During a rejoin, when should number 1 communicate their airspeed?

The appropriate time for number 1 to communicate their airspeed during a rejoin is when it exceeds a 10 KIAS difference from what is expected. This communication is crucial for maintaining safe separation and effective coordination between the aircraft in a formation. A significant change in airspeed—beyond 10 KIAS—can impact the number 2 aircraft's ability to join safely and maintain the formation, as it allows number 2 to adjust their position and airspeed to rejoin effectively. Communicating this information helps ensure that all members of the formation are aware of the dynamics and can execute the maneuvers safely, minimizing the chance of a mishap.