Which of the following is a requirement for safe rejoin operations?

Effective communication regarding airspeed is crucial for safe rejoin operations in formation flying. Each aircraft in a formation needs to maintain a specific airspeed to ensure safe spacing and positioning relative to one another. If one aircraft significantly alters its airspeed without communicating this change, it can lead to a dangerous situation such as a mid-air collision or an unstable formation.

Additionally, understanding each other's airspeed allows pilots to manage their formation efficiently, especially during maneuvers such as joins and breaks. Effective communication fosters situational awareness among pilots, ensuring they are all on the same page regarding their performance, intentions, and any adjustments that may be needed during the operation.

Other options like formation flying at night, maintaining formation without visual contact, and all aircraft maintaining a low altitude do not contribute positively to safe rejoin operations. While night flying adds complexity, it does not automatically imply safety. Similarly, maintaining formation without visual contact is inherently risky because it limits pilots' ability to assess their position relative to each other. Lastly, low altitude flying does not relate directly to the safety of rejoin operations, as altitude can vary based on mission requirements and does not inherently improve or jeopardize safety during rejoin maneuvers.