AIM DECODED

7-4-9. Air Traffic Wake Turbulence Separations

By Chris Palmer, 2x Master Aviation Educator · Gold Seal CFI · Updated May 21, 2026

AIM 7-4-9 All of Chapter 7 →

AIM Text

  1. Because of the possible effects of wake turbulence, controllers are required to apply no less than minimum required separation to all aircraft operating behind a Super or Heavy, and to Small aircraft operating behind a B757, when aircraft are IFR; VFR and receiving Class B, Class C, or TRSA airspace services; or VFR and being radar sequenced.
    1. Typical separation applied to aircraft operating directly behind a super or heavy at the same altitude or less than 1,000 feet below, and to small aircraft operating directly behind a B757 at the same altitude or less than 500 feet below:
      1. Heavy behind super - 5 miles.
      2. Large behind super - 7 miles.
      3. Small behind super - 8 miles.
      4. Heavy behind heavy -3 miles.
      5. Small/large behind heavy - 5 miles.
      6. Small behind B757 - 4 miles.
    2. Also, separation, measured at the time the preceding aircraft is over the landing threshold, is provided to small aircraft:
      1. Small landing behind heavy - 6 miles.
      2. Small landing behind large, non-B757 - 4 miles.
  2. Additionally, appropriate time or distance intervals are provided to departing aircraft when the departure will be from the same threshold, a parallel runway separated by less than 2,500 feet with less than 500 feet threshold stagger, or on a crossing runway and projected flight paths will cross:
    1. Three minutes or the appropriate radar separation when takeoff will be behind a super aircraft;
    2. Two minutes or the appropriate radar separation when takeoff will be behind a heavy aircraft.
    3. Two minutes or the appropriate radar separation when a small aircraft will takeoff behind a B757.
  3. A 3-minute interval will be provided when a small aircraft will takeoff:
    1. From an intersection on the same runway (same or opposite direction) behind a departing large aircraft (except B757), or
    2. In the opposite direction on the same runway behind a large aircraft (except B757) takeoff or low/missed approach.
  4. A 3-minute interval will be provided when a small aircraft will takeoff:
    1. From an intersection on the same runway (same or opposite direction) behind a departing B757, or
    2. In the opposite direction on the same runway behind a B757 takeoff or low/missed approach.
  5. A 4-minute interval will be provided for all aircraft taking off behind a super aircraft, and a 3-minute interval will be provided for all aircraft taking off behind a heavy aircraft when the operations are as described in subparagraphs c1 and c2 above, and are conducted on either the same runway or parallel runways separated by less than 2,500 feet. Controllers may not reduce or waive this interval.
  6. Pilots may request additional separation (i.e., 2 minutes instead of 4 or 5 miles) for wake turbulence avoidance. This request should be made as soon as practical on ground control and at least before taxiing onto the runway.
  7. Controllers may anticipate separation and need not withhold a takeoff clearance for an aircraft departing behind a large, heavy, or super aircraft if there is reasonable assurance the required separation will exist when the departing aircraft starts takeoff roll.

Source: FAA Aeronautical Information Manual · current edition · paragraph 7-4-9.

Research Notes

AIM 7-4-9 covers Volcanic Ash — the operational hazards of volcanic ash to aircraft.

Why volcanic ash is dangerous:

  • Engine ingestion: Ash is silica-rich. In jet engine combustors, it melts and re-solidifies on turbine blades, causing engine flameout
  • Windshield abrasion: Ash particles abrade windshields, reducing visibility
  • Static lines / pitot heating: Ash blocks sensors, causing erroneous airspeed/altitude readings
  • Avionics: Static buildup and erroneous readings

Famous incidents: British Airways 9 (1982 — Mount Galunggung, 4-engine flameout, eventual restart and emergency landing). Air New Zealand DC-10 over Antarctica (1979 — though primarily controlled flight into terrain). KLM 867 (1989 — Redoubt Volcano, 4-engine flameout).

Avoidance strategies:

  • Monitor NOTAMs for volcanic activity
  • Use SIGMET-VA (volcanic ash) reports
  • Use the Volcanic Ash Advisory Center (VAAC) products
  • Avoid downwind of any active volcano
  • If ash encountered, escape promptly (turn back if upwind possible, or proceed clear of ash band)

Reference: AC 00-24C (Thunderstorms); AIM 7-4-9; FAA Volcanic Ash Program.

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