Lead Sled
Sitt'n on the throne...
- Joined
- Apr 1, 2004
- Posts
- 2,066
Absorption of oxygen depends on what is called partial pressure. Partial pressure is the pressure of the air at that altitude, and the percent of oxygen in the atmosphere. So if pressure decreases, you can either add more pressure (only to a certain point), increase the percentage of oxygen, or both. The quick don masks in the cockpit provide both.Immelman said:Interesting. Being an ignorant one who has not yet taken an altitude chamber ride, I am curious about Time of useful conciousness... Why the heck is it much less than 7 seconds? If I exhale with all my might here at sea level - and hold it - without inhaling first - I have no trouble typing a sentence or two here... takes quite a bit longer than 7 seconds. Does the ambient pressure at 470 really 'suck' the O2 out of you that quickly?
The reason that useful consciousness goes down so sharply is that when there is less oxygen outside of the body than inside, the oxygen will flow out of the body because of the law of gaseous diffusion. The less the pressure or oxygen outside, the quicker it will flow out. In my experience in the altitude chamber at 25,000 feet, I lasted about 3 minutes.
Hypoxia isn't the only problem you have with a rapid decompression. The possibility of Decompression Sickness (DCS) also exists. A high percentage of the atmosphere is made out of Nitrogen. Nitrogen is not used at all by the body, but it still gets taken in by the lungs. When the pressure on the body decreases, this nitrogen often comes out of solution and forms in a number of places. The more common places are in the joints (called the bends), in the lungs, in the central nervous system, and under the skin. All of them are quite serious. The way we avoid this while going into the altitude chamber is by breathing 100% oxygen for 30 mins. While breathing oxygen, the amount of nitrogen outside our body is less than inside, so the nitrogen flows outwards thus reducing the risk of DCS.
We routinely operate at FL410 and higher in our bizjets. The highest for me personally is FL450 and I've been there many times. There are many bizjets certified to FL510 and they are able to operate there. The big problem with those kinds of altitudes is if you suddenly loose the cabin you're dead - the time of useful consciousness is just to freaking short to allow any type of meaningful response.
My, highest flight in a glider was 17,500 feet - I had the O2, but ATC wouldn't open the window for me to climb any higher. In a normally aspirated aircraft I've "soared" a 65 hp Aeronca 7AC Champ to 14000' a couple of times.
Personally, I feel that there are far too many pilots out there that approach high-altitude flight with a careless and cavalier attitude. In reality, there has never been (that I am aware of) a catastrophic (total, sudden, & complete) pressurization failure with any aircraft certified to operate at those attitudes. Have there been failures? Of course, but they are very manageable. Does that make the real dangers of high altitude flight any less significant? In my opinion the problem is one of inadequate training. One trip to an Altitude Chamber is a very eye opening experience and they only take civilians to 25000'.
We use the Scott Eros Mask System which uses the inflatable straps. At altitudes above FL430, as far as I'm concerned, it's mandatory to have one, if not both pilots, on 100% O2. Assuming the worst failure, any pressurization problems at FL450+ would probably have to be considered fatal - if not for the crew then for the passengers. Like I said the TUC is just too short to have any type of meaningful response.
As far as what flight crews actually do when it comes to wearing masks...
Let's just say that pilots probably take more liberty with the O2 regulations than any other.
This is probably a good time to put in a plug for altitude chamber training. All of you guys who are flying high performance aircraft (or hope to someday) should get yourselves schedule for a ride in an altitude chamber. The cost is VERY reasonable (read: cheap) and you'll walk away with a new found respect for your physical limitations. There are several chambers located around the country, I'd just give your local FSDO a call and get them to point you in the right direction.
‘Sled