How long can each engine be turned on during a ground start before a cool-off period is required?

The correct duration for keeping each engine on during a ground start before requiring a cool-off period is 60 seconds. This limitation is primarily in place to prevent overheating and to ensure that the engines do not suffer damage from excessive heat buildup. During a ground start, the engines generate heat, and if they are operated continuously for too long without allowing adequate cool-down time, it can lead to increased wear, potential engine damage, or even system failures.

This 60-second limit is a guideline established by the manufacturer based on engine design and thermal characteristics during ground operations. Adhering to this limitation helps maintain engine health and prolongs the operational life of the aircraft. The cool-off period allows for heat dissipation and ensures that the engine components do not exceed their safe operating temperatures.