AIM - AIR 2. 9. - Vortex Strength

Wake Turbulence

Wake turbulence is caused by wing tip vortices and is a by-product of lift. The higher
air pressure under the wings tries to move to the lower air pressure on top of the wings
by flowing towards the wing tips where it rotates and flows into the lower pressure on
top of the wings. This results in a twisting rotary motion which is very pronounced at
the wing tips and continues to spill over the top in a downward spiral. Therefore, the
wake consists of two counter-rotating cylindrical vortices.

Vortex Strength

The strength of these vortices is governed by the shape of the wings, the weight and the
speed of the aircraft; the most significant factor being weight. The greatest vortex
strength occurs under conditions of heavy weight, clean configuration, and slow
speed. Strength of the vortex shows little dissipation at altitude within 2 minutes of the
time of initial formation. Beyond 2 minutes, varying degrees of dissipation occur
along the vortex path; first in one vortex and then in the other. The break-up of
vortices is affected by atmospheric turbulence; the greater the turbulence, the more
rapid the dissipation of the vortices.

Induced Roll

Aircraft flying directly into the core of a vortex will tend to roll with the vortex. The
capability of counteracting the roll depends on the wing span and control
responsiveness of the aircraft. When the wing span and ailerons of a larger aircraft
extend beyond the vortex, counter-roll control is usually effective and the effect of the
induced roll can be minimized. Pilots of short wing span aircraft must be especially
alert to vortex situations even though their aircraft are of the high performance type.

Helicopter Vortices

In the case of a helicopter, similar vortices are created by the rotor blades. However,
the problems created are potentially greater than those caused by a fixed wing aircraft
because the helicopter’s lower operating speeds produce more concentrated wakes
than fixed wing aircraft and the size of the aircraft is not a factor on the intensity of the
vortex. Departing or landing helicopters produce a pair of high velocity trailing
vortices similar to wing tip vortices of large fixed wing aircraft. Pilots of small aircraft
should use caution when operating or crossing behind landing or departing helicopters.

Vortex Avoidance

Avoid the area below and behind other aircraft, especially at low altitude where even a
momentary wake turbulence encounter could be disastrous.

2.9.1 Vortex Characteristics

General

Trailing vortices have characteristics which, when known, will help a pilot visualize
the wake location and thereby take avoidance precautions. Vortex generation starts
with rotation (lifting off of the nosewheel) and will be severe in that airspace
immediately following the point of rotation. Vortex generation ends when the
nosewheel of a landing aircraft touches down.

Because of ground effect and wind, a vortex produced within about 200 feet AGL
tends to be subject to lateral drift movements and may return to where it started.
Below 100 feet AGL, the vortices tend to separate laterally and break up more rapidly
than vortex systems at higher altitude. The vortex sink rate and levelling off process
result in little operational effect between an aircraft in level flight and other aircraft
separated by 1 000 feet vertically. Pilots should fly at or above a heavy jet's flight
path, altering course as necessary to avoid the area behind and below the generating
aircraft. Vortices start to descend immediately after formation and descend at the rate
of 400 to 500 feet per minute for large heavy aircraft and at a lesser rate for smaller
aircraft, but in all cases, descending less than l 000 feet in total in 2 minutes.

Vortices spread out at a speed of about 5 KT. Therefore, a crosswind will decrease the
lateral movement of the upwind vortex and increase the movement of the downwind
vortex. Thus, a light wind of 3 to 7 KT could result in the upwind vortex remaining in
the touchdown zone for a period of time or hasten the drift of the downwind vortex
toward another runway. Similarly, a tail wind condition can move the vortices of the
preceding landing aircraft forward into the touchdown zone.

Since vortex cores can produce a roll rate of 80° per second or twice the capabilities of
some light aircraft and a downdraft of 1 500 feet per minute which exceeds the rate of
climb of many aircraft, the following precautions are recommended.
Pilots should be particularly alert in calm or light wind conditions where the vortices could:

    (a) remain in the touchdown area;

    (b) drift from aircraft operating on a nearby runway;

    (c) sink into takeoff or landing path from a crossing runway;

    (d) sink into the traffic pattern from other runway operations;

    (e) sink into the flight path of VFR flights at 500 feet AGL and below.

AIP - AIR 2. 9. 2 - Wake Turbulence / Considerations

On the ground

(1)       Before requesting clearance to cross a live runway, wait a few minutes when a
            large aircraft has just taken off or landed.

(2)       When holding near a runway, expect wake turbulence.

Takeoff

(1)       When cleared to takeoff following the departure of a large aircraft, plan to
            become airborne prior to the point of rotation of the preceding aircraft and stay
            above the departure path or request a turn to avoid the departure path.

(2)       When cleared to takeoff following the landing of a large aircraft, plan to
            become airborne after the point of touchdown of the landing aircraft.

Enroute VFR

(1)        Avoid flight below and behind a large aircraft. If a large aircraft is observed
            along the same track (meeting or overtaking), adjust position laterally
            preferably upwind.

Landing

(1)       When cleared to land behind a departing aircraft, plan to touchdown prior to
            reaching the rotation point of the departing aircraft.

(2)       When behind a large aircraft landing on the same runway, stay at or above the
            preceding aircraft’s final approach flight path, note the touchdown point and
            land beyond this point if it is safe to do so.

(3)        When cleared to land behind a large aircraft on a low approach or on a missed
            approach on the same runway, beware of vortices that could exist between the
            other aircraft's flight path and the runway surface.

(4)        When landing after a large aircraft on a parallel runway closer than 2 500 feet,
             beware of possible drifting of the vortex on to your runway. Stay at or above
             the large aircraft's final approach flight path, note his touchdown point and land
             beyond if it is safe to do so.

(5)        When landing after a large aircraft has departed from a crossing runway, note
             the rotation point. If it is past the intersection, continue the approach and land
             before the intersection. If the large aircraft rotates prior to the intersection,
             avoid flight below the large aircraft's flight path. Abandon the approach unless
             a landing is assured well before reaching the intersection.

ATC will use the words “CAUTION – WAKE TURBULENCE” to alert pilots to the
possibility of wake turbulence. It is the pilots' responsibility to adjust their operations
and flight path to avoid wake turbulence.

Air traffic controllers apply separation minima between aircraft. See RAC 4.1.1 for
these procedures which are intended to minimize the hazards of wake turbulence.

An aircraft conducting an IFR final approach should remain on glide path as the
normally supplied separation should provide an adequate wake turbulence buffer.
However, arriving VFR aircraft, while aiming to land beyond the touchdown point of a
preceding heavy aircraft, should be careful to remain above its flight path. If
extending flight path, so as to increase the distance behind an arriving aircraft, one
should avoid the tendency to develop a dragged-in final approach. Pilots should
remember to apply whatever power is required to maintain altitude until reaching a
normal descent path. The largest number of dangerous encounters have been reported
in the last half mile of the final approach.

Be alert to adjacent large aircraft operations particularly upwind of your runway. If an
intersection takeoff clearance is received, or parallel and cross runway operations are
in progress, avoid subsequent heading which will result in your aircraft crossing below
and behind a large aircraft.


NOTES     1:     If any of the procedures are not possible and you are on the ground,
                          WAIT! (2 minutes are usually sufficient). If on an approach, consider
                          going around for an other approach.

                   2:    See AIR 1.7 for Jet and Propeller Blast Danger.