What is the first action to take if cabin heat is on during a cabin fire?

Turning the heater off is the most appropriate initial action during a cabin fire when the cabin heat is on. The heater can exacerbate the fire by providing additional fuel and allowing the flames to spread more rapidly throughout the cabin. By shutting off the heater, you minimize the potential for the fire to intensify due to the heat being circulated.

While closing vents can help limit the spread of smoke and reduce oxygen flow to the fire, it is not the most direct control action. Activating the fire extinguisher is also crucial but should follow the immediate step of turning off the heater. Landing immediately is critical if the situation escalates, but the very first response should focus on controlling the source of heat first.