Wake turbulence and separation

How wake turbulence forms

Every aeroplane trails wake turbulence for as long as it produces lift. It consists of two counter-rotating vortices. According to EASA, they can persist for several minutes behind the aeroplane and descend naturally. How they move depends strongly on the wind and the weather. EASA points out that the so-called “heavy” and “super heavy” aircraft are typically more prone to generate stronger vortices.

The FAA manual AIM adds detail: the vortices form from rotation on take-off until touchdown. Their strength depends on weight, speed, wingspan and wing shape. Flaps and other wing configuring devices change the vortices, and a “dirty” configuration makes them decay faster. They are strongest when the generating aircraft is heavy, clean and slow. EASA’s CPL and ATPL learning objectives say the same.

According to the AIM, the danger is the rolling moment, which can exceed the roll control of the aircraft that meets the wake. Pilots of short-span aircraft must be especially alert.

How the vortices behave

This section describes physics and relies on the FAA Aeronautical Information Manual (AIM, chapter 7, section 4), not on EASA rules.

• The vortices of larger aircraft sink at several hundred feet per minute, weaken as they do so and drift with the wind.
• Close to the ground (within 100 to 200 ft) they move sideways over the ground at 2 to 3 kt.
• In a light wind with a cross-runway component of 1 to 5 kt, the upwind vortex can stay in the touchdown zone for some time. A tailwind can carry the vortices of the preceding aircraft forward into the touchdown zone.

In its CPL and ATPL learning objectives, EASA also expects you to describe the wind conditions that are worst for wake turbulence near the ground.

The four EASA categories

The category depends on the maximum certificated take-off mass (AMC1 SERA.8012):

Category	Maximum take-off mass
SUPER (J)	aircraft types listed as such in ICAO Doc 8643
HEAVY (H)	136,000 kg or more
MEDIUM (M)	less than 136,000 kg but more than 7,000 kg
LIGHT (L)	7,000 kg or less

An aeroplane with a maximum take-off mass of 1,100 kg is therefore in category LIGHT. Aircraft in the HEAVY and SUPER categories add “heavy” or “super” directly after their call sign in the first radio contact with an air traffic services (ATS) unit (SERA.14090).

Separation by air traffic control

Air traffic control (ATC) applies wake turbulence separation minima in the approach and departure phases of flight (SERA.8012) when:

• an aircraft is operating directly behind another at the same altitude or less than 300 m (1,000 ft) below it,
• both aircraft are using the same runway or parallel runways less than 760 m (2,500 ft) apart, or
• an aircraft is crossing behind another at the same altitude or less than 300 m (1,000 ft) below it.

EASA gives minimum times for this (AMC2 and AMC3 to ATS.TR.220). For a LIGHT aircraft following a larger one:

Preceding aircraft	Landing	Take-off, same runway	Take-off, from an intermediate part of the runway
SUPER	4 minutes	3 minutes	4 minutes
HEAVY	3 minutes	2 minutes	3 minutes
MEDIUM	3 minutes	2 minutes	3 minutes

The landing values do not apply to arriving VFR flights. The take-off values apply to the same runway and to parallel runways less than 760 m apart.

What applies to VFR arrivals

The separation in SERA.8012(a) does not apply to arriving VFR flights. Instead the air traffic control unit must issue a caution for wake turbulence, on the radio with the phrase CAUTION WAKE TURBULENCE. On a visual approach behind an aircraft of a heavier wake turbulence category, EASA says the pilot-in-command (PIC) is responsible for ensuring that the spacing is acceptable. If more spacing is needed, the pilot should inform ATC and state the requirement.

In the air you are responsible too: an aircraft that must keep out of the way of another under the right-of-way rules shall avoid passing over, under or in front of it, unless it passes well clear and takes into account the effect of wake turbulence (SERA.3210(c)).

What the EASA syllabus asks for

The PPL(A) syllabus lists three points on wake turbulence under operational procedures: cause, relevant parameters, and actions when crossing traffic, during take-off and landing. Under principles of flight, the wake turbulence behind an aeroplane is also part of the syllabus, with its causes, distribution and duration. In the CPL and ATPL learning objectives, EASA names separation as the action to avoid wake turbulence.