Advisory U.S. Deporment of Transportation Circular Federal Autation Administration Subject: AIRCRAFT WAKE TURBULENCE Date: 10/1/91 AC No: 90-23E Initiated by: AFS-430 Change:
IT'S THERE---BUT WHERE ?
DO I GO OVER ? --UNDER ?
BEHIND? --HOW FAR ???
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 90-23D, Aircraft Wake Turbulence, dated 12/15/72.
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 counterrotating vortices trailing from the wing tips. The vortices fron large aircraft pose problems to encountering aircraft. for instance, the wake oi these aircraft can impose rolling moments exceeding the control authority ot the encountering aircraft. further, turbulence generated within the vortices, if encountered at close range, can damage aircraft components and equipment and cause personal injuries.
the vortex wake generated by larger (transport category) aircraft and adjust his/her 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 f essure under the wing. This pressure differential triggers the rollup of he airflow aft of the wing resulting in swirling air masses trailing downstream of the wingtips. After ne rollup is completed, the wake consists of two counterrotating cylindrical vortices (see figure 1). Most of the energy is within a tew teet of the center of each vortex. but pilots should avoid a region within about 100 feet of the vortex core.
FIGURŁ 1. The rolling up process Par 3
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 aircrait 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 tests.
FIGURE 2. Typical vortex flow field outlined by smoke
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 wing span and counter-control responsiveness of the encountering aircraft.
Par 5 3
AC 90-23Е 10/1/91
COUNTER WAKE VORTEX FLOW
CONTROL FIGURE 3. Induced Roll
b. 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 vortex generating aircraft) to counter the imposed roll induced by vortex flow. Pilots of short-span aircraft, even of the high perfors ace type, must be especially alert to vortex encounters. The wake of li ger aircraft requires the respect of all pilots. (See iigures 3 and 1.)
JRE 1. Relative Span Par 6
VORTEX BEHAVIOR. Trailing vortices have certain behavioral characteristics which can help pilot 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 5.)
Touchdown Rotation 8000000000000 000002ac Wake Ends Wake Begins FIGURE 5.
The vortex circulation is outward. upward. and around the wing tips when viewed fros 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 fror the ground. In view of this. if persistent vortex turbulence is encountered.
a slight change of altitude and lateral position (preferably uprind) will provide a flightpath clear of the turbulence.
c. Flight tests have shown that the vortices from larger (transport categoryl 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. )
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. (Figure 7.)
Par 7 5
AVOID I Nominally 500-1000' Sink Rate Several Hundred Ft./Min.
FIGURE 6 ЗК → No Wind Vortex Movement Near Ground - No Wind FIGURE 7
e. 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 of s 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 guartering tailwind requires maximum caution. Pilots should be alert to larger aircraft upwind from their approach and takeoff flightpaths.
6 Par 7
- 3K Wind 6K
- 0 (ЗК-ЗК) (3K+3K) Vortex Movement Near Ground - with Cross Wind FIGURE 8 Tail Wind- Light Touchdown Point Quartering Tailwind 600500006 66666600 FIGURE 9
8. OPERATIONAL PROBLEM AREAS. 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 flightpath of the generating aircraft. Avoid the area below and behind the generating Par 8 7
AC 90-23Е 10/1/91 aircraft, especially at low altitude where even a momentary wake encounter could be hazardous. Pilots should be particularly alert in calr.
wind conditions and maneuvering situations in the vicinity of the airport 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 crossing runway.
d. Sink into the traffic patterns from other airport operations.
e. Sink into the flight path of aircraft operating under Visual Flight Rules and at hemispheric altitudes 500 feet below.
f. 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 flightpaths, whenever possible, to mininize vortex exposure to other aircraft.
9. VORTEX AVOIDANCE PROCEDURES. Unde. certain conditions. airport traffic controllers apply procedures for separating aircraft operating under Instrument Flight Rules. The controllers will also provide to VER 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, nowever, the pilot is expected to adjust his/her operations and flightpath as necessary to preclude serious wake encounters.
procedures are recommended for the situation shown:
a. When landing behind a larger aircraft - same runway ¡gure 10). stay at or above the larger aircraft's final approach flightpath--note touchdown point--iand beyond it.
Wind Touchdown Point → FIGURE 10 Par g
b. 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 larger aircraft s final approach flightpath -- note its touchdown point.
селенем, wind ообобобо, Less than 2500' 00 Touchdown Points FIGURE 11 (figure 12), cross above the larger aircraft's flightpath. When landing behind a larger aircraft - crossing runway ----- FIGURE 12 Par 9
d. When landing behind a departing larger aircraft - same runway (figure 13). note larger aircraft's rotation point--land well prior to rotation point.
Rotation Point' 0000000000 FIGURE 13
e. When landing behind a departing larger aircraft - crossing runway.
note larger aircraft s rotation point--if past the intersection--continue che.
approach--land prior to the intersection (figure 14). If larger aircraft" rotates prior to the intersection. avoid flight below the larger aircraft's flightpath. Abandon the approach unless a landing is ensured well before reaching the intersection (figure 15).
Rotation Point FICURE 14 Par 9
• - Koration Poinı z n0' FIGURE 15
f. 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 this 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 encounter.
Departure - Same Runway Large Airplanes on Small Airplane 00000000 FIGURE 16 Par 9 11
Crossing Departure Courtes d FIGURE 17 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 flightpath 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.
1. En route VER - (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 jaterally, preferably upwind.
12 Par 9