

Old 16th Mar 2016, 16:54
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Changing squawk is a bit of a sticky one. That it alerts controllers of airspace below you is inits favour, but against it is the loss of the data block attached to your "echo", which was the reason given by several of my employers for retaining the given squawk code unless told to change to 7700. But the idea of staying at cruise level depressurised is utterly wrong, unjustifiable and should be ignored. Unfortunately, most airlines have good SOPs and bas, some more of one than the other. Those pilots that have been around a few employers and types, especially those that have a few interesting days out, will know when to ignore an SOP. It's much harder for folk in their first company, especially if that company has an over-onerous approach to SOP adherence.
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Old 17th Mar 2016, 07:02
  #102 (permalink)  
 
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In various airlines, when discussing delays in changing SOP's, we were told they required approval by local XAA's. If true then a local XAA has approved this and they should be questioned and asked for their reasoning in the approval.
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Old 17th Mar 2016, 08:30
  #103 (permalink)  
 
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My recollection from RAF days:

O2 required above 10,000ft; 100% O2 required above 25,000ft; Under pressure above 33,000ft.

Up to 41,000ft unpressurised for the duration of a 180 turn? If the anoxia doesn't get them, the cold will.
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Old 18th Mar 2016, 19:35
  #104 (permalink)  
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Originally Posted by 787 9
Its because it is so unbelivable that I cant tell you who it is, and it is a very reputable airline.
So what is their reason for this procedure?

Presumably they must have had some reason. At a reputable airline, they must understand some of the general emergency handing. Let's say for the sake of argument that they aren't completely stupid, so what made them recommend this?
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Old 20th Mar 2016, 06:03
  #105 (permalink)  
 
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Originally Posted by mustafagander
.... 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.
Well, having been there and done that...the cabin reached cruising altitude well before the descent was even started. The RoC was in excess of 100,000 fpm.

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.
All great plans...but if the cabin climbs that fast you most certainly have a lot of damage. Just holding it wings level may well become the best idea. The cockpit will be going mad with warnings. The most important one of all might not even be that obvious. Believe it or not, the physiological effects might not be as obvious as you expect.

The QF30 A/P disconnected initially because the starboard aileron cables were cut. The others wouldn't engage because one of the ADCs had failed.

As for the OPs initial comment...I wouldn't stay at altitude for one second longer than needed. Mad.
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Old 20th Mar 2016, 06:35
  #106 (permalink)  
 
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Originally Posted by mrdeux
As for the OPs initial comment...I wouldn't stay at altitude for one second longer than needed. Mad.
Fully concur.
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Old 20th Mar 2016, 09:24
  #107 (permalink)  
 
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mrdeux,

What was the failure causing that cabin RoC?

As I understand it the wiring loom down the RHS of the ship was severely damaged with almost all wires severed including R A/P. I'm not sure that the A/P knows or cares whether the RH aileron cables are there or not.
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Old 20th Mar 2016, 10:44
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Hi,

It is not very often that as a non-pilot I have something of value to contribute. As a climber I have made several climbs to 14,000ft+ without oxygen. Addmittedly, we did it slowly and with acclimatisation at 7-10k ft. But we were able to operate at those altitudes for siginifcant periods (relative to the time of any unpressurised flight).

Perhaps more useful is this graph posted on the 'Concorde question' thread in tech log (a really good read BTW) showing the effects of various pressure failures on Concorde. Note the rate at which the cabin pressure rises (quoted from post 68 by ChristiaanJ:


It shows the emergency descent profile (solid line, 'Avion'), and the resulting effect on the cabin altitude (dotted lines) in the cases of one window ('hublot') blowing out with either three or four air conditioning packs ('groupes') operating.

As the graph shows, in the worst case the cabin altitude rises to about 40,000ft for about two minutes before starting to drop again, which is survivable when breathing oxygen.

It was studies like this, that lead to the small windows on Concorde. Keen spotters may actually notice that the windows on the prototypes are bigger than on all the other aircraft

The diagram is taken from "The Concorde Story" by Chris Orlebar, but the original was so pale that it was uncopyable, so I did redraw it, in answer to a question by a French friend (hence the legends in French).

ChristiaanJ
I think what the discussion so far has missed is the impact of the AC packs if they remain functioning.

EG
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Old 20th Mar 2016, 10:59
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Originally Posted by Silberfuchs
Do you want some Basil?
Thank you for the kind offer, Silberfuchs, but I'm now out of the game and can only hope, as pax, that the pilots over lumpy bits know what they're doing.
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Old 20th Mar 2016, 12:14
  #110 (permalink)  
 
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Ex Grunt, operating at 14000' for sustained periods would not be an issue for most fit pilots. The effect of altitude is logarythmic, though, and you have seconds at 40k.

The discussion about peak cabin altitudes is futile and utterly misleading, and quite likely what the manufacturers wanted for certification. Get a big enough hole and your cabin altitude will match the actual altitude in moments, and that does not necessarily mean an irrecoverable structural failure - have a door blow out and you'll have a very fast and complete decompression, but there's a pretty good chance of dealing with it.
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Old 20th Mar 2016, 13:27
  #111 (permalink)  
 
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Originally Posted by mustafagander
mrdeux,

What was the failure causing that cabin RoC?
That was QF30....
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Old 20th Mar 2016, 15:11
  #112 (permalink)  

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Let's look at the title of the thread.

Maintaining cruise altitude while depressurised
= Dead
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Old 20th Mar 2016, 15:18
  #113 (permalink)  
 
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"= Dead"

True for the passengers at 41,000 ft, but I believe the pilots, with their quick don, tight fitting, diluter-demand O2 regulators, could survive until their oxygen supply ran out or they froze to death, which ever occurred sooner.

Possibly, the A/C packs could provide enough heat for survival. I just don't know.
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Old 20th Mar 2016, 16:30
  #114 (permalink)  

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I have to disagree with you there, Wanabee. At that altitude even 100% oxygen won't give you the partial pressure that you need. OK, you MAY be able to retain consciousness, but to get enough O2 to function properly, you need a pressure mask. Wingswinger has it right. We were allowed 35000' for a limited period using 100% oxygen.
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Old 20th Mar 2016, 17:33
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Above 30odd thousand feet, 100% oxy at ambient pressure will merely delay hypoxia, not prevent it. Stay there indefinitely, Wanabee, and you will go unconcious and then die.
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Old 20th Mar 2016, 18:08
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I don't know this for fact, but if our quick don masks are placarded to 40,000 ft then, with a depressurized cabin of 40,000 ft, I would expect them to provide at least the equivalent O2 partial pressure with no mask at 10,000 ft.

Am I wrong?

Last edited by wanabee777; 21st Mar 2016 at 15:21.
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Old 20th Mar 2016, 18:23
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QF30 decompressed at 29,000ft, with no injuries.
Would they have got away with it if they'd been at 41,000ft?
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Old 21st Mar 2016, 14:36
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The thing is that these limits are for inactive people. So yes, technically, there is enough partial pressure of oxygen at 40k at 100%. But pilots dealing with a decompression are going to be on an initial oxygen deficit from the time taken to react to the decompression and get their mask on, and after that will be very stressed and dealing with a great mental workload. So, staying at high cruise levels is still folly.

Emergency descents are often rushed in the sim, causing errors and omissions, which is highly undesirable. Once the mask is on at 100%, the immediate threat is dealt with and the rest of the memory items should be done swiftly but methodically - it should not be rushed. But dawdling and deliberate time wasting is definitely incorrect.
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Old 21st Mar 2016, 14:39
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Lurker, it is not just the height of the decompression, but also the rate, that determines if barotrauma occurs. Individual personal circumstances are also a factor; some people have narrow sinuses or eustatian tubes that will increase susceptibility, as would a head cold. There are also other issues like scuba diving recency, giving increased susceptibility to the bends.
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Old 21st Mar 2016, 16:09
  #120 (permalink)  
 
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In a pilot with a Class 1 medical the oxygen level in the blood will equilibrate with the inspired oxygen partial pressure within 30 seconds breathing normally but within 15 seconds with the hyperventilation likely in such an emergency. So the issue is not 'delaying' hypoxia but the cerebral function with a particular oxygen supply to the brain.

At 40,000 feet the inspired partial pressure of oxygen if breathing 100% is 140 kPa. Breathing 21% oxygen - atmospheric air - at 10,000 feet it is 104 so maintaining altitude from a medical perspective seems OK.......BUT even with a tight fitting mask you wont get 100%. Added to that, the time to loss of consciousness (LOC) is much less than the time to LOC if you breath the same pp of oxygen at sea level - with an explosive decompression it may be a couple of seconds. Complications such as air emboli etc further complicate matters. Hypothermia may well cause breathholding if you are lucky, the heart stopping if not.

So yes if you fit the mask, train with it and slowly depressurise to 40,000 feet with no aircraft defects you will be fine, but in a real life depressurisation, even if of slow onset, I doubt you will be able to function although probably conscious
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