What command indicates activation of a backup start system?

The command that indicates activation of a backup start system is "Activate airstart unit." This phrase is directly associated with initiating a backup mechanism designed for engine starting during situations where the primary start systems fail or are inoperative. In aviation operations, particularly on the ramp, this procedure is critical for ensuring the aircraft can start its engines safely and efficiently, thereby maintaining operational readiness and safety.

When the command to activate the airstart unit is given, it typically implies that the personnel are moving forward to utilize an auxiliary system that provides the necessary airflow or pressure to assist in starting the aircraft engines. This is especially useful during ground operations and in scenarios where other starting methods may not be feasible.

The other options relate to different actions not specifically linked to the activation of a backup start system. Understanding the purpose of each command helps in grasping the operational procedures and safety measures implemented in ramp operations.