Lear 35 and Rapid Decompression
FlyFast:
I flew a Lear 35 at a Part 135 Company for two years. The airplane I flew was used for data and research in the Payne Stewart litigation because they were similar serial numbers. Lawyers and engineers were out on three occasions I know of to take measurements.
I looked at the cabin outflow valve and talked to our mechanics about it.
The thing is nearly 100% plastic. Ours was replaced while I worked there because we had an erratic cabin pressure write-up. It turned out to be cracked nearly 2/3 of the way through. It was just a matter of time until it failed completely.
That valve was less than two years old, and was of the new and improved “Post Payne Stewart” variety. When the valve fails, you now have a 3-inch hole directly from the cockpit into the nose compartment. That is where the 2-4 second estimate for pressure equalization comes from.
I have nearly 6000 hours of military time with a mask on my face. I find military masks relatively comfortable. The problem is, most civilian types are not. The EROS “Alien” masks are pretty good, but at $12,000 each, most operators won’t retrofit them to an older jet. The Lear mask was a piece of Gothic medieval torture—but I wore it anyway.
When I first got there, no one would wear the mask. I made a point of wearing it, or asking the Capt to. I often got the “Why? No one can see us” look. So I told them what the chamber is like, and how quickly it can go from good to bad.
By the time I left, most guys would wear the mask, if even reluctantly. So at least it was being done. The good news in Part 135 is that time above 350 (or 250 if someone goes aft) is usually pretty short.
(You may not have considered that during a rapid decompression, you can get evolved gas problems—ie, the bends, or even nitrogen narcosis—just like rapid ascending from a scuba dive. If your regulator allows pre-breathing oxygen, it will lessen that possibility).
I don’t think the rule is all that archaic. Just look at QANTAS’ oxygen bottle blowout this summer. Every Lear 20 and 30-series has that plastic valve---how many other jets have a similar single-point failure mode?
You never know. And that is why you should wear the mask.