U.S. Department of Transportation Federal Aviation Administration Advisory Circular Subject: Aircraft Wake Vortex Encounter Risk Mitigation Date: 1/13/26 AC No: 90-23H Initiated by: AFS-400 Change:
This advisory circular (AC) presents basic information on wake vortex characteristics, alerts pilots to the hazards of aircraft wake vortex encounters, recommends operational procedures to avoid wake vortex encounters, and provides wake vortex encounter guidance. The contents of this document do not have the force and effect of law and are not meant to bind the public in any way, and the document is intended only to provide information to the public regarding existing requirements under the law or agency policies.
Rober t Reckert for Hugh Thomas Acting Executive Director, Flight Standards Service
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CONTENTS Paragraph Page
1 Purpose of This Advisory Circular (AC)1
2 Audience1
3 Where You Can Find This AC1
4 What This AC Cancels1
5 Summary of Changes1
6 Introduction1
6.1 Pilot Role in Wake Vortex Encounter Mitigation2
7 Wake Vortex Characteristics2
7.1 Wake Vortex Generation2
7.2 Wake Vortex Strength5
7.3 Wake Vortex Decay6
7.4 Wake Vortex Movement6
8 Wake Vortex Encounter Risk9
8.1 Factors Affecting the Likelihood of a Wake Vortex Encounter9
8.2 Factors Affecting the Severity of a Wake Vortex Encounter10
8.3 Angle of Wake Vortex Encounter12
8.4 Encountering Aircraft Energy State13
8.5 Pilot Influence on Likelihood and Severity of a Wake Vortex Encounter14
9 Wake Vortex Encounter Mitigations14
9.1 Departures14
9.2 Arrivals19
9.3 En Route25
9.4 Wake Vortex Encounter Mitigations for Generating Aircraft26
9.5 Wake Vortex Encounter Mitigations Utilized by ATC27
10 Wake Vortex Encounter Recovery Guidance29
10.1 Situational Awareness29
10.2 Startle and Surprise30
10.3
Flight Control Inputs30
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10.4 Other Upset Recovery Considerations31
11 Pilot Wake Vortex Encounter Reporting32
11.1 Reporting to ATC32
11.2 Aviation Safety Reporting System (ASRS)32
11.3 Operator Reporting Policies33
11.4 FAA Hotline33
11.5 Notification of Aircraft Accidents or Incidents33
Appendix A. Administrative InformationA-1
Appendix B. Miscellaneous ATC Wake Separation EffortsB-1
List of Figures
Figure 1. Wake Vortices1
Figure 2. Wake Vortex Generation2
Figure 3. Touchdown and Rotation Points3
Figure 4. Rotorcraft Downwash and Outwash3
Figure 5. Rotorcraft Accelerating from a Hover4
Figure 6. Rotorcraft Approach to an Airport5
Figure 7. Wake Vortex Drift6
Figure 8. Wake Vortex Sink7
Figure 9. Wake Vortex Drift With Zero Crosswind7
Figure 10. Wake Vortex Drift With 3 Knots of Crosswind8
Figure 11. Wake Vortex Drift With 6 Knots of Crosswind8
Figure 12. Light Quartering Tailwind Wake Vortex Hazard9
Figure 13. Climbing Aircraft Wake Vortex Sink10
Figure 14. Loss of Control in Flight11
Figure 15. Counter Roll Control11
Figure 16. Axial Wake Vortex Encounter12
Figure 17. Air Movement Within Wake Vortices13
Figure 18. Crossing Wake Vortex Encounter13
Figur
e 19. Departure Flightpath15
Figure 20. Opposite Direction Departure16
Figure 21. Intersection Departure17
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Figure 22. Crossing Departures17
Figure 23. Missed Approach Wake Vortices18
Figure 24. Departures From Adjacent Airports18
Figure 25. Avoiding Rotorcraft Downwash and Outwash19
Figure 26. Landing After a Larger Aircraft21
Figure 27. Parallel Runways With Threshold Stagger22
Figure 28. Landing After a Departing Aircraft22
Figure 29. Landing After a Crossing Runway Departure (Rotation Point Before
Intersection)23
Figure 30. Landing After a Crossing Runway Departure (Rotation Point After
Intersection)23
Figure 31. Landing After an Arriving Aircraft on a Converging Runway24
Figure 32. Arrival at an Adjacent Airport25
Figure 33. Opposing Flightpaths26
Figure 34. In-Trail Flightpath26
1 1 PURPOSE OF THIS ADVISORY CIRCULAR (AC). This AC presents basic information on wake vortex characteristics, alerts pilots to the hazards of aircraft wake vortex encounters, recommends operational procedures to avoid wake vortex encounters, and provides wake vortex encounter guidance.
Note: This is a guidance document. Its content is not legally binding in its own right and will not be relied upon by the Department as a separate basis for affirmative enforcement action or other administrative penalty.
Conformity with the guidance document is voluntary only. Nonconformity will not affect rights and obligations under existing statutes and regulations.
2 AUDIENCE. This AC is intended to provide guidance on wake vortex encounters to student pilots, certificated pilots, flight instructors, training providers, and pilot examiners.
3 WHERE YOU CAN FIND THIS AC. You can find this AC on the Federal Aviation Administration’s (FAA) website at https://www.faa.gov/regulations_policies/advisory_cir culars and the Dynamic Regulatory System (DRS) at https://drs.faa.gov.
4 WHAT THIS AC CANCELS. AC 90-23G, Aircraft Wake Turbulence, dated February 10, 2014, is canceled.
5 SUMMARY OF CHANGES. This AC has been reorganized to assist pilots in effective Risk-Based Decision Making (RBDM) as it relates to wake vortex encounters.
6 INTRODUCTION. Wake vortices are masses of counter-rotating air created as a byproduct of the generation of lift. Every aircraft that uses a fixed or rotary airfoil to generate lift in flight creates wake vortices of varying initial strength and persistence.
Wake vortices, from similar-sized (i.e., similar characteristics of weight, wingspan, etc.)
or larger-sized generating (lead) aircraft, are a hazard to encountering (following) aircraft that may vary from light turbulence encounters to a loss of control in flight (LOC-I) or in-flight breakup accident. The purpose of this AC is to discuss the behavior, risk, and mitigation techniques for wake vortices. See Figure 1, Wake Vortices.
Figure 1. Wake Vortices
6.1 Pilot Role in Wake Vortex Encounter Mitigation. In accordance with Title 14 of the Code of Federal Regulations (14 CFR) § 91.3(a), the pilot in command (PIC) of an aircraft is directly responsible for, and is the final authority as to, the operation of that aircraft. This responsibility includes wake vortex encounter avoidance and mitigation.
Pilots arriving and departing from uncontrolled airports, flying in uncontrolled airspace, or accepting a clearance for a visual approach are accepting the sole responsibility for wake vortex encounter mitigation. Pilots are encouraged to:
• Learn to visualize the behavior, location, and movements of wake vortices from similar or larger-sized generating aircraft.
• Proactively adjust their flightpath or delay operations as needed to avoid or mitigate a wake vortex encounter.
• Be alert for possible wake vortex encounters, particularly during takeoff, approach, and landing operations.
• Follow the wake vortex encounter avoidance and mitigation guidance contained in this AC, applicable aircraft manuals, and the Aeronautical Information Manual (AIM).
7 WAKE VORTEX CHARACTERISTICS.
7.1 Wake Vortex Generation. Lift is generated by the creation of a pressure differential above and below the airfoil (e.g., wing or rotor). Higher-pressure air below the airfoil seeks to equalize the differential by moving towards the lower-pressure air above. This movement of air rolls upwards and inwards to form two counter-rotating cylindrical vortices trailing behind the airfoil. The disturbed air surrounding the wake vortices is the wake flow field. Studies conducted during approach and landing operations have not shown a discernible difference between the wake vortices of aircraft with and without winglets. See Figure 2, Wake Vortex Generation.
Figure 2. Wake Vortex Generation
7.1.1 Rotation to Touchdown. Airfoils generate wake vortices whenever lift is produced. In airplanes, lift and wake vortices increase in strength substantially from the rotation point, the point where the airplane begins to lift off the surface, to the touchdown point, the point where the aircraft returns to the surface on landing. See Figure 3, Touchdown and Rotation Points.
Figure 3. Touchdown and Rotation Points
7.1.2 Rotorc raft Downwash, Outwash, and Wake Vortices. The spinning rotor blades of helicopters and other rotorcraft, in a hover or slow hover taxi, generate a downward column of air called downwash. When a rotorcraft is in ground effect (i.e., usually less than one rotor diameter distance above the surface), the downward column of air interacts with the surface and produces outwash, a turbulent lateral movement of air. When a rotorcraft is in a hover in ground effect, outwash propagates in all lateral directions in still air. Tip vortices from the rotor blades are pushed down helically, coalesce as they approach the surface, and are driven outwards by the outwash intensifying the turbulent airflow. Downwash and outwash may damage aircraft below and in the vicinity of rotorcraft operations and may produce adverse effects on nearby aircraft operations, including turbulence and induced roll moments. See Figure 4, Rotorcraft Downwash and Outwash.
Figure 4. Rotorcraft Downwash and Outwash
7.1.2.1 When a rotorcraft hovers into the wind or accelerates from a hover, the horizontal flow of air modifies the downwash. As the rotorcraft continues to accelerate, the rotor blades move into less disturbed air, and the downwash transitions to counter-rotating wake vortices. The forward flight speed of the rotorcraft in which the airflow transitions to counter-rotating wake vortices is dependent on multiple factors, including the type of rotorcraft and the environment that it is operating in. When a rotorcraft decelerates, the same forces are exerted in the reverse sequence. Rotorcraft wake vortices may have characteristics that differ from fixed-wing wake vortices (e.g., differing advancing and trailing blade vortices, separation between wake vortices differing when the rotorcraft is in a climb versus descent, etc.) due to the aerodynamic differences between a fixed and rotary airfoil. Rotorcraft wake vortex characteristics such as wake decay rate and wake drift have similar behaviors as wake vortices created by fixed-wing aircraft.
7.1.2.2 In terminal operations where a rotorcraft is arriving or departing, there is a combination of wake vortices, downwash, and outwash. Rotorcraft-generated turbulent airflows, including downwash, outwash, and wake vortices, are a hazard to following similar or smaller-sized aircraft, especially fixed-wing aircraft. Aircraft have suffered LOC-I accidents while attempting to arrive or depart an airport after a rotorcraft transited the area. See Figure 5, Rotorcraft Accelerating from a Hover, and Figure 6, Rotorcraft Approach to an Airport.
Figure 5. Rotorcraft Accelerating from a Hover
5 Figure 6. Rotorcraft Approach to an Airport
7.2 Wak e Vortex Strength. The initial strength of wake vortices (i.e., strength of the wake vortices directly after wake generation) is influenced by the current weight, angle of attack (AOA), airfoil shape, wing loading or disk loading, and wingspan or rotor diameter of the generating aircraft. Wake vortices are the strongest in the immediate vicinity behind and below the generating aircraft and dissipate over time. The wake age (i.e., time elapsed since wake generation) influences the residual strength (i.e., remaining intensity of the wake vortices at a given time after generation).
7.2.1 Initial Strength. The greatest initial strength of a wake vortex nominally occurs when the generating aircraft is at a relatively high AOA, a heavy weight, and in a clean configuration (e.g., landing gear and wing flaps retracted). In the terminal environment, the greatest initial strength of a wake vortex occurs when the generating aircraft is at relatively low speeds.
7.2.1.1 “Super” (e.g., Airbus A380) and “Heavy” (e.g., Lockheed C-5) category aircraft, as defined by FAA Order JO 7360.1, Aircraft Type Designators, have been shown to produce high initial strength wake vortices that take longer to diminish due to the heavier weight and longer wingspan of these aircraft when compared to aircraft of smaller categories.
7.3 Wake Vortex Decay. The decay rate and movement of wake vortices is influenced by the initial strength of the wake vortices, the distance between the pair of wake vortices, and atmospheric conditions, including lateral wind speed and direction, vertical wind speed and direction, atmospheric turbulence, air density, and airmass stability.
Considering a fixed wake vortex decay rate, time becomes the key variable. As time passes after wake generation, the residual strength of a wake vortex diminishes.
7.3.1 Environment Effects on Decay Rate. The environmental conditions that are the most conducive to a low wake decay rate are light winds, low atmospheric turbulence, and a stable airmass. The wake vortex decay rate tends to increase with an increase in wind speed, turbulence, or a decrease in airmass stability.
7.3.2 Aircraft Configuration Effects on Decay Rate. The extension of landing gear, wing flaps, or other wing configuring devices tend to change the wake vortex characteristics of an airplane. The extension of wing flaps tends to reduce the separation between vortices and hastens wake decay. The extension of landing gear tends to hasten the decay of vortices by creating turbulence that interacts with the wake flow field.
7.4 Wake Vortex Movement. Wake vortices nominally sink behind the generating aircraft and are influenced by the surrounding airmass characteristics, including vertical and horizontal winds that transport the vortices. Wake vortices tend to remain spaced apart slightly less than the wingspan of the generating aircraft, or the outboard edge of the wing flaps when deployed. When within ground effect (i.e., less than approximately a wingspan of the generating aircraft from the surface), the counter-rotating wake vortices tend to diverge laterally. See Figure 7, Wake Vortex Drift.
Figure 7. Wake Vortex Drift
7.4.1 W ake Vortex Sink. Wake vortices tend to have the highest sink rate in the immediate vicinity behind the generating aircraft. Wake vortices from large aircraft may descend initially at a rate of several hundred feet per minute. The sink rate of the wake vortices tends to diminish as the wake age increases. Generally, for large generating aircraft, wake vortices tend to level approximately 500 to 900 feet below the flightpath of the generating aircraft. See Figure 8, Wake Vortex Sink.
7 Figure 8. Wake Vortex Sink
7.4.1.1 Wak e vortices from aircraft with high initial wake vortex strength, including “Heavy” and, especially, “Super” category aircraft, may descend more than 1,000 feet before decaying to a less perceptible residual strength.
7.4.1.2 After initially descending, several factors, including ambient thermal lifting, vertical winds, stratification, and interaction with the surface, may cause wake vortices to stop descending or even rise. This rising effect is commonly referred to as wake bounce and may result in wake vortices rising into the glidepath of an approach.
7.4.2 The Influence of Ground Effect on Wake Vortices. When wake vortices are in ground effect, the wake vortices tend to diverge laterally. In a zero-crosswind scenario, the wake vortices nominally diverge laterally at a speed of 2 to 3 knots. See Figure 9, Wake Vortex Drift With Zero Crosswind.
Figure 9. Wake Vortex Drift With Zero Crosswind
7.4.2.1 W hen wake vortices are in ground effect, a light crosswind of approximately 3 knots tends to decrease the outward lateral movement of the upwind vortex and increase the outward lateral movement of the downwind vortex. This action results in the upwind vortex remaining near the glidepath and runway longer and displacing the downwind vortex at an increased rate. The downwind vortex may influence operations on an adjacent runway. See Figure 10, Wake Vortex Drift With 3 Knots of Crosswind.
8 Figure 10. Wake Vortex Drift With 3 Knots of Crosswind
7.4.2.2 S tronger crosswinds of 6 knots and above may displace one or both vortices to an adjacent runway. For example, a 6-knot crosswind component added to the downwind vortex nominally diverging laterally at 3 knots has a groundspeed of 9 knots. In this scenario, the downwind vortex displaces laterally 1,800 feet in 2 minutes. See Figure 11, Wake Vortex Drift With 6 Knots of Crosswind.
Figure 11. Wake Vortex Drift With 6 Knots of Crosswind
7.4.2.3 A tailwind condition may force the wake vortices of a generating aircraft forward from where the vortices were generated, resulting in wake vortices closer to an approach glidepath than would otherwise occur in a no-wind scenario.
7.4.2.3.1 A light quartering tailwind may cause a hazardous landing condition for encountering aircraft. The crosswind component resists the upwind vortex tendency to move outwards and keeps the upwind vortex laterally near the approach and runway centerline while the tailwind component forces the wake vortices closer to the glidepath and further down the runway. See Figure 12, Light Quartering Tailwind Wake Vortex Hazard.