Why should avionics equipment be turned off during engine starting and shutdown?

Turning off avionics equipment during engine starting and shutdown is primarily important to prevent damage from surge currents and voltage spikes. During these phases, the aircraft's electrical system can experience fluctuations in voltage as the engines are started or shut down. These fluctuations can generate electrical noise and transient spikes that may exceed the tolerance levels of sensitive avionics components.

Modern avionics systems are designed to operate within specific voltage ranges. When voltage surges occur, they can potentially cause malfunctions, damage, or reduced lifespan of the electronic systems. By powering down these systems prior to engine operation, the risk of exposure to these irregular electrical conditions is minimized, ultimately protecting the equipment and ensuring reliable operation once the aircraft is in flight.

While other factors such as saving battery life, reducing pilot distraction, and minimizing electrical noise are important considerations, they are secondary to the primary goal of protecting the avionics equipment from electrical disturbances associated with engine startup and shutdown.