What should you turn off first in the event of an idle mixture failure?

In the event of an idle mixture failure, turning off the fuel shutoff valve first is crucial for managing the situation effectively. When the mixture control is set to idle and fails, the engine may receive an excessive fuel mixture that can lead to a loss of power or even engine flooding. By shutting off the fuel supply, you prevent additional fuel from entering the engine, which can help to regain control over the engine performance.

The other options, while they may be relevant in certain emergency procedures, do not directly address the immediate problem of excess fuel causing the idle mixture failure. For example, turning off the master switch would deactivate all electrical systems, including lights and radios, which is not necessary when the primary concern is managing engine performance. Similarly, while cabin heat might help in certain scenarios, it does not resolve the immediate issue of fuel management. Adjusting airspeed might be a factor in flight dynamics but does not address the engine's fuel supply directly. Therefore, the priority should be to stop the fuel flow to regain control over the engine's operation.