What is the starter cranking limit for engines #3 to #5 on the ground?

The starter cranking limit for engines #3 to #5 on the ground is defined to ensure engine start reliability while preventing damage to the starter and the engine itself from prolonged cranking. The correct choice specifies a duration of 90 seconds for cranking and a resting period of 5 minutes before the next cranking attempt.

This 90 seconds ON cranking period is effective in providing enough time to start the engine successfully under normal conditions. However, limiting the cranking time is vital because excessive cranking can lead to overheating of the starter motor and other components. The 5 minutes OFF period allows for adequate cooling and recovery of the starter before it is used again, ensuring that the starter does not overheat and remains within operational limits.

In contrast, other options suggest different ON and OFF periods that may not provide the same level of protection or efficiency for the starter system, either extending the cranking time beyond recommended limits or reducing the necessary cooling interval. Adhering to the specified limits is crucial for maintaining the operational integrity of the engines and the aircraft's performance.