I can’t find any historical information on why and when the US changed from 24000 MSL to 18000 MSL as the transition altitude. If anyone has a reference, please post it. It occurred sometime between 1963 and 1989. Class A was adopted in the US as an airspace designation in late 1991 and has always included the airspace between 18000 MSL and FL600.
I vaguely recall reading somewhere (sorry, no reference), that this had to do with the mid-air over Grand Canyon sometime in the 50s (?). Seems like it was 18k in the eastern US and 24k in the West and one contributing factor to this accident was confusion about the altimeter setting. Again, this is from rather vague memory, perhaps this helps to find a reference.
has always included the airspace between 18000 MSL and FL600
And above that? Didn’t Concorde cruise at FL620?
Obviously it works in the USA with its very streamlined and uniform ATC service, but I wonder if there aren’t parts of Europe which would need to have their ATC re-organised a bit, to provide the regular QNH updates.
Above FL600 in the USA is Class E, so you can fly VFR there. I suppose you could do a Z flight plan, with the IFR portion covering the climb FL180-FL600 
but I wonder if there aren’t parts of Europe which would need to have their ATC re-organised a bit, to provide the regular QNH updates.
Not really, as you mostly talk to someone anyway. And if not, there’s always the ATIS of some airport in the vicinity, after all there aren’t many parts (if any) of Europe where you are more than 50 miles from an airport with ATIS.
OK, I finally found references and rational for the change in the Federal Register. Prior to CFR 14 part 91, the FAA defined its rules in Civil Aviation Regulations and Part 60.25 contained the rule for altimeter settings. At and below 23500 MSL one used altimeter settings. Above FL240 29.92 was used. There was a Special Rule added that recognized separation requirements for Jets were required and sections of the US airspace above FL240 became identified as Positive Control Areas, meaning that a transponder was mandated (64 code Mode 3/A) and all flight in the PCA was IFR. In 1963, the FAA switched to the current FAR structure and essentially did not change the relevant regulations. In 1964 they made two changes, they reduced the altitude for altimeter setting to 18000 MSL and defined all above to be FL based on 29.92. They also lowered V routes and established J routes at the 18000 MSL boundary. The reason stated for adopting 18000 MSL as the dividing point was that general aviation aircraft for the most part had service ceilings at or below 18000 MSL with higher performance turbine equipment normally operating above this altitude. Another reason for the 18000 split had to do with the range and frequency congestion of the V airways if they were to support navigation above 18000 MSL. VOR stations were designated as H (high), L (low), and T (terminal), with the H ones used in the J routes. Fewer of the H were required to support the J routes and L were used for the V routes, but used lower power. The positive control area was lowered to 18,000 and it eventually became class A in late 1991.
In the Final Rule document, the FAA describes their rational and discusses any recommendations and whether or not they are adopted. There is no mention in any of the relevant Notices of Final Rules anything regarding mountains, so I conclude this is an old wives tale.
Great post NCYankee!
The reason stated for adopting 18000 MSL as the dividing point was that general aviation aircraft for the most part had service ceilings at or below 18000 MSL
That makes total sense and is as true in 1960 as it is today.
Among pistons, only turbocharged aircraft generally flies higher.
And even then you get into 23.1447 which says that a mask has to be worn with any installed oxygen system when flying above 18k. My interpretation of 23.1447 and the use of the word “installed” is that that regulation does not apply to portable kits, so if you can get good performance with a cannula at 25k, that’s legal. But IMHO portable o2 kits were rarely used in the old days, because there were no demand regulators back then, no oxymiser cannulas, and anybody using a portable cylinder with a plain cannula would empty the cylinder quicker than his fuel tank… and most pilots would have tried to avoid the use of a mask due to (a) the hassle and (b) masks with a mike are expensive.
