I know you gave us a whopping in the Ashes LM, but no need to take your bat and ball and retire. Just not cricket old man.
FTH versus Critical may be down to different usages on either side of the pond.
major seversky | american aircraft | american public | 1942 | 2045 | Flight Archive
"Our main fighters," he writes, "the Kittyhawk and Tomahawk and the Airacobra, are stymied by engine shortcomings above the critical altitude of 16,000ft., though British and Axis aircraft function above 30,000ft.
Note that all three aircraft mentioned are supercharged rather than turbo. FTH or Critical Altitude very rarely get a mention in flight manuals eg
P-51 the British manual mentions FTH in passing, no mention of anything in the American
B-29 only mention is a line on performance graphs indicating "Wastegate closed"
P-38 one sentence saying critical altitude is when the wastegate closes
Beaufighter no mention
Lancastrian no mention
B-24 only refers to "Altitude", though it is referring to what is FTH or Critical Altitude
Merlin 130 as fitted to the DH Hornet only refers to "Altitude", as with the B-24. Merlin chart reproduced below. Test bed figures.
-----------------------------------------------Boost
-----------------------Gear
--BHP
----RPM
---lb/sq. in
--Altitude
Max. T-O power
-------MS
--1,670
--3,000
---18
---------SL
Max. Combat power
---MS
--2,030
--3,000
---25
-------1,250
-------------------------FS
--1,890
--3,000
---25
------13,750
Max. Climbing power
--MS
--1,420
--2,850
----12
-----11,000
-------------------------FS
--1,350
--2,850
---12
------22,000
Max. cruising power
---MS
--1,205
--2,650
-----9
------11,000
-------------------------FS
--1,190
--2,850
-----9
------25,000
Once again we strike language difficulties. Gear refers to supercharger speed, and MS equates to low blower and FS high blower.
Edited to add, see
P-51 Mustang Performance for the usage of the term Critical Altitude with respect to testing the P-51 (supercharger) by Army Air Forces Material Command,Wright Field, Dayton, 18 May 1943. You will see the same used in the tests by the same organization on the P-38 (turbos)
P-38 Performance Trials
On the facts presented it would seem Critical Altitude = Full Throttle Height.