Wake Turbulence

What wake turbulence is

Every wing that lifts an aircraft leaves two vortices behind it, one at each tip, which slowly descend and spread. The heavier the aircraft, the stronger they are. A smaller aircraft that enters these vortices suffers three effects¹:

• violent rolling, which may exceed the aircraft's control authority to resist it, and is the greatest danger;
• loss of height or of rate of climb;
• structural stress.

The risk is greatest close to the ground. On approach and take-off, the aircraft is slow, low and has little margin of speed, thrust and height to recover¹.

Wake turbulence is invisible. No one can say precisely where it is². That is why the rules do not ask you to see it: they ask you to keep enough distance or time for it to dissipate or descend before the next aircraft passes.

Helicopters

Helicopters also produce vortices, proportionally more intense than those of aeroplanes of the same weight. When hovering or air taxiing, the rotor blows high-speed air out to about three times the rotor diameter. Keep helicopters doing this clear of LIGHT aircraft³.

Categories

Wake turbulence minima depend on each aircraft's category, defined by certificated maximum take-off mass⁴:

The category of each type is in ICAO Doc 8643⁴ and comes in item 9 of the flight plan, after the type (A320/M, B77W/H, A388/J).

SUPER and HEAVY aircraft must say "super" or "heavy" immediately after the callsign on initial contact with each unit⁵.

*Brasília Center, TAM 8084 heavy.

When to apply

Wake turbulence minima exist to reduce the hazard of wake turbulence and do not replace the normal minima. The larger of the two always applies. If the IFR minimum is larger than the wake minimum, apply the IFR minimum⁷.

Besides the cases defined on this page, you may apply a wake minimum in any situation where you see a possible wake hazard, even if no specific rule requires it⁸.

With ATS surveillance

With ATS surveillance, wake minima are distances, measured on the screen like the radar minimum⁹:

Pairs not in the table (MEDIUM behind MEDIUM, or any aircraft behind a LIGHT) have no wake minimum. The radar minimum applies to them.

These minima apply on approach, on departure and en route below FL 240, and only when¹⁰:

• the following aircraft follows the preceding aircraft's track at the same altitude or less than 1,000 ft below;
• the following aircraft crosses the preceding aircraft's track at the same altitude or less than 1,000 ft below; or
• both use the same runway, or parallel runways less than 760 m apart.

Wake turbulence descends. That is why the rule only looks at aircraft at the same altitude or up to 1,000 ft below the other. An aircraft above the other's track, or more than 1,000 ft below it, does not need the wake minimum.

On the runway and without surveillance

Without surveillance, and in the tower, wake minima are times. They apply between aircraft using the same runway or nearby runways, and do not depend on the tower having radar.

Landings

An aircraft landing behind a heavier one needs¹¹:

Departures

An aircraft departing behind a heavier one needs¹²:

The "full length" column applies when both aircraft are using¹³:

• the same runway;
• parallel runways less than 760 m apart;
• crossing runways, if the projected flight path of the second aircraft crosses that of the first at the same altitude or less than 1,000 ft below; or
• parallel runways 760 m or more apart, with the same flight path crossing condition.

The "from an intersection" column applies when the following aircraft departs from an intermediate part of the same runway, or of a parallel runway less than 760 m away. It starts its take-off roll further along and gets airborne closer to the point where the preceding aircraft rotated. That is why the minimum goes up by 1 minute¹⁴.

Displaced landing threshold

With a displaced landing threshold and crossing flight paths, an aircraft departing after a heavier one has landed, or landing after a heavier one has departed, needs the same times as a full-length departure: 2 min (H behind J, M or L behind H, L behind M) or 3 min (M or L behind J)¹⁵.

Opposite direction

When a heavier aircraft makes a low approach or a missed approach, and a lighter one will take off on the same runway in the opposite direction, or land in the opposite direction on the same runway or on a parallel runway less than 760 m away, apply¹⁶:

When wake separation is not required

Control does not need to apply wake separation in two cases¹⁷:

• VFR landing on the same runway behind a SUPER, HEAVY or MEDIUM aircraft;
• IFR on a visual approach, after reporting the preceding aircraft in sight and being instructed to follow it and maintain own separation.

In these cases, and whenever you consider it necessary, you must issue a wake turbulence caution. From then on, it is up to the pilot to decide whether the spacing from the heavier aircraft is acceptable and, if not, to request more separation¹⁸.

PUA 646, cleared visual approach runway 35, maintain own separation from preceding B747, caution wake turbulence.

Cautions and pilot requests

The wake turbulence caution gives the pilot what they need to protect themselves: the type of aircraft generating the wake and where it is¹⁹.

PT KTU, caution, wake turbulence B747, twelve o'clock, 7 miles, passing FL 120 climbing.

PT ABC, caution, wake turbulence from Lear Jet departing to Campinas.

SUPER aircraft call for extra attention²⁰:

• Warn the aircraft following a SUPER that is climbing or descending within 15 NM of it.
• If a pilot requests a lateral offset, more separation or another level to avoid the wake of a SUPER crossing their route, accommodate it whenever possible.