How long should personnel stay clear when applying external or APU power due to potential movement of control surfaces?

When applying external power or APU power, personnel should maintain a clear distance of 4 seconds to ensure safety due to the possibility of unexpected movement of control surfaces. This time frame allows personnel to react to any sudden movement that might occur as the systems come online. Control surfaces can be activated unintentionally during power application, creating a hazardous situation for anyone in proximity.

Staying clear for this designated time helps ensure that personnel are at a safe distance, allowing for any required safety protocols to take place. Adequate training and awareness of aircraft systems are crucial in maintaining safety standards while managing power sources.