The carb heat was certainly wired in 'Hot' in 1954, when the fuel in use had a rating of around 70 to 80 (anyone got a copy of the early PN?) which is what I'd have thought the Gypsy Major was intended for.
binbrook,
I have a copy of RAF PILOT'S NOTES FOR CHIPMUNK T10 A.P. 4308A-P.N. February 1950. The operation of the "Air Intake Heat Control" is detailed in the Part I - Descriptive Section, but there is no mention of when to use it in Part II - Handling or Part IV - Emergencies. There is also no mention of baling out, which is surprising.
The RAF PILOT'S NOTES CHIPMUNK T. Mk. 10 A.P.4308A-P.N. 3rd Edition June 1966, which is the document I was issued, has an expanded 'Descriptive Section':
On some aircraft this control may be wired to HOT if the air temperature is below 30° C; if COLD air is used below this temperature, carburettor icing may occur, indicated by rough running and loss of power.
The 'Testing the Engine' section does not mention the carburettor heat but 'Checks before takeoff' section includes:
Carburettor air-intake as required
The 'Handling in flight' section includes:
As the engine is prone to carburettor icing, it is recommended that the air-intake heat control should be wired in the HOT position in temperatures below 30° C. If the control is used in the COLD position, icing may occur, indicated by rough running or loss of power. If icing does not clear shortly after selecting HOT air, manipulation of the throttle may assist.
The 'Checks before landing' section, which is now the famous "My Friend Fred ..." mnemonic includes:
Mixture ... Rich. Carburettor air as required.