Greetings all -
It has been with bemusement that I have followed the exchanges on declaring MAYDAY and loss of pressure.
Pressurization (or lack of) at altitude
Time of Useful Consciousness (TUC) without oxygen for a typical human can be 20-30 minutes at 18000', 3-5 minutes at 25000', and 30-60 seconds at 35000'. That from the Lancair manual. I do not have my Air Force physiology manual handy, but those numbers are similar to what we teach. Here are some important points for us to remember:
- The TUC can be cut by 1/2 or more after a rapid decompression. Most people think of "explosive" decompression when they hear the term "rapid". I was taught in both the air carrier and military environments that if the cabin pressure was climbing faster than the airplane could, treat it as a "rapid" event. A slow leak is one thing - anything involving sudden needle movements, or notable 'whumps', etc., are "rapid."
- The TUC numbers were mostly derived from testing young male test subjects in military altititude chambers many years ago. So, the test subjects were late teens to late 20s, in extremely good physical condition - in fact the only 'detractor' was {my speculation} that many of them probably were smokers, due to the demographics of this society in general and the military specifically 30 or 40 years ago. It is a known fact that our bodies don't do as well in adverse conditions as we age. Smoking dramatically decreases TUC due to the decreased capacity of white blood cells to carry O2. Even past smoking history will decrease your capacity compared with someone who has never smoked. Alcohol and other drugs offer similar negative benefits. So - considering that the test subjects could probably run 5 flights of stairs without being winded, any of us (me!) that can't do that without breathing heavily might want to adjust the 'book' TUC numbers.
- You can tell that there are significant differences in TUC as altitude increases. That translates into a deadly trap for flying without oxygen at lower altitudes. If you happen to be above YOUR body's tolerance altitude {again, this may be significantly lower than it was 'last week' if you are recovering from the flu, for example}, but you are at a relatively low altitude (less than 18000'), the degradation of brain function is typically so gradual as to not be detected UNTIL YOU RECOVER. If you have ridden in an altitude chamber, you know what I mean. If you haven't, trust me. For those that haven't, the best way to determine that something isn't right is "if you are feeling too bad or too good" - euphoria or depression/drowsy/headache. Other symptoms can be found in the AIM. Riding in the chamber gives you the opportunity to safely determine YOUR personal hypoxia symptoms - mine include hot flashes - almost to a burning intensity. Tingling of extremeties, blue lips and fingernails are a couple other common symptoms. The good news is that symptoms disappear very rapidly upon application of oxygen.
- The AF requires we wear oxygen above 10000' cabin altitude. I personally am quite comfortable with the Pt 91 limits, but I am still in pretty good condition - I fly jets for a living and pull g's on a routine basis, etc.. {I think anyone flying w/o O2 willingly above 12500 is really asking for disaster} Go back and review Payne Stewart's Lear accident (think I got the name correct) -- that is a classic illustration of how insidious hypoxia can be.
Emergency, MAYDAY, and so forth
- The pilot to controller glossary defines "emergency" as a distress or urgency condition. "MAYDAY" means imminent or grave danger when repeated three times. "PAN-PAN-PAN" should be used for urgency conditions (AIM 6-3-1). The AIM also advises "some are reluctant to report an urgency condition when they encounter conditions which may not be immediately perilous, but are potentially catastrophic." (6-1-2).
- I don't think I've ever heard "PAN-PAN-PAN" in the US, but have on several occasions overseas. Here in the US, most of us simply tell ATC something like "center, Cool 09 is declaring an emergency" - and we do it to alert them that we have "issues" we would like to resolve with minimal interference. If I thought I was then going to have to abandon the aircraft (a luxury not afforded us in the IV/P), I would update that to MAYDAY - grave danger.
- My personal decision matrix on what I "say" after a depressurization {I've had several in jets - none in the IV/P} depends on my ability to hold the cabin altitude below 18000. If I can descend rapidly enough to do that, I don't declare anything. If I can't, I declare. That point is partly derived from a USAF requirement to have a flight surgeon visit if a loss of pressurization results in a cabin altitude above 18000'. Now, here's a kicker - you are at FL250 and experience a "rapid-D". Immediately after donning your O2 mask, you tell ATC "hey, we've lost pressure & need to descend below FL180 please" and they say something like "okay, we've got crossing traffic, expect lower when clear" ... hmmm... my next call (as soon as they release the mic) will be "center, Cool 09, Emergency, descending to FL180, request lower". They will move the crossing traffic somewhere else, trust me.
- Somebody's personal experience on what they survived "with no problem" is but one data point given that circumstance, on that day. And did they REALLY know what impairment they had? Remember, hypoxia is insidious - remember Payne Stewart.
Personally, I will stick with the test data and the altitude chamber experiences.
Hopefully this is useful to someone.
Jim
Lancair Instructor
Jim McIrvin
cell 210-275-7780
email pilot195@rocketmail.com or mcirvinj@swbell.net
www.geocities.com/pilot195
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