Ask The Pilot

[jb747]

I've been to Quito in the past couple of weeks, and am now in Cuzco. When planes are pressurized to 8,000 feet, what is the process for changing the pressurization to 9,000 or 11,000 ft?

In modern aircraft, altitude of the destination airport is either entered onto the pressurisation controller, or sent via the FMC, and this then controls the target altitudes used by the pressurisation system. Most likely, it will allow the cabin to climb (at around 300 fpm) during the descent.

Also, why do Dash 8's not have oxygen masks? Is it because they only fly at 25,000 feet?

Exactly. Not required for operations below that level, because it is assumed that the aircraft will reach safe altitudes in a short enough time. Note, though, the pilots have oxygen.

Even in QF30, in which the aircraft very rapidly depressurised to 29,000 feet, the aircraft was descended quickly enough that the passengers, EVEN WITHOUT ANY OXYGEN, would never have hit the end of their 'time of useful consciousness'.

 

[rechoboam]

In modern aircraft, altitude of the destination airport is either entered onto the pressurisation controller, or sent via the FMC, and this then controls the target altitudes used by the pressurisation system. Most likely, it will allow the cabin to climb (at around 300 fpm) during the descent.



Exactly. Not required for operations below that level, because it is assumed that the aircraft will reach safe altitudes in a short enough time. Note, though, the pilots have oxygen.

Even in QF30, in which the aircraft very rapidly depressurised to 29,000 feet, the aircraft was descended quickly enough that the passengers, EVEN WITHOUT ANY OXYGEN, would never have hit the end of their 'time of useful consciousness'.

But oxygen is one thing, having no atmospheric pressure is a separate problem, no? The unpressurised masks pax have (in contrast to the pilots') wont do anything useful at 30,000 feet.

 

[el_rooto]

In modern aircraft, altitude of the destination airport is either entered onto the pressurisation controller, or sent via the FMC, and this then controls the target altitudes used by the pressurisation system. Most likely, it will allow the cabin to climb (at around 300 fpm) during the descent.



Exactly. Not required for operations below that level, because it is assumed that the aircraft will reach safe altitudes in a short enough time. Note, though, the pilots have oxygen.

Even in QF30, in which the aircraft very rapidly depressurised to 29,000 feet, the aircraft was descended quickly enough that the passengers, EVEN WITHOUT ANY OXYGEN, would never have hit the end of their 'time of useful consciousness'.

Did all of the oxygen masks work on QF30 even though there was a tank missing?

 

[jb747]

But oxygen is one thing, having no atmospheric pressure is a separate problem, no? The unpressurised masks pax have (in contrast to the pilots') wont do anything useful at 30,000 feet.

The 'time of useful consciousness' figures are based upon having no mask.

But, I presume you are referring to pressure breathing, and yes, the pilots masks will do that, and no the pax masks are probably of limited use at very high altitudes.

There is a discussion of the medical aspects of it all here:
the effects of altitude on pilots

 

[jb747]

Did all of the oxygen masks work on QF30 even though there was a tank missing?

A (very) few masks failed to drop. That's not unexpected, and extra masks are provided in virtually every bin. From memory about 60 masks were activated in excess of the number people on board. Presumably the cabin crew may have activated some extras, but I expect that many of the passengers, not knowing exactly what to expect, activated more than one.

The masks are dropped by the oxygen pressure itself (in that gaseous style of system), so they don't all drop at once, but as the pressure hits the trigger point at each row, they come down a row at a time. So, if you're at the back of the aircraft, it comes back at you like a wave. All of the masks were provided with oxygen - - nobody had a mask with no flow.

The missing bottle made no real difference to the pressure provided by the system, although the damage caused to the wiring meant that it could not be turned off when no longer needed, and the plumbing damage meant that it was all expended around the time of landing.

 

[smeacs]

The 'time of useful consciousness' figures are based upon having no mask.

But, I presume you are referring to pressure breathing, and yes, the pilots masks will do that, and no the pax masks are probably of limited use at very high altitudes.

There is a discussion of the medical aspects of it all here:
the effects of altitude on pilots

Thanks for that link was interesting, So how often would crews suffer hypoxia to an extent requiring O2?

 

[jb747]

Thanks for that link was interesting, So how often would crews suffer hypoxia to an extent requiring O2?

Never, unless there is something wrong with the system.

Subtle hypoxia is extremely dangerous, and has given us quite a few fairly high profile accidents. An RAAF F18 up near Gove, Payne Stewart, a Kingair (I think) out of Perth, and the Greek 737 all come to mind.

Pressure breathing though, is more something that you could see in a military environment rather than a civil one. The mask in my avatar was very much capable of it...but only did so if the cabin altitude exceeded 25,000 feet. Pressurisation profiles in military aircraft are very different to civil...in the A4 (and again if memory serves correctly), the cabin was pressurised to half of the outside altitude...so we would regularly be at altitudes that would mean we could not, even for a few moments, remove the mask.

Pressure breathing is extremely uncomfortable, and very tiring, and is one of the items that the RAAF used to practice in their depressurisation chambers.

 

[smeacs]

I assume pressure breathing is essentially where the mask is positive pressure relative to the air outside the mask? Is it similar to positive pressure breathing apparatus we might see on the ground?

 

[jb747]

I assume pressure breathing is essentially where the mask is positive pressure relative to the air outside the mask? Is it similar to positive pressure breathing apparatus we might see on the ground?

I don't know what you'd use on the ground, or what you'd use it for.

The A4 mask operated under a very slight overpressure (and pure oxygen) at all times...about an 1" of water pressure (now that's an old scale), but pressure breathing was multiple pounds. The upshot is that any time you relax, oxygen is forced into your lungs, and you have to forcibly exhale. The intent is to provide the oxygen in your lungs at a high enough partial pressure that you'll actually get some oxygen transfer to the bloodstream.

 

[medhead]

I don't know what you'd use on the ground, or what you'd use it for.

The A4 mask operated under a very slight overpressure (and pure oxygen) at all times...about an 1" of water pressure (now that's an old scale), but pressure breathing was multiple pounds. The upshot is that any time you relax, oxygen is forced into your lungs, and you have to forcibly exhale. The intent is to provide the oxygen in your lungs at a high enough partial pressure that you'll actually get some oxygen transfer to the bloodstream.

BA is used for fire fighting and in certain noxious work environments. I'm not sure that they use pure O2 (it is a poison afterall) or if there is a need to force a certain partial pressure.

I was thinking about my CPAP that has a decent pressure behind it, but I figure that it is still at atmospheric pressure and has a bleed port. So completely different.


Sent from my iPhone using Aust Freq Fly app so please excuse the lack of links.

 

[anat0l]

about an 1" of water pressure (now that's an old scale)

Some context setting:

According to my calculations, if I did my math right, 1" (1 inch) of water is equivalent to a pressure of 0.25 kilopascals (kPa).

Standard atmospheric is about 100 kPa which corresponds to about 10 m of water.

At higher altitudes, the atmospheric pressure decreases to 75 kPa (8,000 ft or 2.4 km), 70 kPa (10,000 ft or ~3 km) and 30 kPa (30,000 ft or 9.1 km).

 

[harvyk]

I'm not sure that they use pure O2 (it is a poison afterall) or if there is a need to force a certain partial pressure.

AFAIK Pure O2 is only a poison at ground level. At different pressures it can become necessarily to breath in pure O2. This is according to a friend of mine who is a diver and uses O2 for some of his deep dives.

I would imagine in a very low pressure environment (eg at say 30,000ft) a very high amount of O2 would be necessary to get the required amount of oxygen into blood, as the amount of O2 (in litres) which your body requires wouldn't change, but actually getting the required amount of O2 per breath would be very difficult in a low pressure environment given there would most likely be less air coming in per breath.

 

[smeacs]

BA is used for fire fighting and in certain noxious work environments. I'm not sure that they use pure O2 (it is a poison afterall) or if there is a need to force a certain partial pressure.

I was thinking about my CPAP that has a decent pressure behind it, but I figure that it is still at atmospheric pressure and has a bleed port. So completely different.


Sent from my iPhone using Aust Freq Fly app so please excuse the lack of links.

Yeah firefighting is what I am familliar with and I beleive that is pressureised to around 5milibar. And no we just breath air in a cylinder if it was O2 it would be like strapping a bomb to your back.

 

[jb747]

I don't think I'd get too carried away with it being a poison. It is, after all, used at 100% in many military aircraft all of the time. NASA had no problem using it at 19 psi...though they did rather underestimate the flammability aspects.

 

[medhead]

I don't think I'd get too carried away with it being a poison. It is, after all, used at 100% in many military aircraft all of the time. NASA had no problem using it at 19 psi...though they did rather underestimate the flammability aspects.

My comment about oxygen toxicity was only in relation to BA and/or industrial OHS. You already explained why oxygen was required and used in a military environment, so I wasn't questioning that at all.

Of course, the military does lots of things that aren't really applicable to civilian situations. Live fire opposed obstacle crossing comes to mind. Swimming APCs in the surf is another.


Sent from my iPhone using Aust Freq Fly app so please excuse the lack of links.

 

[ColinP]

It's been a very long time since I did my PADI open water diving course and I'm happy to be corrected but I seem to remember that pure O2 becomes poisonous/toxic/ unhealthy/ lethal/ call it what you like, at around 1.6 - 2 atmospheres depending on duration of exposure.

 

[markis10]

It's been a very long time since I did my PADI open water diving course and I'm happy to be corrected but I seem to remember that pure O2 becomes poisonous/toxic/ unhealthy/ lethal/ call it what you like, at around 1.6 - 2 atmospheres depending on duration of exposure.

Pretty good memory, cell death occurs at 1.6ata!

 

[jb747]

Well, I think we've now totally ruled out the possibility of excess oxygen causing problems on a flight. Excess other things, not so sure.....

 

[dfcatch]

O2 toxicity occurs at 1.6 pO2.

So essentially oxygen toxicity is something you have to worry about at depth, not above ground.

In diving - the max depth you can breath 100% O2 is only 6 meters. So it's not used for deep diving at all (unless you want to die), with the exception of a shallow decompression tank for use at the end of a deep dive. (Beyond PADI stuff here). Normal air can be safely breathed to about 57m in most cases without having to worry about O2 toxicity.

There is a secondary form of pulmonary oxygen toxicity which can occur from prolonged exposure at elevated partial pressures - which is why advanced O2 medical providers will assess exposure times in treatment.

The key issue with either O2 toxicity or hypoxia is the partial pressure of the oxygen - which is a factor of the pressure (altitude/depth) related; and the percentage of O2 present in the breathing gas.

Firefighting BA is normal air (21% O2, 79% Nitrogen), however is provided via a positive pressure mask to ensure that no elements of the "irrespirable atmosphere" can be inhaled.

Edit: oh - and it's not usually any physical damage from the O2 toxicity that kills you underwater.... It's the convulsions and unconsciousness that causes you to spit your regulator out, aspirate water, and drown.

 

[johnny_the_knife]

AFAIK Pure O2 is only a poison at ground level. At different pressures it can become necessarily to breath in pure O2. This is according to a friend of mine who is a diver and uses O2 for some of his deep dives.

O2 toxicity can be an issue in two ways. Both are related to dose being made up of partial pressure and duration of exposure.

Central nervous system o2 tox is an issue in hyperbaric environments such as scuba and some industrial scenarios. As a rule, partial pressures of up to 1.4 are considered acceptable for the working part of a dive and 1.6 for decompression. Usually the first sign of a cns o2 hit is a convulsion, which is certainly sub optimal if you are under water.

The second type of o2 tox is pulmonary and can happen at any usable ppo2 (there is a lower limit at which point asphyxiation occurs due to a lack of o2). My understanding is that your lungs are slowly destroyed by the oxidizing nature of the gas. This needs long term exposure and can be a problem in medical situations but is also a consideration in scuba.

In terms of scuba, pure o2 is used mainly for decompression, and only shallower than 6m. Other high o2 mixes are used, referred to as nitrox, to both extend bottom times (usually not more than 40% o2) and speed up decompression. Pure oxygen is also used in some military and commercial applications at depths of 6m and less.

Note: ppo2 in this post measured in bars.