I have two ways to solve this problem:
1: if autopilot reads temperature below +2C from the airspeed sensor; switch on pitot heating. - this does not really regulate power, and needs to use more than required power, because the measured point is not the tube itself.
2:(I would prefer) - a simple circuit (few components, that can reliably be mounted to the pitot itself); if temperature or pitot tube itself <2C , then adjust heating current to keep 3degC
voltage ~14v , maximum needed effect ~4W - if you say op-amp +4 resistors, NTC, + a power transitor/FET, then I can do that already.
What I am looking for , is small design, if all can be epoxied to pitot - then the heat loss in a transistor, if in linear mode, can be used for something good - and component count should be kept at a minimum