Unpressurised scheduled passenger transport?
Maybe this is the wrong forum but I rarely visit the others...
I am curious how does this work out in practice. I am talking about the Islander/Trilander type business. They can't go much above 10k feet, obviously don't have oxygen, yet they fly under IFR which means Eurocontrol routes, most of those are above FL070 and many above FL130, and such an aircraft is bound to spend much of its time in IMC and turbulence.