What does ICAO use as a basis for holding time?

The correct choice is based on the International Civil Aviation Organization's (ICAO) standards for determining holding patterns and times. ICAO specifies that holding times are calculated primarily using the outbound leg of the hold. This is significant because the outbound leg duration is used to ensure that aircraft maintain safe separation and adhere to prescribed flight procedures while circling within the holding pattern.

The outbound leg's time estimation considers the standard speed of the aircraft and the distance traveled during this segment. This practice ensures uniformity across various aircraft types and operational scenarios and helps in managing air traffic effectively.

Understanding this principle is crucial for pilots and air traffic controllers as it influences the way they manage airspace around gateways and during approach phases, as well as how they communicate and coordinate between each other to maintain safety and efficiency in flight operations.