# Wake Turbulence Information on wake turbulence is placed in this chapter as aviators are likely to experience turbulent conditions while operating around other aircraft. Successful aviators understand and recognize conditions conducive to wake turbulence and take appropriate countermeasures. Larger aircraft create more turbulence and are greater hazards. *** ## In-Flight Hazard Every aircraft in flight generates wake turbulence. This disturbance is caused by a pair of counter-rotating vortices trailing from the wing tips. It is possible the wake of another aircraft can impose rolling moments exceeding the control authority of the aircraft. Additionally, if encountered at close range, wake turbulence can damage the aircraft and/or cause personal injury to the occupants. It is important to imagine the location of the vortex wake generated by other aircraft and adjust the flight path accordingly. *** ## Ground Hazard Hazardous turbulence is not only encountered in the air. During ground operations and takeoff, jet engine blast (thrust stream turbulence) and rotorwash can cause damage and disturbance if encountered at close range. Exhaust velocity versus distance studies at various thrust levels have shown a need for light aircraft and helicopters to maintain an adequate separation behind large turbojet aircraft. *** ## Vortex Generation Lift is generated by the creation of a pressure differential over the wing surfaces. This pressure differential triggers rollup of airflow aft of the wing resulting in swirling air masses trailing downstream of the wingtips. After rollup is completed, the wake consists of two counter-rotating cylindrical vortices. Most of the energy is within a few feet of the center of each vortex; however, pilots must avoid the region within approximately 100 feet of the vortex core. ### Strength Vortex strength is governed by the weight, speed, and shape of the generating aircraft’s wing. The basic factor is weight; vortex strength increases proportionately with an increase in aircraft operating weight. Peak vortex speeds up to almost 300 feet per second have been recorded. The greatest vortex strength occurs when the generating aircraft is heavy, clean, and slow. ### Behavior Trailing vortices have certain behavioral characteristics which can help aviators visualize wake location and take avoidance precautions. * Vortices are generated from the moment an aircraft leaves the ground because 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. * Vortex circulation is outward, upward, and around the wing tips when viewed either ahead or behind the aircraft. If persistent vortex turbulence is encountered, a slight change of altitude and lateral position (preferably upwind) should provide a flight path clear of the turbulence. * Flight tests have shown vortices from aircraft sink at a rate of up to several hundred FPM, slowing their descent and diminishing in strength with time and distance behind the generating aircraft. Atmospheric turbulence hastens breakup. Aviators should fly at or above the preceding aircraft’s flight path, altering their course as necessary to avoid the area behind and below the generating aircraft. * A crosswind decreases lateral movement of the upwind vortex and increases movement of the downwind vortex. This results in the upwind vortex remaining in the touchdown zone for a period of time and increases drift of the downwind vortex toward another runway. *** ## Induced Roll and Counter Control Army aircrews should be aware of the following conditions to avoid the adverse effects of larger aircraft vortices. The available power may be insufficient to counter turbulent air near the runway environment. ### **Induced Roll** In rare instances, a wake encounter can cause in-flight structural damage of catastrophic proportions. The most common hazard is associated with induced rolling moments which can exceed the roll control capability of the encountering aircraft. During flight tests, aircraft have been intentionally flown directly up trailing vortex cores of larger aircraft. These tests prove the capability of an aircraft to counteract the roll imposed by the wake vortex primarily depends on wing span and counter control responsiveness of the encountering aircraft. ### **Counter Control** Counter control is usually effective and induced roll is minimal in cases where the encountering aircraft extends outside the affected area of the vortex. It is more difficult for aircraft smaller than the aircraft generating the vortex to counter the imposed roll induced by vortex flow. Although aviators of short span aircraft and helicopters must be especially alert to vortex encounters, the wake of larger aircraft requires the respect of all aviators. *** ## Operational Problem Areas Wake turbulence encounters can result in one or more jolts with varying severity depending on several factors, including 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. **Avoidance Strategies:** * Pilots should avoid the area below and behind the preceding aircraft, especially at low altitude where even a momentary wake encounter could be hazardous. * Aviators must be particularly alert in calm wind conditions and maneuvering situations in the vicinity of the airfield where vortices could remain in the touchdown area, drift from aircraft operating on nearby runways, sink into takeoff or landing paths from crossing runways, sink into traffic patterns from other airport operations, or sink into the flight path of aircraft operating VFR. Aviators should visualize the location of the vortex trail behind a larger aircraft/helicopter and use proper vortex avoidance procedures to achieve safe operation. Whenever possible, it is equally important for aviators of larger aircraft/helicopters to plan or adjust their flight paths to minimize vortex exposure to other aircraft. ### Helicopters In a slow hover taxi or stationary hover near the surface, helicopter main rotor systems generate downwash producing high-velocity outwash vortices to a distance approximately three times the diameter of the rotor. Aviators should avoid operating within three rotor diameters of any helicopter in a slow hover taxi or stationary hover. In forward flight, departing or landing helicopters produce a pair of strong, high-speed trailing vortices similar to wingtip vortices of larger fixed-wing aircraft. Aviators must use caution when operating or crossing behind landing and departing helicopters. ### Jet Engine Exhaust 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. It is also desirable to align the aircraft to face any possible jet engine blast effects. Runway hold lines are established according to FAA standards, with distances varying based on factors such as runway type and the wingspans of aircraft served by the runway. *** ## Vortex Avoidance Techniques Under certain conditions, airport traffic controllers apply procedures for separating aircraft operating under instrument flight rules. They also provide VFR aircraft with the position, altitude, and direction of larger aircraft followed by the phrase “caution–wake turbulence.” Whether or not a warning has been given, aviators are expected to adjust their operations and flight paths as necessary to avoid serious wake encounters. **Recommended Procedures:** **Landing Behind a Larger Aircraft on the Same Runway:** * Stay at or above the larger aircraft's final approach flight path and note the touchdown point, then land beyond it. **Landing Behind and Offset from a Larger Aircraft Landing on a Parallel Runway Closer Than 2,500 Feet:** * Consider possible vortex drift onto your runway, stay at or above the larger aircraft's final approach flight path, note its touchdown point, and land beyond it. **Landing Behind a Larger Aircraft on a Crossing Runway:** * Cross above the larger aircraft's flight path. **Landing Behind a Departing Larger Aircraft on the Same Runway:** * Note the larger aircraft's rotation point and land well prior to the rotation point. **Departing Behind a Larger Aircraft:** * Note the larger aircraft's rotation point and rotate prior, then continue to climb above the larger aircraft's climb path until turning clear of its wake. **Intersection Takeoffs on the Same Runway:** * Remain alert for adjacent large aircraft operations, particularly upwind of the runway. **Departing or Landing After a Larger Aircraft Executes a Low Approach, Missed Approach, or Touch-and-Go Landing:** * Ensure an interval of at least two minutes has elapsed before takeoff or landing. **En Route VFR:** * Avoid flight below and behind a larger aircraft's path, adjusting position laterally if necessary. ### Aviator Responsibility **Flight Discipline** * Government and industry groups are actively working to minimize or eliminate hazards of trailing vortices. However, it is the responsibility of the aviator to exercise the necessary flight discipline to ensure vortex avoidance during VFR operations. Aviators must visualize and employ avoidance procedures for wake turbulence with the same degree of concern as in collision avoidance. **Acceptance of Instructions** * Aviators are reminded that in operations conducted behind all aircraft, acceptance of instructions from ATC implies acknowledgment that the aviator ensures safe takeoff and landing intervals and accepts responsibility for providing wake turbulence separation. This includes: * Traffic information. * Instructions to follow an aircraft. * Acceptance of a visual approach clearance. **Identification of Heavy Aircraft** * When information about heavy aircraft is known, ATC specifies the word "heavy." **VFR Departures Behind Heavy Aircraft:** * For VFR departures behind heavy aircraft, air traffic controllers are required to use at least a 2-minute separation interval. However, an aviator may initiate a request to deviate from the 2-minute interval and indicate acceptance of responsibility for maneuvering the aircraft to avoid wake turbulence hazard. #### **Air Traffic Control Wake Turbulence Separation** Aircrews should exercise caution and adhere to ATC separation requirements, considering extra time and fuel requirements during planning when operating around large aircraft due to wake turbulence. ### Behind Heavy Jets Due to the potential effects of wake turbulence, controllers are mandated to apply specific minimum separation for aircraft operating behind heavy jets and, in certain scenarios, behind large non-heavy aircraft. * **Separation Requirements:** * When operating directly behind a heavy jet at the same altitude or less than 1,000 feet below: * Heavy jet behind heavy jet: **4 miles** * Small/large aircraft behind heavy jet: **5 miles** * Separation for small aircraft, measured when the preceding aircraft is over the landing threshold: * Small aircraft landing behind heavy jet: **6 miles** * Small aircraft landing behind large aircraft: **4 miles** * Departing aircraft separation: * Departures behind a heavy jet require either a **2-minute interval** or the appropriate **4 or 5 mile radar separation** in the following situations: * From the same threshold. * On a crossing runway where projected flight paths cross. * From the threshold of a parallel runway when staggered ahead of the adjacent runway by less than 500 feet, and when runways are separated by less than 2,500 feet. Aviators, after considering potential wake turbulence effects, have the option to specifically request a waiver of the 2-minute interval. Controllers may acknowledge this request as acceptance of responsibility for wake turbulence separation by the aviator, and if traffic conditions permit, they may issue takeoff clearance accordingly. ### Behind Larger Aircraft **3-Minute Interval** * When a small aircraft intends to take off: * From an intersection on the same runway (same or opposite direction) behind a departing large aircraft. * In the opposite direction on the same runway behind a large aircraft takeoff or low/missed approach. * This 3-minute interval may be waived upon specific aviator request. **Exceptions** * Controllers may not reduce or waive the 3-minute interval if the preceding aircraft is a heavy jet and operations are on either the same runway or parallel runways separated by less than 2,500 feet. **Additional Separation Request** * Aviators may request additional separation, specifically two minutes instead of 4 or 5 miles for wake turbulence avoidance. This request should be made as soon as practical on ground control and at least before taxiing onto the runway. **Anticipation by Controllers** * Controllers may anticipate separation and need not withhold a takeoff clearance for an aircraft departing behind a large/heavy aircraft if there is reasonable assurance that required separation exists when the departing aircraft starts its takeoff roll. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://229.gitbook.io/229-hub/knowledge/army-aviation-fundamentals/multi-aircraft-operations/wake-turbulence.md?ask= ``` The question should be specific, self-contained, and written in natural language. 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