When should Mode C validation occur for each aircraft?

Mode C validation is an essential practice in air traffic control to ensure that the altitude reporting from an aircraft's transponder is accurate. This validation helps controllers to maintain safe separation between aircraft and to properly manage traffic flows.

The ideal time for Mode C validation is during the acceptance of an inter-facility handoff. When a controller receives an aircraft from a previous facility, they need to verify that the altitude being reported corresponds to the aircraft's actual altitude. This is crucial as it confirms the accuracy of the altitude readout before the aircraft continues into a different airspace sector, where further integration into traffic routing decisions is necessary. Proper Mode C validation at this point helps to ensure safety and prevent potential conflicts with other aircraft.

Validation immediately after landing, during peak traffic hours, or after reaching cruising altitude would not provide the timely assurance needed for altitude accuracy during critical phases of flight, such as during descent, approach, and initial climb, when aircraft are more densely packed in a controlled airspace.