Aircraft Wake Turbulence
AC NO: 90-23E
Date: October 1, 1991
Initiated by: AFS-430
Department of Transportation
Federal Aviation Administration
Every aircraft in flight generates a wake. Historically, when pilots
encountered this wake in flight, the disturbance was attributed to
"prop wash." It is known, however, that this disturbance is
caused by a pair of counter rotating vortices trailing from the wing
tips. The vortices from large aircraft pose problems to encountering
aircraft. For instance, the wake of these aircraft can impose rolling
moments exceeding the roll control capability of some aircraft. Further,
turbulence generated within the vortices, if encountered at close range,
can damage aircraft components and equipment and cause personal
injuries. The pilot must learn to envision the location of the vortex
wake generated by large aircraft and adjust his flight path accordingly.
- VORTEX GENERATION.
Lift is generated by the creation of a pressure differential over the
wing surfaces. The lowest pressure occurs over the upper wing surface
and the highest pressure under the wing. This pressure differential
triggers the rollup of the airflow aft of the wing resulting in swirling
air masses trailing downstream of the wingtips. After the rollup is
completed, the wake consists of two counter rotating cylindrical
vortices (see figure 1). Most of the energy is within a few feet of the
center of each vortex, but pilots should avoid a region within about 100
feet of the vortex core.
The strength of the vortex is governed by the weight, speed, and shape
of the wing of the generating aircraft. The vortex characteristics of
any given aircraft can also be changed by extension of flaps or other
wing configuring devices. However, as the basic factor is weight, the
vortex strength increases proportionately with increase in aircraft
operating weight. Peak vortex tangential speeds up to almost 300 feet
per second have been recorded. The greatest vortex strength occurs when
the generating aircraft is heavy-clean-slow. Figure 2 shows smoke
visualization of a vortex photographed during early smoke tower fly-by
- In rare instances, a wake encounter could cause in-
flight structural damage of catastrophic proportions. However, the
usual hazard is associated with induced rolling moments which can
exceed the rolling capability of the encountering aircraft. In
flight experiments, aircraft have been intentionally flown directly
up trailing vortex cores of large aircraft. It was shown that the
capability of an aircraft to counteract the roll imposed by the wake
vortex primarily depends on the wing span and counter-control
responsiveness of the encountering aircraft.
- Counter-control is usually effective and induced roll
minimal in cases where the wing span and ailerons of the
encountering aircraft extend beyond the rotational flow field of the
vortex. It is more difficult for aircraft with short wing span
(relative to the generating aircraft) to counter the imposed roll
induced by vortex flow. Pilots of short span aircraft, even of the
high performance type, must be especially alert to vortex
encounters. The wake of larger aircraft requires the respect of all
pilots. (See figures 3 and 4.)
Trailing vortices have certain behavioral characteristics which can help
a pilot visualize the wake location and thereby take avoidance
OPERATIONAL PROBLEM AREAS.
are generated from the moment aircraft leave the ground, since
trailing vortices are a by-product of wing lift. Prior to takeoff or
landing, pilots should note the rotation or touchdown point of the
preceding aircraft. (See figure 5.)
- The vortex circulation is outward, upward and around
the wing tips when viewed from either ahead or behind the aircraft.
Tests with large aircraft have shown that the vortices remain spaced
a bit less than a wing span apart drifting with the wind, at
altitudes greater than a wing span from the ground. In view of this,
if persistent vortex turbulence is encountered, a slight change of
altitude and lateral position (preferably upwind) will provide a
flight path clear of the turbulence.
tests have shown that the vortices from larger (transport category)
aircraft sink at a rate of several hundred feet per minute, slowing
their descent and diminishing in strength with time and distance
behind the generating aircraft. Atmospheric turbulence hastens
breakup. Pilots should fly at or above the preceding aircraft's
flightpath, altering course as necessary to avoid the area behind
and below the generating aircraft. However, vertical separation of
1,000 feet may be considered safe. (See figure 6.)
- When the vortices of larger aircraft sink close to the
ground (within 100 to 200 feet), they tend to move laterally over
the ground at a speed of 2 or 3 knots. (Figure 7.)
- A crosswind will decrease the lateral movement of the
upwind vortex and increase the movement of the downwind vortex
(Figure 8). Thus, a light wind with a cross-runway component of 1 to
5 knots (depending on conditions) could result in the upwind vortex
remaining in the touchdown zone for a period of time (figure 9) and
hasten the drift of the downwind vortex toward another runway.
Similarly, a tailwind condition can move the vortices of the
preceding aircraft forward into the touchdown zone. The light
quartering tailwind requires maximum caution. Pilots should be alert
to large aircraft upwind from their approach and takeoff
A wake encounter is not necessarily hazardous. It can be one or more
jolts with varying severity depending upon the direction of the
encounter, weight of the generating aircraft, size of the encountering
aircraft, distance from the generating aircraft, and point of vortex
encounter. The probability of induced roll increases when the
encountering aircraft's heading is generally aligned or parallel with
of the generating aircraft. Avoid the area below and behind the
generating aircraft, especially at low altitude where even a momentary
wake encounter could be hazardous. Pilots should be particularly
alert in calm wind conditions and situations where the vortices could:
- Remain in the touchdown area.
- Drift from aircraft operating on a nearby runway.
- Sink into takeoff or landing path from a crossing
- Sink into the traffic patterns from other airport
- Sink into the flight path of VFR flights operating at
the hemispheric altitudes 500 feet below.
- Pilots of all aircraft should visualize the location
of the vortex trail behind large aircraft and use proper vortex
avoidance procedures to achieve safe operation. It is equally
important that pilots of larger aircraft plan or adjust their flight
paths to minimize vortex exposure to other aircraft.
VORTEX AVOIDANCE PROCEDURES.
Under certain conditions, airport traffic controllers apply procedures
for separating aircraft operating under Instrument Flight Rules. The
controllers will also provide to VFR aircraft, with whom they are in
communication and which in the tower's opinion may be adversely affected
by wake turbulence from a larger aircraft, the position, altitude and
direction of flight of larger aircraft followed by the phrase
"caution - wake turbulence." Whether or not a warning has been
given, however, the pilot is expected to adjust his/her operations and
as necessary to preclude serious wake encounters. The following vortex
avoidance procedures are recommended for the situation shown:
-- note his touchdown point.
When landing behind a larger aircraft - same runway (figure 10),
stay at or above the large aircraft's final approach flight path --
note touchdown point -- land beyond it.
- When landing behind a larger aircraft - when parallel
runway is closer than 2,500 feet (figure 11), consider possible
vortex drift onto your runway. If you have visual contact with the
larger aircraft landing on the parallel runway, whenever possible,
stay at or above the large aircraft's final approach
When landing behind a larger aircraft - crossing runway
(figure 12), cross above the larger aircraft's
When landing behind a departing larger aircraft - same
runway (figure 13), note larger aircraft's rotation point -- land well
prior to rotation point.
When landing behind a departing larger aircraft -
crossing runway, note larger aircraft's rotation point -- if past the
intersection -- continue the approach -- land prior to the
intersection (figure 14). If larger aircraft rotates prior to the
intersection, avoid flight below the larger aircraft's
Abandon the approach unless a landing is ensured well before reaching
the intersection (figure 15).
When departing behind a larger aircraft: Note larger
aircraft's rotation point -- rotate prior to larger aircraft's
rotation point -- continue climb above the larger aircraft's climb
path until turning clear of his wake (Figure 16). Avoid subsequent
headings which will cross below and behind aircraft (figure 17). Be
alert for any critical takeoff situation which could lead to a vortex
Intersection takeoffs - same runway, be alert to
adjacent large aircraft operations particularly upwind of your runway.
If intersection takeoff clearance is received, avoid subsequent
heading which will cross below a larger aircraft's path.
Departing or landing after a larger aircraft executing a
low missed approach or touch-and-go landing. Because vortices settle
and move laterally near the ground, the vortex hazard may exist along
the runway and in your flight path after a larger aircraft has
executed a low missed approach or a touch-and-go landing, particularly
in light quartering wind conditions. You should assure that an
interval of at least 2 minutes has elapsed before your takeoff or
En route VFR - (1,000-foot altitude plus 500 feet).
Avoid flight below and behind a larger aircraft's path. If a larger
aircraft is observed above on the same track (meeting or overtaking),
adjust your position laterally, preferably upwind.
A hovering helicopter generates a downwash from its main rotor(s)
similar to the "prop wash" of a conventional aircraft.
However, in forward flight, this energy is transformed into a pair of
strong, high-speed trailing vortices similar to wing-tip vortices of
larger fixed-wing aircraft. Pilots should avoid helicopter vortices
since helicopter forward flight airspeeds are often very low which
generate exceptionally strong vortices (figure 18).
JET ENGINE EXHAUST.
During ground operations, jet engine blast (thrust stream turbulence)
can cause damage and upsets if encountered at close range. Exhaust
velocity versus distance studies at various thrust levels have shown a
need for light aircraft to maintain an adequate separation during ground
operations (figure 19).
Engine exhaust velocities, generated by larger jet aircraft during
ground operations and initial takeoff roll, dictate the desirability
of lighter aircraft awaiting takeoff to hold well back of the runway
edge at the taxiway hold line. Also, it is desirable to align the
aircraft to face any possible jet engine blast effects.
Additionally, in the course of running up engines and taxiing on the
ground, pilots of larger aircraft should consider the effects of
their jet blasts on other aircraft, vehicles, and maintenance and
servicing equipment. An illustration of exhaust velocities behind a
typical "wide-body" or jumbo jet is shown in figure 19.
- The Federal Aviation Administration has established
standards for the location of runway hold lines. For example, runway
intersection hold short lines are established 250 feet from the
runway centerline for precision approach runways served by approach
category C and D aircraft. For runways served by aircraft with
wingspans over 171 feet, such as the B-747, taxiway hold lines are
280 feet from the centerline of precision approach runways. These
hold line distances increases slightly with an increase in field
Government and industry groups are making concerted efforts to minimize
or eliminate the hazards of trailing vortices. However, the flight
disciplines necessary to ensure vortex avoidance during visual
operations must be exercised by the pilot. Vortex visualization and
avoidance procedures should be exercised by the pilot using the same
degree of concern as in collision avoidance since vortex encounters
frequently can be as dangerous as collisions.
- Pilots are reminded that in operations conducted
behind all aircraft, acceptance from Air Traffic Control of traffic
information, instructions to follow an aircraft, or the acceptance
of a visual approach clearance, is an acknowledgment that the pilot
will ensure safe takeoff and landing intervals and accepts the
responsibility of providing his own wake turbulence separation.
- For VFR departures behind heavy aircraft, air traffic
controllers are required to use at least a 2-minute separation
interval unless a pilot has initiated a request to deviate from the
2-minute interval and has indicated acceptance of responsibility for
maneuvering his aircraft so as to avoid the wake turbulence hazard.
Operational Tips for Light Aircraft
How to Avoid Vortex Wake
- Lift Off Short of Large Aircraft Rotation
- Land Well Beyond Large Aircraft Touchdown
- Pass Over Flight Path of Large Aircraft, or At
Least 1000' Under.
- Stay to Windward of Large Aircraft Flight
- Keep Alert, Especially on Calm Days When
Vortices Persist Longest.
David S. Potter
Acting Director, Flight Standards Service
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