Maintaining cruise altitude while depressurised
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 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!
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 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!
Last edited by Easy Street; 21st Mar 2016 at 17:16.
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Aluminium Shuffler
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.
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.
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.
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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.
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?
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.
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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.
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.
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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.
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.
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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.
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.
Which is why, IMHO, TA Only ASAP is more important than 7700.
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.
