He practically smacked my hand away and said "leave it on". He left the carb heat on until approx 300ft AGL and then he set it to COLD.
I was confused by all of this. His explanation was that it was their standard operating procedure and that it reduced the likelihood of carb ice accumulation during the critical take-off phase. Clearly there was no arguing with him so we finished up the next circuit with a full stop.
Since the use of carburetor heat tends to reduce the output of the engine and to increase the operating temperature, carburetor heat should not be used when full power is required (as during takeoff) or during normal engine operation, except to check for the presence or to remove carburetor ice.
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Carburetor Heat
Seems an odd trade off: A certain reduction in take off power to deal with the remote risk of ice having accumulated during the approach and preventing the engine from developing full power.
It seems to me that if carbie heat has been applied properly before the reduction in power, the probabilities of ice accumulating during the approach are extraordinarily low. However, it should be noted that the rote-learned procedure is usually to apply carby heat immediately before pulling the throttle. But it takes time for carby heat to heat the carby. If the probabilities of icing are high, carby heat should be applied up to 30 seconds before the throttle is pulled.