Which is a key requirement when specifying a transition for a SID?

The requirement that it must be necessary is crucial when specifying a transition for a Standard Instrument Departure (SID). This necessity ensures that the transition supports safe and efficient operations within the airspace system. Each SID and its associated transitions are designed to facilitate aircraft movement in a manner that enhances safety while minimizing complexity and potential conflicts with other air traffic.

By ensuring that a transition is necessary, air traffic controllers can help maintain an efficient flow of traffic while accommodating the unique operational needs of various aircraft types. This necessity encompasses considerations such as airspace restrictions, the performance capabilities of the aircraft, and the overall traffic environment.

The requirement for necessity goes hand in hand with operational efficiency and safety, ensuring that flight plans remain practical for pilots while adhering to regulatory and procedural standards. It emphasizes the importance of creating SIDs that are not just theoretically sound, but also operationally applicable and valuable in real-world conditions.