What action should be taken if the ammeter shows a full negative deflection and the voltage light is illuminated?

When the ammeter shows a full negative deflection and the voltage light is illuminated, it indicates a potential failure in the electrical system, such as a battery or alternator issue. The correct action in this scenario is to turn off the avionics master switch. This action helps to reduce the electrical load on the system and can prevent further damage or draining of the batteries.

Turning off the avionics master switch allows the pilot to prioritize essential electrical needs and maintain control of the aircraft while assessing the situation. The negative deflection on the ammeter signifies that the electrical system is not functioning correctly, and leaving unnecessary systems powered could lead to total electrical failure.

While landing the aircraft immediately might seem prudent, it is not always the immediate necessary action as long as the aircraft is in a safe flying condition. Increasing altimeter settings is irrelevant in the context of electrical failure, as it does not address the problem at hand. Engaging the autopilot would also not be wise since relying on an automated system during a potential loss of electrical capacity could risk losing control.

Therefore, the right response focuses on minimizing electrical drain and ensuring aircraft safety until a more thorough assessment can be made.