It's mainly hypothetical, but I could probably foresee a couple of places where this could happen. One is the northern route crossing the atlantic (Frobisher Bay-Sondre Strömfjord-Kulusuk) where no HF is required but depending on altitude, you're out of VHF range. Or anywhere else in sparsely populated or trafficked areas; Northern Canada, Africa, Greenland, Siberia, parts of Asia or the Pacific.
There is obviously a very hypothetical danger of flying into terrain in a place where rising terrain and lowering temp and pressure conjure if you're stuck on an old altimeter setting.
Or to give a more recent example, I took off from Lydd and went to Bembridge at 5400ft, which is just 100ft below the Class A veil. Only after I'd landed did I realize that any press altitude change along the way that I was oblivious to could have made me bust the A. Not that the QNH's were not available to me had I wanted to, but nevertheless. Sometimes you're busy watching the scenery.
Last edited by AdamFrisch; 13th Sep 2010 at 04:54.