Wake turbulence separation: 2 or 3 minutes, and the DODIA rule
Wake turbulence minima look like a table with a lot of cells, and that is exactly how the exam sets it up: the same pair of aircraft, a different situation, a different answer. They collapse into a handful of rules once you know the categories and the memory trick.
The categories
Aircraft are grouped by maximum certificated take-off mass: HEAVY is 136 000 kg or more, MEDIUM is less than that and more than 7 000 kg, and LIGHT is 7 000 kg or less. Time-based wake separation is needed only when the aircraft that follows is in a lighter category than the one ahead: MEDIUM behind HEAVY, LIGHT behind HEAVY, and LIGHT behind MEDIUM. A following aircraft in the same or a heavier category needs no wake minimum.
DODIA: the five situations
| Letter | Situation | Minimum |
|---|---|---|
| D | Departure (same runway, or parallel runway less than 760 m away) | 2 minutes |
| O | Opposite direction: the heavier aircraft makes a low or missed approach and the lighter one takes off or lands in the opposite direction | 3 minutes |
| D | Displaced landing threshold (departure after an arrival, or arrival after a departure, paths expected to cross) | 2 minutes |
| I | Intermediate take-off from part of the runway | 3 minutes |
| A | Arrival, same runway | 2 minutes, but 3 minutes for a LIGHT behind a MEDIUM or HEAVY |
Source: ICAO Doc 4444 (PANS-ATM) 5.8. The values above are for the HEAVY / MEDIUM / LIGHT scheme and non-radar, time-based separation.
Parallel and crossing runways
- Parallel runways less than 760 m apart are treated as the same runway.
- Parallel runways 760 m or more apart: wake separation is needed for departures only when the flight paths cross at the same altitude or less than 1 000 ft below.
- Crossing runways: the same 1 000 ft rule applies. If the second aircraft’s path crosses the first’s at the same altitude or less than 1 000 ft below, apply the departure minimum of 2 minutes.
Where students lose marks
- The one-minute difference. A full-length departure needs 2 minutes. An intermediate take-off needs 3. The extra minute is for the shorter distance.
- Arrival is the only case where the category pair changes the answer. 2 minutes by default, 3 minutes only if the follower is LIGHT.
- “No minimum” is a valid answer. A HEAVY behind a MEDIUM, or a MEDIUM behind a MEDIUM, needs no wake separation in this scheme.
- 760 m and 1 000 ft are the only two distances. They are easy to swap for 1 000 m and 760 ft in the answer options.
Study aid, not the regulation. Radar wake separation uses distances and a different category scheme in many States; this page covers the time-based minima only.
Quick answers
What is the wake turbulence separation for a departure behind a HEAVY on the same runway?
2 minutes for a MEDIUM or LIGHT aircraft following a HEAVY, and 2 minutes for a LIGHT following a MEDIUM.
What is DODIA?
A memory aid for the five situations: Departure, Opposite direction, Displaced threshold, Intermediate take-off and Arrival.
When is 3 minutes required?
For a take-off from an intermediate part of the runway, when the lighter aircraft uses the opposite-direction runway after the heavier one’s low or missed approach, and for a LIGHT landing behind a MEDIUM or HEAVY.
How far apart are parallel runways treated as one runway?
Less than 760 m (2 500 ft). From 760 m up, wake separation is needed only when the flight paths cross at the same altitude or less than 1 000 ft below.
Make it stick
Fly the Wake Turbulence Separation drill: every item is a streak you have to land.