Under NAS regulations, what is the basis for holding time?

The basis for holding time in air traffic control procedures is primarily determined by the inbound leg of the holding pattern. This is because the inbound leg is the segment of the holding pattern where the aircraft is stabilized and aligned for approach towards the holding fix. Calculating the holding time based on the inbound leg ensures that the aircraft remains safe and can effectively manage its speed and descent profile while awaiting further instructions or clearance for descent and landing.

The inbound leg provides a consistent frame of reference for pilots and air traffic controllers to gauge how long the aircraft can remain in a holding pattern without compromising operational safety or efficiency. A clear understanding of the inbound leg's parameters—such as the aircraft's speed and the expected aircraft separation—allows for more precise coordination of traffic in congested airspace.

Other factors like outbound leg, aircraft type, and weather conditions can influence holding patterns and adjustments needed therein, but they do not serve as the primary basis for determining holding time. The focus remains on the inbound leg to provide a standard and reliable measure for maintaining safe distances and flow of air traffic.