How many seconds should you leave the APU/OFF/EXT PWR switch in the 'off' position before switching to APU?

Leaving the APU/OFF/EXT PWR switch in the 'off' position for 2 seconds before switching to APU allows for proper system stabilization and prevents potential electrical anomalies. This short delay allows any residual power or settings from the previous mode to dissipate, ensuring that the transition to the APU mode occurs smoothly and reliably.

A delay of 2 seconds strikes a balance between allowing enough time for stabilization without unnecessary extension of downtime, which could impact overall operations. The other durations may not provide the necessary time for effective transition, either being too short to ensure stabilization or unnecessarily prolonging the switch process. Understanding this timing is important for ensuring the safe and efficient operation of the APU and the overall electrical systems of the aircraft.