Subject: AIRCRAFT WAKE

TURBULENCE

Date: 02/20/02

Initiated by: AFS-430

AC No.: 90-23F

Change:

1. PURPOSE. This advisory circular (AC) is intended to alert pilots to the hazards of aircraft wake

turbulence and recommends related operational procedures.

2. CANCELLATION. AC 90-23E, Aircraft Wake Turbulence, dated October 1, 1991, is

canceled.

3. INTRODUCTION. Every aircraft in flight generates a wake. Historically, when pilots

encountered this wake 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 control authority of the encountering aircraft. Further,

turbulence generated within the vortices 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 larger (transport category) aircraft and adjust his/her flight path accordingly.

4. 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
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swirling air masses trailing downstream of the wing-tips. After the rollup is completed, the wake

consists of two counterrotating 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.

FIGURE 1. THE ROLLING UP PROCESS

5. VORTEX STRENGTH. 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.

6. INDUCED ROLL.

a. 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 roll control capability of the encountering aircraft. In flight experiments, aircraft have been

intentionally flown directly up trailing vortex cores of larger aircraft. It was shown that the capability

of an aircraft to counteract the roll imposed by the wake vortex primarily depends on the wingspan and

counter-control responsiveness of the encountering aircraft.
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FIGURE 2. INDUCED ROLL

b. Counter-control is usually effective and induced roll minimal in cases where the wingspan and

ailerons of the encountering aircraft extend beyond the rotational flow field of the vortex. It is more

difficult for aircraft with short wingspans (relative to the vortex 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 2 and 3).
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FIGURE 3. WAKE VS. AIRCRAFT WEIGHT

7. VORTEX BEHAVIOR. Trailing vortices have certain behavioral characteristics which can help

pilots visualize the wake location and thereby take avoidance precautions.

a. Vortices 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 4).
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FIGURE 4. ROTATION AND TOUCHDOWN POINTS OF PRECEDING AIRCRAFT

b. 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 wingspan apart, drifting with the wind at altitudes greater than a wingspan 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.

c. Flight 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 flight path, 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 5).
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FIGURE 5. DESCENT OF VORTICES FROM LARGER AIRCRAFT

d. 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 (see figure 6).

FIGURE 6. MOVEMENT OF VORTICES FROM LOW FLYING LARGE AIRCRAFT

e. A crosswind will decrease the lateral movement of the upwind vortex and increase the

movement of the downwind vortex (figure 7). 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 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

larger aircraft upwind from their approach and takeoff flight paths.
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FIGURE 7. EFFECT OF 3-KNOT CROSSWIND ON VORTICES FROM LOW FLYING

AIRCRAFT

FIGURE 8. EFFECT OF 6-KNOT CROSSWIND ON VORTICES FROM LOW FLYING

AIRCRAFT

8. OPERATIONAL PROBLEM AREAS.

a. 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 the flight path 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 maneuvering situations in the vicinity of the

airport where the vortices could:

(1) remain in the touchdown area,

(2) drift from aircraft operating on a nearby runway,
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(3) sink into takeoff or landing path from crossing runway,

(4) sink into the traffic patterns from other airport operations, or

(5) sink into the flight path of aircraft operating under Visual Flight Rules (VFR) and at

hemispheric altitudes 500 feet below.

b. Pilots of all aircraft should visualize the location of the vortex trail behind larger 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, whenever possible, to minimize vortex exposure to other

aircraft.

9. VORTEX AVOIDANCE PROCEDURES. Under certain conditions, airport traffic

controllers apply procedures for separating IFR aircraft. If a pilot accepts a clearance to visually

follow a preceding aircraft, the pilot accepts responsibility for separation and wake turbulence

avoidance. 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.” After issuing the caution for wake

turbulence, the airport traffic controllers generally do not provide additional information to the

following aircraft unless the airport traffic controllers know the following aircraft is overtaking

the preceding aircraft. WHETHER OR NOT A WARNING OR INFORMATION HAS BEEN

GIVEN, HOWEVER, THE PILOT IS EXPECTED TO ADJUST AIRCRAFT OPERATIONS

AND FLIGHT PATH AS NECESSARY TO PRECLUDE SERIOUS WAKE ENCOUNTERS.

When any doubt exists about maintaining safe separation distances between aircraft to avoid

wake turbulence, pilots should ask the control tower for updates on separation distance and

aircraft groundspeed.

a. When landing behind a larger aircraft - same runway (figure 9) - stay at or above the larger

aircraft’s final approach flight path. Note touchdown point - land beyond it.
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FIGURE 9. AVOIDANCE PROCEDURES LANDING BEHIND LARGER AIRCRAFT ON

THE SAME RUNWAY

b. When landing behind a larger aircraft - when parallel runway is closer than 2,500 feet

(figure 10) - 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 larger aircraft’s final

approach flight path. Note its touchdown point.

FIGURE 10. AVOIDANCE PROCEDURE LANDING ON PARALLEL RUNWAYS CLOSER

THAN 2,500 FEET
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c. When landing behind a larger aircraft - crossing runway (figure 11) - cross above the larger

aircraft’s flight path.

FIGURE 11. AVOIDANCE PROCEDURE FOR LANDING BEHIND

LARGER AIRCRAFT THAT IS USING A CROSSING RUNWAY

d. When landing behind a departing larger aircraft - same runway (figure 12) - note larger

aircraft’s rotation point, and land well before the rotation point.
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FIGURE 12. AVOIDANCE WHEN LANDING BEHIND A DEPARTING AIRCRAFT ON

THE SAME RUNWAY

e. When landing behind a departing larger aircraft - crossing runway - note larger aircraft’s

rotation point. If rotation is past the intersection, continue the approach and land before the

intersection (figure 13). If larger aircraft rotates prior to the intersection, avoid flight below the larger

aircraft’s flight path. Abandon the approach unless a landing is ensured well before reaching the

intersection (figure 14).

FIGURE 13. AVOIDANCE FOR LANDING BEHIND DEPARTING LARGER AIRCRAFT

ON A CROSSING RUNWAY ROTATION POINT PAST THE INTERSECTION
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FIGURE 14. AVOIDANCE FOR LANDING WHEN LARGER DEPARTING AIRCRAFT

ROTATES PRIOR TO THE INTERSECTION

f. When departing behind a larger aircraft - same runway - note larger aircraft’s rotation point, and

rotate prior to larger aircraft’s rotation point. Continue climb above the larger aircraft’s climb path

until turning clear of this wake (figure 15). Avoid subsequent headings which will cross below and

behind aircraft (see figure 16). Be alert for any critical takeoff situation which could lead to a vortex

encounter.

FIGURE 15. DEPARTING SAME RUNWAY BEHIND A LARGER AIRCRAFT
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FIGURE 16. CRITICAL TAKEOFF SITUATION AND CROSSING DEPARTURE COURSES

g. 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.

h. 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 ensure that an

interval of at least 2 minutes has elapsed before your takeoff or landing.

i. 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.

10. HELICOPTERS. 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 wake turbulence (see figure 17).
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FIGURE 17. HELICOPTER VORTICES

11. JET ENGINE EXHAUST.

a. 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.
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b. 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.

12. PILOT RESPONSIBILITY.

a. 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 VFR 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.

b. Pilots are reminded that in operations conducted behind all aircraft, acceptance of instructions

from air traffic control (ATC) in the following situations is an acknowledgment that the pilot will

ensure safe takeoff and landing intervals, and accepts the responsibility for providing wake turbulence

separation.

(1) Traffic information,

(2) Instructions to follow an aircraft, and

(3) The acceptance of a visual approach clearance.

c. For operations conducted behind heavy aircraft, ATC will specify the word “heavy” when this

information is known. Pilots of heavy aircraft should always use the word “heavy” in radio

communications.

d. Heavy and large jet aircraft operators should use the following procedures during an approach

to landing. These procedures establish a dependable baseline from which pilots of in-trail, lighter

aircraft may reasonably expect to make effective flight path adjustments to avoid serious wake vortex

turbulence.

(1) Pilots of aircraft that produce strong wake vortices should make every attempt to fly on the

established glidepath, not above it; or, if glidepath guidance is not available, to fly as closely as

possible to a “3 to 1” glidepath, not above it.

EXAMPLE:

Fly 3,000 feet at 10 miles from touchdown, 1,500 feet at 5 miles, 1,200 feet

at 4 miles, and so on to touchdown.
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(2) Pilots of aircraft that produce strong wake vortices should fly as closely as possible to

approach course centerline or to the extended centerline of the runway of intended landing as

appropriate to conditions.

e. Pilots operating lighter aircraft on visual approaches in-trail to aircraft producing strong wake

vortices should use the following procedures to assist in avoiding wake turbulence. These procedures

apply only to those aircraft that are on visual approaches.

(1) Pilots of lighter aircraft should fly on or above the glidepath. Glidepath reference may be

furnished by an ILS, by a visual approach slope system, by other ground-based approach slope

guidance systems, or by other means. In the absence of visible glidepath guidance, pilots may very

nearly duplicate a 3-degree glideslope by adhering to the “3 to 1” glidepath principles.

EXAMPLE:

Fly 3,000 feet at 10 miles from touchdown, 1,500 feet at 5 miles, 1,200 feet at

4 miles, and so on to touchdown.

(2) If the pilot of the lighter following aircraft has visual contact with the preceding heavier

aircraft and also with the runway, the pilot may further adjust for possible wake vortex turbulence by

the following practices:

(a) Pick a point of landing no less than 1,000 feet from the arrival end of the runway.

(b) Establish a line-of-sight to that landing point that is above and in front of the heavier

preceding aircraft.

(c) When possible, note the point of landing of the heavier preceding aircraft and adjust

point of intended landing as necessary.

EXAMPLE:

A puff of smoke may appear at the 1,000-foot markings of the runway, showing

that touchdown was that point; therefore, adjust point of intended landing to

the 1,500-foot markings.

(d) Maintain the line-of-sight to the point of intended landing above and ahead of the

heavier preceding aircraft; maintain it to touchdown.

(e) Land beyond the point of landing of the preceding heavier aircraft.

(3) During visual approaches pilots may ask ATC for updates on separation and groundspeed

with respect to heavier preceding aircraft, especially when there is any question of safe separation from

wake turbulence.
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(4) Aircraft Classes. For the purposes of Wake Turbulence Separation Minima, ATC

classifies aircraft as Heavy, Large, and Small as follows:

(a) Heavy-Aircraft capable of takeoff weights of more than 255,000 pounds whether or not

they are operating at this weight during a particular phase of flight.

(b) Large-Aircraft of more than 41,000 pounds, maximum certificated takeoff weight, up

to 255,000 pounds.

(c) Small-Aircraft of 41,000 pounds or less maximum certificated takeoff weight.

13. PILOT AWARENESS INTERVENTION. There is a small segment of the aviation community

that have become convinced that wake vortices may “bounce” up to twice their nominal steady state

height (with a 200-foot span aircraft the “bounce” height could reach approximately 200 feet above

ground level (AGL)). This conviction is based on a single unsubstantiated report of an apparent

coherent vortical flow that was seen in the volume scan of a research sensor. No one can say what

conditions cause vortex bouncing, how high they bounce, at what angle they bounce, nor how many

times a vortex may bounce. On the other hand, no one can say for certain that vortices never

“bounce.” Test data have shown that vortices can rise with the air mass in which they are embedded.

Wind shear, particularly, can cause vortex flow field “tilting.” Also, ambient thermal lifting and

orographic effects (rising terrain or tree lines) can cause a vortex flow field to rise. In view of the

foregoing, pilots are reminded that they should be alert at all times for possible wake vortex encounters

when conducting approach and landing operations. The pilot has the ultimate responsibility for

ensuring appropriate separations and positioning of the aircraft in the terminal area to avoid the wake

turbulence created by a preceding aircraft.

James J. Ballough

Director, Flight Standards Service