What does Block 21 refer to in a flight strip?

Block 21 on a flight strip designates the next fix or coordination fix. This information is crucial for air traffic controllers, as it enables them to track the aircraft's anticipated path and facilitate proper coordination with other air traffic in the area. By knowing the next fix, controllers can effectively manage airspace and ensure safe separation between multiple aircraft, especially during critical phases of flight such as approach and departure.

Understanding the next fix is part of maintaining situational awareness in busy airspace, allowing controllers to anticipate potential conflicts and communicate necessary adjustments to pilots. This element of the flight strip plays an essential role in the overall safety and efficiency of air traffic operations.