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787 9
11th Mar 2016, 20:21
There is an airline over the last 3 months that has maintained a procedure on certain routes at cruise altitudes up to 41000ft, that if a depressurisation occurs, cruise altitude must be maintained until the completion of a 180 degree turn (over 4 mins at 480 knots).
This has recently been discontinued but I am interested in any comments and whether any other airlines around the world would or have entertained such a procedure.

misd-agin
12th Mar 2016, 00:57
Where's the location that requires that?

safelife
12th Mar 2016, 05:29
Legal (certification) requirement is to commence an emergency descend within one minute.
Any more time would require an approval, for that an analysis of the available oxygen would be required.
Another constraint are the passenger oxygen masks. Most are effective only below 25000 ft so the time to get there is limited.

Stan Woolley
12th Mar 2016, 06:10
Most are effective only below 25000 ft so the time to get there is limited.

That's the first time I've ever heard that. Where do you get your information?

Stan Woolley
12th Mar 2016, 06:13
This has recently been discontinued but I am interested in any comments and whether any other airlines around the world would or have entertained such a procedure.

I'm not surprised. That would surely be quite an effective way of making sure you had killed all your passengers! :8

wanabee777
12th Mar 2016, 06:31
I don't recall the exact altitude, but from what I remember from altitude chamber classes while in the Air Force, one has to "pressure breath" 100% O2 when the ambient pressure drops below a certain level.

Only the cockpit masks are designed to do this.

The passenger masks are not designed, nor are able, to force oxygen under pressure through the lungs and into the hemoglobin above a certain pressure level.

I seem to remember that the critical level below which the passenger masks start to become effective is somewhere around 30,000 ft. (could be mistaken)

OK4Wire
12th Mar 2016, 06:44
30,000'? IRC

AerocatS2A
12th Mar 2016, 06:46
Cockpit oxygen masks are not pressure masks. They are capable of a slight positive flow for the purpose of keeping smoke out of the mask and goggles but they do not provide pressurized o2 to the lungs.

wanabee777
12th Mar 2016, 06:51
30,000 ft sounds about right.

Obviously, you would want to descend well below that level for the flight attendants and passengers to get adequate amounts of O2 into their blood stream.

wanabee777
12th Mar 2016, 06:54
Cockpit oxygen masks are not pressure masks. They are capable of a slight positive flow for the purpose of keeping smoke out of the mask and goggles but they do not provide pressurized o2 to the lungs.

That's news to me.

wiggy
12th Mar 2016, 07:14
Likewise wanabee

Certainly the placard on the regulator our 777 flightcrew Eros masks states that pressure breathing is available up to 35?-?40K . Not at work at the moment so can't check the exact figure.

captainsmiffy
12th Mar 2016, 07:36
The airbus Eros masks are not pressure masks.....but they mix ambient and oxygen below approx 33,000 feet - providing 100% oxygen above this level. I think this is what the previoys respondents are thinking about.

fireflybob
12th Mar 2016, 07:37
So the temperature has plunged to circa -55 deg C and you're going to sit there for 4 minutes - give me a break - sheer self survival shouts go down!

captainsmiffy
12th Mar 2016, 07:48
Pressure breathing equipment requires a different training regime. You do not, and will not, have pressure breathing equipment on an aeroplane for which you are not trained in its use. You have a mask that has an overpressure for the smoke scenarios and it provides 100% oxygen above approx 30000 feet. It mixes this with ambient below this level.

These masks increase the amount of O2 available to you and do NOT work by increasing the pressure of the O2 to get it into your haemoglobin. It is the partial pressure of the O2 within the gas (atmosphere) that does this. The O2 partial pressure decreases as we climb, which is why more O2 is required. We are NOT pressure breathing in a decompression.

wanabee777
12th Mar 2016, 08:02
Maybe I'm confusing the present day O2 regulators with the ones we used to have on the 727.

captainsmiffy
12th Mar 2016, 08:14
Very much doubt that. You will need to have done a course on pressure breathing if it were fitted. It isnt the sort of thing that you learn 'on the job' when you have just depressurised.

787 9
12th Mar 2016, 08:16
Where's the location that requires that?

I'm reluctant to say due to repercussions but interested to know if any other airlines did it and what the forum thinks of the idea.

787 9
12th Mar 2016, 08:18
I'm not surprised. That would surely be quite an effective way of making sure you had killed all your passengers! :8

That was my view. Not all, but some.

Chesty Morgan
12th Mar 2016, 08:25
Proper pressure breathing kit requires you to forcibly breath out against a lot more positive pressure than is provided by airline O2 masks. The stuff on airliners, whilst it might be called pressures breathing, most definitely isn't.

wanabee777
12th Mar 2016, 08:28
The only courses I ever had involving pressure breathing O2 were over 40 years ago when we were required to go to the altitude chamber every 2 or 3 years while I was in the military. No training, in that regard, with my airline.

I don't recall the act of "pressure breathing" O2 to be all that complicated or involved.

Aluminium shuffler
12th Mar 2016, 08:35
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.

wanabee777
12th Mar 2016, 08:39
There is no way that the partial pressure of 100% oxygen is enough at jet cruise levels without pressure breathing.

That has always been my understanding as well.

wiggy
12th Mar 2016, 09:14
I'm now told our masks are placarded to 40K...

..and like others, done the chamber stuff but not with an airline.

wanabee777
12th Mar 2016, 09:18
I'm now told our masks are placarded to 40K...

That seems odd.

Since I'm retired, I can't check what our placard says.

zlin77
12th Mar 2016, 09:31
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...

Discorde
12th Mar 2016, 09:41
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.

wanabee777
12th Mar 2016, 09:45
.... 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...Breathing O2 through the passenger masks or "walk around" bottles would have no effect at cabin altitudes above 30,000 ft. JMHO

ManaAdaSystem
12th Mar 2016, 10:27
Breathing O2 using the passenger masks or "walk around" bottles would have no effect at cabin altitudes above 30,000 ft. JMHO

If i recall correctly, the Helios flight was at or above 30000 ft and the only conscious person on that flight was a flight attendant using a portable O2 bottle.

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?

wiggy
12th Mar 2016, 10:37
Breathing O2 using the passenger masks or "walk around" bottles would have no effect at cabin altitudes above 30,000 ft. JMHO

Not sure about no effect, it might improve the prospects a fit individual, seated, being able to function - it's certainly got to be better than ambient at 30K...

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 .....

wanabee777
12th Mar 2016, 11:40
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 (http://hyperphysics.phy-astr.gsu.edu/hbase/ptens2.html)

https://www.khanacademy.org/science/chemistry/gases-and-kinetic-molecular-theory/ideal-gas-laws/v/partial-pressure

Tinribs
12th Mar 2016, 11:55
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

WeekendFlyer
12th Mar 2016, 12:15
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.

wanabee777
12th Mar 2016, 12:42
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.

How does this permit operations above 40,000 ft?

The max operating altitude of the 777 is 43,100 ft.

Basil
12th Mar 2016, 13:32
Another thing working against us, as this (http://www.pftforum.com/blog/alveolar-o2-and-altitude/) 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.

http://www.pftforum.com/blog/wp-content/uploads/2015/08/PAO2_PO2_Altitude_Percent_Change.png


p.s. I've skied at 12,000 feet and did notice a little shortness of breath but didn't stay there to experiment :8

arearadar
12th Mar 2016, 13:59
For what reason would you maintain altitude and make a 180 degree turn

Basil
12th Mar 2016, 14:21
For what reason would you maintain altitude and make a 180 degree turn
SAM threat?

CONSO
12th Mar 2016, 14:39
SAM threat?
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 ??

PersonFromPorlock
12th Mar 2016, 15:08
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.

Airbubba
12th Mar 2016, 15:38
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:

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.


There were some definite CRM issues with this crew:

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....


https://www.fss.aero/accident-reports/look.php?report_key=975

funfly
12th Mar 2016, 16:11
As a passenger, if we are at 40,000ft and loose pressure then I am going to die?

galaxy flyer
12th Mar 2016, 16:20
wannebe,

Because the certification standard is that the cabin NEVER exceeds 40,000, the masks are adequate. The standard implies starting down within one minute and cabin doesn't get above 25,000'. The design is such that only an explosive event would cause an instant 40,000 and that would be a catastrophic unsurvivable event.

GF

draglift
12th Mar 2016, 16:23
Funfly you asked As a passenger, if we are at 40,000ft and loose pressure then I am going to die?


Answer no as long as the crew start a descent fairly quickly. The BAC 111 did not have oxygen masks (except some delivered to the American Airlines.) They could cruise at up to FL350 and in the event of a rapid decompression you had to get down quickly to keep the passengers conscious. While being trained on another aircraft I remember being taught that above about FL380 if there was a rapid decompression and you did not have a mask and the plane did not descend you would be unconscious after about 30 seconds and brain damaged after 2 minutes.

Herod
12th Mar 2016, 16:57
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.

787 9
12th Mar 2016, 17:18
The requirement was due to terrain but with no work done on escape routes, this was the fix.

wanabee777
12th Mar 2016, 21:46
Wikipedia has some good info on TUC.

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

http://www.skybrary.aero/images/thumb/TUC.jpg/400px-TUC.jpg

https://en.wikipedia.org/wiki/Time_of_useful_consciousness

jack11111
12th Mar 2016, 22:39
I doubt the veracity of the opening statement of this thread.

Wageslave
12th Mar 2016, 23:18
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.

787 9
13th Mar 2016, 00:05
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.

Band a Lot
13th Mar 2016, 03:57
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

Basil
13th Mar 2016, 11:14
with no work done on escape routes, this was the fix.
The best and clearest escape route charts I've seen were those published by Cathay Pacific. I should have scanned them before I left.

Tinribs
13th Mar 2016, 11:47
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

ExSimGuy
13th Mar 2016, 11:54
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:ugh:

Herod
13th Mar 2016, 17:26
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?

Discorde
13th Mar 2016, 18:40
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?

Ozlander1
14th Mar 2016, 00:35
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.

wanabee777
14th Mar 2016, 00:44
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.

WingNut60
14th Mar 2016, 01:01
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

Semaphore Sam
14th Mar 2016, 02:03
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

CONSO
14th Mar 2016, 02:29
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

787 9
14th Mar 2016, 04:43
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

wanabee777
14th Mar 2016, 06:26
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?

mustafagander
14th Mar 2016, 09:26
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.

safelife
14th Mar 2016, 10:51
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.

wanabee777
14th Mar 2016, 12:50
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.

I hope you're right.

So, in regard to this thread's title, how long can an airliner maintain cruise altitude (in this case FL410) while depressurized?

Intruder
14th Mar 2016, 13:55
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.

Captain Calamity
14th Mar 2016, 14:08
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.

Lonewolf_50
14th Mar 2016, 15:13
I doubt the veracity of the opening statement of this thread. That's a least two of us. It has generated a few pages of response, which perhaps is what was desired.

wanabee777
14th Mar 2016, 16:56
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.

Even with a diluter-demand (not necessarily a pressure-demand) regulator, shouldn't the flight deck crew be able to remain conscious until their oxygen supply runs out?

http://www.boldmethod.com/learn-to-fly/aircraft-systems/oxygen-systems/

http://www.boldmethod.com/images/learn-to-fly/aircraft-systems/oxygen-systems/altitude-limits-large.jpg

Captain Calamity
14th Mar 2016, 17:39
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.

wanabee777
14th Mar 2016, 18:05
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.

daikilo
14th Mar 2016, 19:40
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.

Captain Calamity
14th Mar 2016, 20:08
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.

wanabee777
14th Mar 2016, 20:19
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.

787 9
14th Mar 2016, 20:33
That's a least two of us. It has generated a few pages of response, which perhaps is what was desired.
Can you say why you don't believe it because it is factual.

Its because it is so unbelivable that I cant tell you who it is, and it is a very reputable airline.

Ignignokt
14th Mar 2016, 20:37
Let's look at the likely event leading to a decompression. In the event of window failure...

Out of curiosity, decompression comes up quite regularly on a site like AV Herald, though the cause is not always included. Is window failure a common occurrence? I would imagine something like a faulty door seal (or something less glamorous along those lines) is behind most of those.

Captain Calamity
14th Mar 2016, 20:37
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.

787 9
14th Mar 2016, 20:56
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.
My latest comment related to Lonewolf_50 not believing the opening statement of this thread.

Ian W
14th Mar 2016, 22:41
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?

Once you have done your 'aviate' which would include immediate descent and a 'navigate' turn away from any conflicting traffic you can see on TCAS, the best thing to do is squawk 7700. Even in busy metroplexes a 7700 will immediately result in controllers turning everyone out of your way. If you are at say FL360 and start descent for 10,000ft that's possibly 4 different sectors as horizontal layers you are dropping through. The coordination the controllers have to do is difficult and all of those sectors are potentially busy and may not respond immediately. However, with 7700 breaking through any filtering and your Mode C winding down fast the information is already there for all the controllers to see and they will react at once. The supervisors are normally automatically alerted and everyone moves to unload the sector(s) that you are in, That is just by you making one selection to 7700.

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.

jugofpropwash
14th Mar 2016, 22:56
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?

Una Due Tfc
15th Mar 2016, 00:23
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

PersonFromPorlock
15th Mar 2016, 00:25
I may have missed it, but has anyone so far pointed out the other problems that a loss of pressurization, especially an RD or ED, involves? Bends, chokes, trapped-gas dysbarisms, CNS.... taking a few hundred people, not all of them healthy, from 3000ft to 40,000 feet in a few seconds is going to create all kinds of physiological havoc that calls for descending to a lower altitude ASAP.

wanabee777
15th Mar 2016, 01:39
UDT,

Ref your remarks...

Our transponders should automatically squak 77**, (I don't think the last two digits matter), once a loss of cabin pressure greater than 10,000ft is detected.

wanabee777
15th Mar 2016, 01:44
PFP,

Your point is valid.

In the pressure chamber (actually negative pressure chamber) training that I remember, we were all instructed to open our mouths before they conducted the explosive decompression demonstration. "What the hell was that!!!"

LOUD bang...Immediate fog...Instant cold...

Some participants had their fillings pop out.

The flatulent smell was pungent.

We all had a "bloated" sensation as the air rushed out of our lungs.

galaxy flyer
15th Mar 2016, 01:55
PFP,

Agreed but the certification standard does NOT anticipate the cabin getting above about 25,000' for long, if at all.

GF

Aluminium shuffler
15th Mar 2016, 03:23
The chart showing effective altitudes for different mask types is disingenuous. It assumes a rapid descent from those altitudes, not sustained flight at them; the masks will significantly retard the onset of hypoxia, giving a much increased time of useful conciousness at those levels, but will certainly not prevent hypoxia if the level is maintained.

harrryw
15th Mar 2016, 06:54
12 Minutes Passenger Oxygen in an NGX.
4 minutes holding level does not leave much for descent.
And a lot of people with slightly decreased lung function (older passengers) would have been pushed well into the O2 desaturation curve.

mustafagander
15th Mar 2016, 09:27
Igni
Window failures are fortunately rare. As I understand it, that is the design case for certification, see the small windows on Concorde.

Door seals leaking is usually more a problem of acoustics than pressurisation.

Bus Junkie
15th Mar 2016, 10:21
At Vietnam Airlines it is no problem: Maintenance screwed up the MEL, the crew took off and recognized the A350 wasn't pressurizing passing FL140, no problem they continued to climb to cruise altitude. Once level they decided to descend.

deltahotel
15th Mar 2016, 13:10
Person from Porlockis right. You should minimise any time above 25000 - above that there is a real and serious riskof decompression sickness. Any subaqua divers around? They'll tell us all about bends, chokes, creeps, staggers etc caused as the N2 in the blood comes out of solution on depressurisation.

The only time I did a chamber run above 25000, we prebreathed 100% O2 for 60 minutes to purge N2.

You really don't want to be mucking around up there.

Lonewolf_50
15th Mar 2016, 13:47
Its because it is so unbelivable that I cant tell you who it is, and it is a very reputable airline. I confess to you that it is very hard for me to grasp that a serious company in this industry would have such a practice or SOP. There is so much information and info on best practices on that matter that it just doesn't make sense to me, particularly as it relates to both passenger health/risk and crew health/risk.

Prodigal Dragon
15th Mar 2016, 15:42
Hypoxia, and the lack of practical training received by our contractor pilots, is one of our biggest Risks to Life (RtL). Consequently, we recently went to RAF Henlow to receive some excellent training. I haven't done this level of training since leaving the RAF over 12 years ago, and I would thoroughly recommend it to those with a need and a budget. Furthermore, they have a part-task trainer, so you don't even have to go into a hypobaric chamber anymore.

The advice to stay at height - especially if the cabin alt is above 25,000 ft - is unwise. The RAF use 25,000ft as a cut off for pressure sickness, with 10,000 ft being the safe altitude before the risk of hypoxia increases significantly.

Pressure breathing is vastly different to having an over-pressure. Draegar Hoods are not effective above a cabin alt of 25,000 ft.

Aviate, navigate and communicate while you can. That should call for an immediate, controlled descent to below 10,000 ft, depending on the SALT.

The SAMs may get a 'cheap kill', but lack of oxygen will certainly send you to 'the great bar in the sky'. And debating about what to do in such a situation won't help your cause.

Fly safely!

wanabee777
15th Mar 2016, 16:21
Does Draeger make PBE's for flightcrew use?

I couldn't find information ref PBE's with self-contained O2 provisions on their web site.

ExSimGuy
15th Mar 2016, 16:45
"self contained" - Why? :(

The aircraft I worked on all had a separate supply of O2 for the flight deck :ugh:

Chesty Morgan
15th Mar 2016, 16:52
PBEs are essentially for use fighting fires or smoke. Empty positioning flights, cargo flights may require the pilots to do so.

RAT 5
15th Mar 2016, 17:29
I confess to you that it is very hard for me to grasp that a serious company in this industry would have such a practice or SOP. There is so much information and info on best practices on that matter that it just doesn't make sense to me, particularly as it relates to both passenger health/risk and crew health/risk.

So there you are; a seasoned captain, perhaps via a few airlines and perhaps even military. You've spent your whole life training and practicing Emergency Descents and Pressurisation problems. They've all been to the common standard of descent without unnecessary delay. I didn't say rush.
Now you end up at an airline with this as an SOP, and the worse happens. What are you going to do? It is your own life at stake + all the pax. You have more than enough O2, but they............Could this be the time to enforce "deviation from SOP for flight safety reasons SOP"?

suninmyeyes
15th Mar 2016, 18:36
Una Due Tfc


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.



Over the years we have had the debate of whether we should squawk 7700 when making an emergency descent. This has varied from yes we should squawk 7700 to alert ATC or no we shouldn't as ATC will lose all screen data on us, aircraft type, callsign etc. Generally we understand the preference to be not to squawk 7700 unless ATC request it or if we are unable to contact them. Consequently to squawk 7700 in emergency descent was removed from the checklist a while ago.

mustafagander
16th Mar 2016, 09:46
Sunny,

The latest iteration of the B747 FCTM says, inter alia, for "rapid descent" squawk 7700 as soon as can be done after the memory items are completed.

wiggy
16th Mar 2016, 10:17
FWIW whilst our 777 QRH and FCTM is silent on the issue our Ops Manual specifically states 7700 should be squawked in the event of an emergency descent to ensure the aircraft is "visible" to the controller(s) of lower sectors.

Sillert,V.I.
16th Mar 2016, 10:37
To me, the last five pages of posts serve as a chilling reminder of a very good reason why it's not a good idea to overfly a war zone at any altitude.

I'm now wondering how many commercial routes involve flight above terrain where the MSA is above FL250.

suninmyeyes
16th Mar 2016, 16:42
Mustafagander and Wiggy


Thanks chaps. I see it disappeared from the checklist but has reappeared in the FCTM.

Aluminium shuffler
16th Mar 2016, 16:54
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.

RAT 5
17th Mar 2016, 07:02
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.

Wingswinger
17th Mar 2016, 08:30
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.

YRP
18th Mar 2016, 19:35
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?

mrdeux
20th Mar 2016, 06:03
.... 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.

wanabee777
20th Mar 2016, 06:35
As for the OPs initial comment...I wouldn't stay at altitude for one second longer than needed. Mad.

Fully concur.

mustafagander
20th Mar 2016, 09:24
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.

ExGrunt
20th Mar 2016, 10:44
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:

http://img.photobucket.com/albums/v324/ChristiaanJ/Decompression.gif
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 :8

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

Basil
20th Mar 2016, 10:59
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.

Aluminium shuffler
20th Mar 2016, 12:14
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.

mrdeux
20th Mar 2016, 13:27
mrdeux,

What was the failure causing that cabin RoC?

That was QF30....

Herod
20th Mar 2016, 15:11
Let's look at the title of the thread.

Maintaining cruise altitude while depressurised = Dead

wanabee777
20th Mar 2016, 15:18
"= 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.

Herod
20th Mar 2016, 16:30
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.

Aluminium shuffler
20th Mar 2016, 17:33
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.

wanabee777
20th Mar 2016, 18:08
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?

OldLurker
20th Mar 2016, 18:23
QF30 decompressed at 29,000ft, with no injuries.
Would they have got away with it if they'd been at 41,000ft?

Aluminium shuffler
21st Mar 2016, 14:36
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.

Aluminium shuffler
21st Mar 2016, 14:39
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.

Radgirl
21st Mar 2016, 16:09
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

Easy Street
21st Mar 2016, 16:26
Apologies if I've missed it, but I don't think anyone contributing to this thread has considered the "other" altitude-related medical condition - not hypoxia, but decompression sickness.

Irrespective of the mixture or pressure of breathing air, if you spend enough time above 18,000' cabin altitude then symptoms of DCS (otherwise known as "the bends") can start to appear, although the risk at that altitude is confined mainly to those who have been diving recently. The figure most commonly quoted for the population at large is 25,000'. This is why the RAF uses that as its maximum cabin altitude, and therefore operating ceiling for unpressurised aircraft like the Tucano. Aircraft which operate for extended periods at cabin altitudes of greater than 25,000' employ partial- or full-body pressure suits to provide counterbalancing force for two purposes: 1) on the chest to allow prolonged pressure breathing and 2) over as much of the body as possible to prevent the development of DCS. As cabin altitude increases further, only a full-body suit such as that worn by U-2 pilots or astronauts is capable of preventing the latter. The FAA has produced a useful factsheet on DCS here (http://www.faa.gov/pilots/safety/pilotsafetybrochures/media/dcs.pdf) which sets out causes, symptoms, effects and treatments.

Prolonged flight above 25,000' cabin altitude would risk DCS in either crew or passengers. This would be particularly unwise on flights departing diving destinations as the prescribed waiting times between diving and flying only take into account the pressure conditions expected on a normal flight. But there is no practical way of telling which of your passengers are vulnerable to DCS until they start complaining with symptoms. Unlike mild hypoxia, from which passengers should make a full recovery without medical treatment after descent, DCS may require protracted and expensive therapy and may result in permanent injury - not something your average airline is going to want to foot the bill for!

Mr Optimistic
21st Mar 2016, 20:57
SLF. I might have missed it, but irrespective of altitude, why would the SOP always require a 180 degree turn?

Ian W
23rd Mar 2016, 11:32
Aluminium Shuffler
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.

This may be the case on some of the more antediluvian systems but most air traffic automation systems work on a 'track' system where the automation has associated the flight data information with the aircraft position reports from the many and various surveillance systems. In the US selecting 7700 will add an 'emergency indicator' that blinks on the existing track datablock and no flight data information is lost. I would expect that the same is true of the automation systems in Europe and Australia.

Even if flight data is lost the great advantages of 7700 breaking through all filters far outweigh the possible loss of information on the processed radar display datablock in less advanced systems. As soon as you are level at 10,000ft you can work with ATC to revert to your original squawk but at least you got there with everyone being deconflicted from you. :ok:

Aluminium shuffler
23rd Mar 2016, 15:03
Thanks Ian. It was advice given to me by "home" ATC controllers on a radar room visit, but is over 15 years old, so the system may well have been replaced with one as you describe when they built Shanwick.

Discorde
23rd Mar 2016, 18:07
In busy airspace would ATC be able to respond quickly enough to multiple potential conflicts generated by an aircraft descending without clearance at very high rate through many levels? Perhaps this scenario has never arisen, but presumably aviation regulators will have considered the possibility. If so, what would their recommendations be for the conflicting requirements of rapid descent versus collision avoidance? Do ATCOs practise this exercise during simulator exercises?

Una Due Tfc
24th Mar 2016, 12:22
As Ian said, modern ATC systems will not lose your data from the label if you squawk an emergency code. It'll set off an audible and visual alarm in any ATC center which has a radar feed that can see you. We do indeed train for aircraft doing emergency descents, loss of comms and the other one, as well as setting up fuel dumping and so on. The squawk is far more important than the RT call as it allows sectors below you to deviate any traffic that may effect you before you enter that sector. Even descending at 8-10k FPM from cruise would give a low level or approach sector a couple of minutes to achieve this. If you lose separation with traffic within a few thousand feet then realistically there's nothing we could have done as by the time we get the avoiding action out of our mouth you have probably already descended through their level.

suninmyeyes
24th Mar 2016, 13:48
I had the opportunity to ask Swanwick today about squawking 7700 in an emergency descent, for those that don't know they are the air traffic centre that handles aircraft in English and Welsh airspace.


Their reply is that it is preferable to squawk 7700 but only after crew have stabilised the aircraft. (This fits in with the "people" part of prioritising plane, path, people.) With regards to the callsign/squawk all the original information is still retained after squawking 7700. I did not know this. A big change since the West Drayton days.

RAT 5
24th Mar 2016, 15:25
In busy airspace would ATC be able to respond quickly enough to multiple potential conflicts generated by an aircraft descending without clearance at very high rate through many levels? Perhaps this scenario has never arisen, but presumably aviation regulators will have considered the possibility. If so, what would their recommendations be for the conflicting requirements of rapid descent versus collision avoidance?


I think dream on about 'regulators' getting involved. The manufacturers write the procedure. Why does Boeing insist upon 10,000' level off? Because their TA is 18,000'. In EU that is still above TA and everyone is on STD. But I was told, in an RST, that I was wrong to go to FL 80.

If you lose separation with traffic within a few thousand feet then realistically there's nothing we could have done as by the time we get the avoiding action out of our mouth you have probably already descended through their level.

Which is why, IMHO, TA Only ASAP is more important than 7700.

Ian W
25th Mar 2016, 10:20
Rat 5
If you lose separation with traffic within a few thousand feet then realistically there's nothing we could have done as by the time we get the avoiding action out of our mouth you have probably already descended through their level.

Which is why, IMHO, TA Only ASAP is more important than 7700.

So you are over the center of the UK at FL380 over exceptionally busy climbing and descending traffic in and out of London metroplex, having left the Atlantic east bound to Paris and you get an explosive decompression. You would seriously go to the 3000 ft transition altitude just barreling on down and hope everyone will do something? I assure you that if you spend a couple of seconds setting 7700 everyone will be far safer. Traffic below you in your path 50 - 60 miles ahead will be turned away without any coordination necessary.
If you look at my initial response I said you should look at TCAS and avoid any traffic in your immediate vicinity. In areas of busy and complex routes turning 30 right and other such rules of thumb will not work, just aim for what TCAS tells you is a more empty area and 7700. People will then move out of your way.