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
Page 2 Par 4 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.
Par 6 Page 3 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).
Page 4 Par 6 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).
Par 7 Page 5 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).
Page 6 Par 7 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.
Par 7 Page 7 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,
Par 8Page 8
(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.
Par 9 Page 9 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
Page 10 Par 9
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.
Par 9 Page 11 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
Page 12 Par 9 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.