9 Figure 12. Light Quartering Tailwind Wake Vortex Hazard
8 WAK E VORTEX ENCOUNTER RISK. Wake vortices from a generating aircraft may pose a hazard to encountering aircraft of similar and smaller size. The risk of wake vortices is the composite of the likelihood and severity of an encounter. No two wake vortex encounters are identical due to the variables involved that affect both the likelihood and severity of an encounter.
8.1 Factors Affecting the Likelihood of a Wake Vortex Encounter. Wake vortices are generated with differing strengths and decay and drift at differing rates. The time between the generating aircraft and an encountering aircraft is a major factor for the likelihood of an encounter. The likelihood of a wake vortex encounter is dependent upon the proximity of the wake vortices to the flightpath of the potential encountering aircraft and the residual strength of the wake vortices at the time of intercept. The likelihood of a wake vortex encounter may increase when the following occurs:
8.1.1 The encountering aircraft is below the generating aircraft in trail or crosses below and behind the generating aircraft. This includes a generating aircraft climbing or descending in front of and through the flightpath of the encountering aircraft, or the encountering aircraft climbing or descending behind and through the flightpath of the generating aircraft. The closer the encountering aircraft is below and behind the generating aircraft in time and distance, the less time the wake vortex has had to decay and the higher the likelihood of an encounter.
8.1.2 The movement of the wake vortices, including lateral drift and vertical sink, may create scenarios where wake vortices are maintained in a critical area such as the glidepath or the runway, or both, for a longer period. Wake vortices may also drift or sink into adjacent aircraft operations. Examples of drift causing a higher likelihood of encounter include:
8.1.2.1 A light quartering tailwind that resists the outward movement of the upwind vortex and pushes the vortices forward in the approach and runway environment toward the touchdown zone (TDZ).
8.1.2.2 A light wind with a crosswind component of 1 to 5 knots that resists the outward movement of the upwind vortex causing the upwind vortex to remain in the final approach, runway, and TDZ area longer. The crosswind accelerates the downwind vortex away from the runway and, depending on conditions, towards an adjacent runway, approach, or departure area.
8.1.2.3 Wake vortices that may drift and sink into the takeoff or landing flightpaths of a crossing runway or into traffic patterns of adjacent airports.
8.1.2.4 Wake vortices that may sink into en route traffic, even when legally separated
(e.g., aircraft separated by 500 feet vertically with an aircraft maintaining a 14 CFR § 91.159 visual flight rules (VFR) cruising altitude and an opposite direction aircraft maintaining a 14 CFR § 91.179 instrument flight rules (IFR) cruising altitude). Additional exposure may occur when a generating aircraft has transited in a climb or descent in front of the flightpath of an encountering aircraft (or the encountering aircraft climbs or descends behind the generating aircraft). See Figure 13, Climbing Aircraft Wake Vortex Sink.
Figure 13. Climbing Aircraft Wake Vortex Sink
8.2 F actors Affecting the Severity of a Wake Vortex Encounter. Wake vortex encounters present a spectrum of severity that ranges from light turbulence to a LOC-I, or in-flight breakup. The turbulence generated by wake vortices may cause injuries to occupants of an encountering aircraft and may damage aircraft components and equipment. Wake vortices from a similar or larger-sized generating aircraft may induce a roll force or series of counter-rotating roll forces on an encountering aircraft that exceeds the counter roll authority (i.e., roll control used to counter the induced roll caused by a wake vortex).
Wake vortex-induced roll forces exerted on an encountering aircraft may result in a situation where the pilot is required to perform an upset recovery. The severity of a wake vortex encounter is affected by the residual strength of the wake vortices at the point of encounter, the characteristics of the encountering aircraft, the encounter angle (i.e., the relative angle between the flightpath of the encountering aircraft and the wake vortex),
11 the energy state of the encountering aircraft, and the ability of the pilot to recover from a wake vortex encounter. See Figure 14, Loss of Control in Flight, and Figure 15, Counter Roll Control.
Figure 14. Loss of Control in Flight
Figu re 15. Counter Roll Control
8.2.1 Whe n the wake vortices residual strength is higher, the forces imparted on the encountering aircraft are higher. The residual strength of the wake vortex at the point of intercept with the encountering aircraft is a function of the amount of wake decay deducted from the initial wake strength. The amount of wake decay depends on the wake vortex age (i.e., time elapsed since wake vortex generation), and the effect of the environment that existed during that time.
8.2.2 The capability of an aircraft to counteract the wake-induced roll depends on the counter roll control and wingspan of the encountering aircraft. Encountering aircraft with a relatively short wingspan, even in a high-performance aircraft, tends to be influenced more by a wake vortex than an aircraft with a larger wingspan. Encounters where the wingspan and roll control surfaces of the encountering aircraft extend beyond the rotation
12 of the wake vortex, counter roll control is usually more effective, and the induced roll is decreased.
8.3 Angle of Wake Vortex Encounter. The relative angle between the flightpath of the encountering aircraft and the wake vortex influences the severity of the encounter.
8.3.1 If the encountering aircraft is in line (i.e., in-trail or passing below the generating aircraft) or converging at a shallow angle with the wake vortices, turbulence and wake-induced roll is imparted on the encountering aircraft. These axial type encounters may have an interaction with the wake flow field characterized by turbulence that is longer lasting than other types of encounters. This turbulent interaction, known as the washboard effect or nibbling, occurs when the encountering aircraft is within the wake flow field but not in the core of a wake vortex. An encountering aircraft may be expelled from the wake vortex in an axial encounter due to the rotation of the wake vortex. If an encountering aircraft continues into the core of the wake vortex, the induced roll force is usually brief in comparison to the overall interaction with the wake flow field. See Figure 16, Axial Wake Vortex Encounter.
Figure 16. Axial Wake Vortex Encounter
8.3.2 Axi al encounters may cause the encountering aircraft to roll significantly in one direction and then roll in the other direction as the aircraft passes from one wake vortex to the other counter-rotating vortex. For example, an aircraft passes into the right-side wake vortex of the generating aircraft that induces a roll to the left. The pilot inputs a counter-roll force to the right. The aircraft exits the right-side wake vortex and enters the left-side wake vortex, causing the aircraft to roll to the right. In this case, the roll to the right is possibly exacerbated by the pilot control inputs from the previous vortex recovery actions. Severity may increase in this scenario as the second roll force nominally rolls the aircraft further than the first, and the pilot may overstress the aircraft with a quick control reversal. It is also possible to encounter the same wake vortex and corresponding roll direction more than once.
8.3.3 During a crossing encounter, where the aircraft crosses the wake flow field at a larger angle, turbulence may be experienced due to the vertical air movements of the wake vortices. Crossing encounters are brief due to the speed at which the encountering aircraft passes through the wake flow field. In a crossing encounter the encountering aircraft may experience turbulence that may be severe but will experience less wake-induced roll
13 when compared to an axial encounter. See Figure 17, Air Movement Within Wake Vortices, and Figure 18, Crossing Wake Vortex Encounter.
Figure 17. Air Movement Within Wake Vortices
Fi gure 18. Crossing Wake Vortex Encounter
8.4 E ncountering Aircraft Energy State. The energy state of the encountering aircraft influences the severity of the encounter. A pilot has three sources of energy available to manage or manipulate the flightpath of an aircraft. These three sources include kinetic energy (i.e., airspeed), potential energy (i.e., altitude above the ground level), and chemical energy (i.e., engine thrust). The balance of these combined energies describes the energy state of an aircraft at any given time. Refer to FAA-H-8083-3, Airplane Flying Handbook, for more information regarding aircraft energy states.
8.4.1 An encountering aircraft in the takeoff or approach-to-landing flight phase is at a lower energy state (e.g., low airspeed and low altitude) and may have a higher severity encounter with wake vortices than the same aircraft with a higher energy state. The time allotted for a pilot to recover from a wake vortex encounter is compressed when the aircraft is closer to terrain or obstacles.
8.4.2 An encountering aircraft that is above maneuvering speed at a high kinetic energy relative to the “envelope” of the Vg diagram is more susceptible to a pilot action that may overstress the aircraft during recovery. Rapid and large alternating control inputs, especially in combination with large changes in pitch, roll, or yaw, may result in structural failures at any speed, including below maneuvering speed.
8.5 Pilot Influence on Likelihood and Severity of a Wake Vortex Encounter. The training, knowledge, decision making, awareness, and skill of a pilot may significantly influence both the likelihood and severity of a wake vortex encounter.
9 WAKE VORTEX ENCOUNTER MITIGATIONS. Wake vortices are generally a hazard invisible to the human eye but may be detected visually during times of high humidity or when wake vortices interact with smoke or clouds. Pilots are encouraged to learn to envision the location and movements of the vortices generated by other aircraft and to adjust their flightpath accordingly to mitigate the likelihood and/or severity of a wake vortex encounter. The best risk mitigation for wake vortex encounters is avoidance.
Wake mitigations for aircraft behind the flightpath of a similar or larger-sized aircraft include:
9.1 Departures. The following are recommendations for pilots in the departure phase of flight.
9.1.1 All Departures. For all departures, pilots are encouraged to:
1. Be alert for any departure situation that may lead to a wake vortex encounter.
2. Be aware of how the expected wind direction and speed along the anticipated departure path may affect wake vortices from a preceding aircraft.
3. Avoid following or crossing behind and below the flightpath of a similar or larger-sized aircraft without adequate time to allow for wake vortex decay and drift.
4. Adjust aircraft operations and flightpath as necessary to preclude serious wake vortex encounters.
5. Pace their departure, including expected departure flightpath, to give ample time and distance for wake vortex decay and drift, especially if their flightpath crosses at or below the flightpath of a similar or larger-sized aircraft or rotorcraft.
6. Avoid the flightpath of a preceding aircraft by altering their flightpath above or laterally upwind, or both, of the flightpath of the generating aircraft.
9.1.1.1 At controlled airports, a pilot may request additional time for wake turbulence separation. A pilot is encouraged to make this request before taxiing onto the runway.
9.1.1.2 At uncontrolled airports, including at a part-time controlled airport when the control tower is not in operation, the pilot in command (PIC) is solely responsible for wake vortex encounter avoidance and mitigation, in accordance with 14 CFR § 91.3(a).
9.1.2 Departure from the Same or Closely Spaced Parallel Runway Following a Departure.
When departing after a similar or larger-sized aircraft has taken off on the same runway, or on a closely spaced parallel runway (i.e., parallel runway separated by less than 2,500 feet), where conditions exist for the wake to drift into their flightpath, pilots are encouraged to:
1. Delay operations to allow for wake decay and drift, as needed. For reference, aircraft taking off from the same runway or a parallel runway are to be separated by air traffic control (ATC) in accordance with FAA Order JO 7110.65, Air Traffic Control. For a small aircraft, prior to the issuance of a takeoff clearance, ATC requires an interval of at least 2 minutes to have elapsed since the takeoff of a Boeing 757-size aircraft and 3 minutes since the takeoff of an Airbus A380-size aircraft. ATC will not clear a small aircraft to line up and wait behind a “Heavy” or “Super” category aircraft.
2. Take note of the takeoff rotation point of the preceding aircraft as well as the departure flightpath.
3. Rotate prior to the rotation point of the preceding aircraft and continue climbing above the flightpath of the preceding aircraft or alter their flightpath upwind until above or clear of the flightpath of the preceding aircraft.
4. Avoid a flightpath that crosses below and behind the departure flightpath of the preceding aircraft. See Figure 19, Departure Flightpath.
Figure 19. Departure Flightpath
9.1.3 D eparture from the Same or Closely Spaced Parallel Runway Following an Arrival.
When departing after a similar or larger-sized aircraft has arrived on the same or closely spaced parallel runway, where conditions exist for wake to drift into their flightpath, pilots are encouraged to:
1. Take note of the landing touchdown point of the preceding aircraft.
2. Rotate after the point of touchdown of the preceding aircraft, or delay operations to allow for wake decay and drift.
3. Avoid a flightpath that crosses below and behind the descent path of the preceding aircraft.
9.1.4 Departure Following an Opposite Direction Departure from the Same or Closely Spaced Parallel Runway. When departing after a similar or larger-sized aircraft has taken off from the opposite end of the same or closely spaced runway, where conditions exist for wake vortices to drift into their flightpath, pilots are encouraged to delay operations to allow for wake decay and drift. See Figure 20, Opposite Direction Departure.
Figure 20. Opposite Direction Departure
9.1.5 D eparture from a Runway Intersection. When departing from an intersection from the same runway, or from a closely spaced parallel runway with a staggered threshold, often referred to as an offset parallel runway, where conditions exist for wake to drift into their flightpath, after a similar or larger-sized aircraft has departed, pilots are encouraged to:
1. Take note of the takeoff rotation point of the preceding aircraft as well as the departure flightpath.
2. Rotate prior to the rotation point of the preceding aircraft and continue climbing above the flightpath of the preceding aircraft or alter course upwind until above or clear of the flightpath of the preceding aircraft.
3. Avoid a flightpath that crosses below and behind the departure flightpath of the preceding aircraft. See Figure 21, Intersection Departure.
17 Figure 21. Intersection Departure
9.1.6 De parture from a Crossing Runway Following a Departure. When departing after a similar or larger-sized aircraft has taken off from a crossing runway, pilots are encouraged to:
1. Take note of the takeoff rotation point of the preceding aircraft, the departure flightpath, and envision the drift of the wake vortices.
2. Avoid a flightpath that crosses below and behind the flightpath of the preceding aircraft.
3. Rotate and climb above the flightpath of the preceding aircraft or alter course upwind until above or clear of the flightpath of the preceding aircraft, or both. See Figure 22, Crossing Departures.
Figure 22. Crossing Departures
9.1.7 De parture from a Crossing Runway Following a Landing Aircraft. When departing after a similar or larger-sized aircraft has landed on a crossing runway, pilots are encouraged to:
1. Take note of the point of touchdown of the preceding aircraft and the approach path, and envision the drift of the wake vortices.
2. Avoid a flightpath that crosses below and behind the preceding aircraft.
3. Rotate and climb above the flightpath of the preceding aircraft or alter course upwind until above or clear of the flightpath of the preceding aircraft, or both.
9.1.8 Departure from Converging Runways. Converging runways have various layouts and hazards. Pilots are encouraged to avoid a flightpath that crosses below and behind a similar or larger-sized aircraft.
9.1.9 Departures Following an Aircraft that Performs a Go-Around. When departing after a similar or larger-sized aircraft performs a missed approach, low approach, go-around, balked landing, or touch and go, the wake vortex hazard may exist along the runway and in your flightpath. For a small aircraft, prior to the issuance of a takeoff clearance, ATC requires an interval of at least 3 minutes to have elapsed since the takeoff of a Boeing 757-size aircraft and 4 minutes since the takeoff of an Airbus A380-size aircraft.
See Figure 23, Missed Approach Wake Vortices.
Figure 23. Missed Approach Wake Vortices
9.1.10 D eparture from an Adjacent Airport. When departing from an airport adjacent to another airport with a similar or larger-sized aircraft operations, pilots are encouraged to avoid wake vortices from preceding arriving and departing aircraft from both airports. See Figure 24, Departures from Adjacent Airports.
Figure 24. Departures from Adjacent Airports
9.1.11 Departure Following Rotorcraft Operations. The effects of rotorcraft wake vortices, downwash, and outwash may be stronger than expected compared to fixed-wing aircraft of a similar size. Pilots of fixed-wing aircraft are encouraged to:
1. Regard rotorcraft of a similar size as a larger aircraft for wake vortex avoidance.
2. Avoid the area within three times the rotor diameter, of the nearest main rotor, of a rotorcraft in a hover or slow hover taxi. See Figure 25, Avoiding Rotorcraft Downwash and Outwash.
3. Avoid wake vortices from an arriving or departing rotorcraft that may be present in different locations than the expected location of wake vortices from fixed-wing aircraft.
Figure 25. Avoiding Rotorcraft Downwash and Outwash
9.2 A rrivals. The following are recommendations for pilots in the departure phase of flight.
9.2.1 All Arrivals. For all arrivals, pilots are encouraged to:
1. Be alert for any arrival situation that may lead to a wake vortex encounter.
2. Adjust aircraft operations and flightpath as necessary to preclude serious wake vortex encounters.
3. Space the arrival, including the expected approach path of their aircraft, to give ample time and distance for wake vortex decay and drift.
4. When on approach, take note of wind speed and direction, runway threshold locations, relative glidepaths, touchdown and rotation points of similar and larger-sized aircraft.
5. Avoid following or crossing below the flightpath of a similar or larger-sized aircraft without adequate time and distance to allow for wake vortex decay and drift.
6. Avoid flying below the flightpath of a preceding aircraft by delaying operations to allow for wake decay and drift, as well as altering their flightpath above or altering their flightpath laterally upwind, or both.
7. When following a similar or larger-sized aircraft, stay at or above the electronic or visual glidepath, as appropriate. Otherwise, consider remaining at or above a nominal 3-degree glidepath using the “3 to 1” glidepath guidance. To approximate the altitude of a 3-degree glidepath, for every nautical mile (NM) away from the runway, add 300 feet above the touchdown zone elevation (TDZE) (e.g., approximately 1,500 feet above TDZE at 5 NM). Exceptions exist where a 3-degree glidepath (i.e., 318 feet per NM descent) is too low, including operations near mountainous terrain.
8. Maintain a line of sight to an aim point that is beyond the expected touchdown point of the preceding aircraft, no less than 1,000 feet from the threshold of the arrival runway.
9. Note the touchdown point of the preceding aircraft (e.g., a puff of smoke from tire contact of the preceding aircraft) and adjust the aim point as necessary.
10. Maintain the line of sight to the aim point and stay above the flightpath of the preceding aircraft to touchdown.
11. If there is any doubt as to the safety of the arrival, make the decision early to perform a missed approach, go-around, or a balked landing.
12. If a touchdown point extension due to wake vortex encounter mitigation does not allow sufficient distance to safely stop the aircraft prior to the end of the useable runway or would cause an unstabilized approach, delay the approach to landing to allow ample time and distance for wake decay and drift before landing at an appropriate touchdown point.
9.2.1.1 Visual Approaches Following Another Aircraft. Acceptance of ATC instructions to follow another aircraft, or to maintain visual separation from another aircraft, is an acknowledgment that the pilot will maneuver the aircraft as necessary to avoid the other aircraft and to maintain adequate wake turbulence separation and avoidance. In a radar environment with ATC, pilots may query ATC for updates on separation and groundspeed with respect to a preceding aircraft.