I don't think Mooneys are the only SE planes certified for flight into icing conditions.
Also there are no apparent regs on masks above FL180 if one is using portable oxygen systems. In fitted oxygen systems you are legally required to follow the flight manual supplement on the system, but there is no such reg covering portable systems. There may be one but when a year or so ago a colleague and I did a detailed comparative airborne test (FL180) of oxygen demand delivery systems (basically, Mountain High and Precise Oxygen) versus constant flow options, we could not find such a "must use a mask above FL180" reg for portable systems. I have the writeup if anybody wants it.
I've been to FL200 using a cannula and it was perfectly fine. Blood o2 at 95% or so. One just needs to be careful with one's breathing (do not stop, obviously) and it raises issues with close supervision of passengers especially children. If you are at FL250 with cannulas and somebody starts to breathe through their mouth rather than nose, they will soon (within tens of seconds) be pretty unwell.
The value in an unpressurised plane which can go to say FL250 is that during the 1-5% of flights when you need to go that high to remain above the muck, you can do it. (The actual % will vary on your attitude to stuff like flying through frontal weather.) But you won't go that high (unpressurised) for any other reason; oxygen refills are a major hassle in Europe and even on a turbo engine the MPG won't get much better above FL200. Non-turbo, best MPG is c. FL080-100. I fly a TB20 and if IFR will always cruise at FL100 and climb only if needed to remain VMC on top. In Europe, tops rarely exceed FL160.
Last edited by IO540; 24th Dec 2008 at 19:55.