Bookworm,
Honestly, not all of us are of English origin! Maybe not even you, I don't know! So, I apologize for inaccuracy in my writing (if any!). If you compare my definition as I was taught many years ago and that one of ICAO, what do you get?
Same **** different wrap! One gives QNH, another one elevation! Or other way round!
The point is I still don't believe that airline pilots would use barometric altimeter for precision approach as the only mean of establishing altitude or height! Without radio altimeter, I don't think that any serious company would allow aircraft to fly.
For years, airport where I worked was considered to have elevation of 2800 feet. Then they sent a WGGS team of experts equipped with many electronic gadgets, they measured everything and – surprise! They established that the elevation is actually 2920 feet! 120 feet difference, 120 feet the ground was closer than expected! I state: Should aircraft used barometric altimeter only, somebody would have crashed for all those years of operation in marginal weather! Full stop!
Non-precision approach is different story as minima’s are much greater. I worked at the airport, which had MDA 1000 AGL. This would give enough time and height to compensate for all possible errors of barometric altimeter, temperature compensation etc and still to go around safely! I really don’t know if crews ever applied temperature compensation. Would they have enough time to calculate in extreme weather conditions? Can you explain what would happen if temperature inversion is in place?
I also have spent many hours “flying” an airline flights from the third seat in the cockpit. Not that I knew everything what was happening in the cockpit, but I had impression that the crew used only radio altimeter in final stages, also selecting the bug at prescribed minima which would activate alarm at the minima – and I have experienced really bad weather conditions!
What would happen when you fly in uncontrolled airspace and land at unmanned aerodrome? We’re not talking only about well established ATC system in USA and Europe. Look at Africa. There’s nobody to give you temperature; QNH, hmmmm, what’s that!!!???? But, this is another topic!