Originally Posted by
Noeyedear
I flew a couple of models of Turbo Commander. It was SOP to turn off the O2 bottle (in the rear compartment) after each day or night of flying to prevent loss through system leakage. Easy enough to forget to turn back on.
Not implying that this is what occurred.
The pressurisation in a few that I flew was woeful. A combination of tired air cycle machines and issues with the underfloor tubing that delivered the air would often mean a 17,000' cabin at 25,000'. And that was with Hot Air temp on full. It was worse with cold temps selected.
If the 695 pressurisation system is anything like the C441, there is a real trap when cold air is selected. The ducting can slowly become blocked with ice in one of the filters. Pressurisation is normal in the climb, but at altitude there is a subtle loss of cabin pressure as inflow reduces.
This nearly killed me on a long distance ferry flight. It was only from experience in the dreaded hypoxia chamber many years before that I recognised my symptoms in time to get the oxygen mask on. Another 20 or 30 seconds and it would have been lights out.
I wrote an article on this many years ago. In the analysis, an expert explained my failure to spot the warning light (no aural alert) as tunnel vision which is also a symptom of hypoxia. I remembered blue fingernails and dizziness, inability to concentrate, euphoria etc, but not the vision impairment effect.
A visit to the hypoxia chamber is no longer necessary for civilian pilots before flying pressurised aircraft; maybe it was never mandatory? It should be, particularly for single pilot operations - it’s more relevant than some of the other stuff we now have to do.