One possible story, though not very likely, is that the regulator does not do its job properly so that at higher rev's the voltage goes up; according to ohm's law this will also increase the heater current, the heater being esxsentially a resistance. If on top of that the heater has lower resistance than intended, or the breaker is too sensitive, tripping could occur.
A thought that crossed my mind as well. But CBs are typically overdimensioned, and by the time this scenario would play out, the voltage would be so high that the battery would be knackered by now. Not to mention all the other faults that this may cause.
One other thing: Isn't a heater element like this some sort of negative coefficient resistor type something thinghy? In other words, as the element heats up, its resistance increases, which means a lower current draw, which means less heat being produced. So in effect this becomes a self-balancing system that keeps a certain temperature automatically?
My money would be on the pitot heater element.
Normally I'd agree, but the OP reported that on the ground the pitot heater heated up normally. That would suggest, to me, that the pitot heater element itself is OK.
Oh well, let's see what the OP reports back.