Written with AI assistance and may contain mistakes. Verify against the official Transport Canada publications before relying on anything here.

Carburettor heat operating technique

Carb heat drill by phase of flight (TP 1102). Icing recognition: see Carburettor icing.

Phase	Condition	Carb heat
Start	-	"Cold": avoids damaging heat system if engine backfires (TP 1102 prestart check)
Before takeoff	High humidity and summer ambient temp below 28 C	Apply immediately before takeoff
Takeoff	-	Avoid. Exception: very low temperature areas, own special procedures
Climb	Full-throttle	Heat costs power. Hold airspeed, accept lower rate of climb
Climb	Below full throttle	Advance throttle as heat applied to hold desired RPM or manifold pressure
Cruise / in-flight prevention	With carburettor air temperature (CAT) gauge	Partial heat as needed to keep CAT out of icing range
Cruise / in-flight prevention	Without gauge	Full heat whenever necessary
Cruise / in-flight prevention	Ice persists after period of full heat	Gradually increase power for greatest heat output
Cruise / in-flight prevention	Some conditions	Check more often than usual 10 to 15 minutes, or fly with heat control full hot
Descent	Icing present or suspected	Full heat, periodically add power so engine heat prevents or clears ice
Descent	Long descent	Full heat early, periodic power (TP 1102)
Descent	Power-off descent	Routine use varies by aircraft; consult AFM

Full heat:

Costs about 9% of engine power on average (TP 1102): heated air less dense, ram air lost.
Manifold pressure drops, mixture richens.
Air is warm and unfiltered (TP 1102 Fig 2-5 text).
Richer mixture can cause roughness. Re-lean: with throttle available, restore prior RPM, lean and readjust until smooth, readjust after each later throttle or carb heat change. At low power (e.g. circuit), leaning may not be practical.

Fixed-pitch prop RPM (PHAK ch.7; TP 1102):

Heat applied:

RPM response	Meaning
Drops, then stays constant	No ice
Gradual RPM rise after drop	Ice melting

Trouble in flight:

Carb ice shows as smooth, steady power loss, or obvious roughness with rapid power drop. Can stop engine outright.
Ice suspected: apply full heat at once.
Prevention: know day's icing potential, watch and listen for power loss or roughness, apply carb heat periodically.

Trap: carb heat avoided on takeoff, often used on descent (prevent/clear ice); do not swap.

Practice