What details are included in Block 21 on a flight strip?

Block 21 on a flight strip is specifically designated for information about the next fix or coordination fix for an aircraft. This is essential for air traffic control as it provides controllers with immediate awareness of where the aircraft is scheduled to go next, which facilitates better coordination and sequencing in busy airspace.

Including details about the next fix allows controllers to efficiently manage traffic flows and aids in maintaining safe separation between aircraft. This information is critical for planning the aircraft's approach and ensuring that adjustments can be made promptly, should any changes in the flight path be necessary.

The other details might be important for various aspects of flight operation, but they do not belong in Block 21. For instance, the pilot's estimated time over a fix is relevant to their navigation but is managed in a different context. Altitude information serves parallel purposes for tracking an aircraft’s altitude but is represented elsewhere on the flight strip. Finally, the complete route of flight is included but is documented in sections that provide a comprehensive picture of the flight plan rather than the specific next fix detail captured in Block 21.