What should you do if you experience an engine fire during flight?

In the event of an engine fire during flight, the appropriate action to take is to set the mixture to idle cut-off. This action effectively cuts off the fuel supply to the engine, which is crucial for extinguishing the fire. By reducing the fuel flow, you're minimizing the potential for the fire to intensify.

Cockpit safety protocols emphasize that reducing or eliminating the fuel supply is vital to controlling engine fires. The other options involve actions that could either exacerbate the situation or do not directly address the fire. For example, increasing power could risk spreading the fire by providing more fuel to the burning engine, while opening cabin heat and air might introduce more oxygen to the fire. Reducing airspeed could have strategic merits in some scenarios, but it does not directly affect the fire. Therefore, cutting the mixture is the most effective and immediate response to mitigate the risk of an engine fire.