AIM Text
Tower controllers establish the sequence of arriving and departing aircraft by requiring them to adjust flight or ground operation as necessary to achieve proper spacing. They may “HOLD” an aircraft short of the runway to achieve spacing between it and an arriving aircraft; the controller may instruct a pilot to “EXTEND DOWNWIND” in order to establish spacing from an arriving or departing aircraft. At times a clearance may include the word “IMMEDIATE.” For example: “CLEARED FOR IMMEDIATE TAKEOFF.” In such cases “IMMEDIATE” is used for purposes of air traffic separation. It is up to the pilot to refuse the clearance if, in the pilot's opinion, compliance would adversely affect the operation.
Source: FAA Aeronautical Information Manual · current edition · paragraph 4-4-13.
Research Notes
AIM 4-4-13 covers Runway Separation — the spacing required between aircraft on or approaching a runway.
The separation responsibility: Towers are responsible for runway separation. They sequence aircraft, issue takeoff clearances at proper intervals, and provide "line up and wait" clearances when needed.
Wake turbulence separation: A critical factor when separating aircraft on/near the same runway. Heavy/Super aircraft create wake turbulence that can flip a smaller aircraft. ATC builds wake-turbulence-based separation into runway operations:
- Small behind Heavy: 4-5 NM (2-3 minutes typically)
- Small behind Super (A380): 6 NM
- Heavy behind Heavy: 4 NM
- Helicopter behind any: special procedures
The 'caution wake turbulence' phrase: ATC will warn pilots when the preceding aircraft was a heavy/super. The PIC may accept the spacing (visual separation maintained) or request additional spacing ("unable, request additional spacing for wake").
Visual separation: A pilot can accept visual separation from another aircraft — "Cessna Two-Three-Uniform, traffic in sight, maintaining visual separation." This shifts separation responsibility to the pilot.
Reference: AIM 7-3 (Wake Turbulence); AIM 4-4-13.