Induced Roll and Counter Control

2.3.1. Induced Roll. While it is possible for a wake vortex encounter to cause catastrophic in-flight structural damage, the most common hazard is the induced rolling moment that exceeds the roll control capability of the aircraft you are flying. During flight tests, the capability of an aircraft to counteract the roll imposed by the wake vortex of the preceding aircraft primarily depends on the wing span and counter control responsiveness of the encountering aircraft.

2.3.2. Counter Control. Counter control is usually more effective and the induced roll reduced in cases where the wing span and ailerons of the encountering aircraft are extended beyond the rotational flow field of the vortex. Aircraft with short wing spans (relative to the vortex generating aircraft) will usually find it more difficult to counter the imposed roll produced by vortex flow. Therefore, pilots flying aircraft with short wing spans, regardless of their individual performance capabilities, must be particularly cognizant of and respect the strength of the wake vortices produced by much larger and heavier aircraft.

2.3.3. Vortex Behavior. Trailing vortices have certain behavioral characteristics that can help pilots visualize the wake location and thereby take the necessary precautions to avoid them.

Since vortices are the by-product of lift, they are generated from the moment an aircraft rotates for take-off until touchdown at the end of a flight. Thus, prior to conducting a takeoff or landing, pilots should note the rotation or touchdown point of the preceding aircraft (see figure 2.1). 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 the vortices remain spaced

Figure 2.1. Touchdown and Rotation Points when Following Larger Aircraft.

a bit less than a wing span apart drifting with the wind at altitudes greater than a wing span from the ground. Therefore, if you encounter turbulence that is being generated by the preceding aircraft, a slight change of altitude and/or lateral position (preferably upwind) will normally provide a flight path clear of the turbulence.

2.3.4. Flight tests show that vortices from large (transport category) aircraft during climb-out, cruise, and descent sink at a rate of several hundred ft/min. The vortex strength and sink rate diminishes with time and distance. Atmospheric turbulence will also hasten the breakup of aircraft generated disturbances. Therefore, you 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. As a general rule, vertical separation of 1,000 feet may be considered safe (see figure 2.2).

Figure 2.2. Aircraft Generated Vortex Descent Profile.

2.3.4.1. When the vortices of a 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 to 3 knots.

2.3.4.2. A crosswind will decrease the lateral movement of the upwind vortex and increase the movement of the downwind vortex. Thus, a light wind with a cross-runway component of 1 to 5 knots (depending on conditions) can result in the upwind vortex remaining in the touchdown zone 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.

Pilots should therefore be alert to larger aircraft upwind from their approach and takeoff flight paths.

2.3.4.2.1. CAUTION: The light quartering tailwind requires maximum caution regarding the effects of wake turbulence.

2.4. Operational Problem Areas. Wake turbulence may cause aircraft upsets similar to

traversing an area of turbulence with the severity of the encounter dependent on 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 possibility of an induced roll

increases when the encountering aircraft’s heading is generally aligned with or parallel to the flight path of the generating aircraft.

2.4.1. Pilots should be particularly alert in calm wind conditions and maneuvering situations in the vicinity of the airport where the vortices could:

2.4.1.1. Remain in the touchdown area.

2.4.1.2. Drift from aircraft operating on a nearby runway.

2.4.1.3. Sink into the takeoff or landing path from a crossing runway.

2.4.1.4. Sink into the traffic patterns from other airport operations.

2.4.1.5. Sink into the flight path of aircraft operating VFR.

2.4.2. Pilots of all aircraft should attempt to visualize the location of the vortex trail behind a larger aircraft and exercise wake turbulence 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.

2.5. Vortex Avoidance Procedures. Under certain conditions, air traffic controllers apply procedures for separating aircraft operating under IFR. The controllers will also provide

precautionary wake turbulence information to VFR aircraft which in the tower’s opinion may be adversely affected by the vortexes generated by a larger aircraft. ATC will give the position, altitude and direction of flight of larger aircraft followed by the phrase “CAUTION—WAKE TURBULENCE.” Whether or not a warning is given, pilots are expected to adjust their flight path(s) to avoid serious wake encounters. Vortex avoidance procedures are recommended for the following situations:

2.5.1. Landing Behind a Larger Aircraft. When landing behind a larger aircraft on the same runway; stay at or above the larger aircraft’s final approach flight path; note the touchdown point and then, if safety permits, land beyond it (see figure 2.3).

Figure 2.3. Recommended Flight Path Landing Behind a Larger Aircraft.

2.5.1.1. CAUTION: Avoid the area below and behind the preceding aircraft especially at low altitude where even a momentary wake encounter could be catastrophic.

2.5.2. Parallel Runway Considerations. When landing behind and offset from a larger aircraft landing on a parallel runway separated by less than 2,500 feet, consider the possibility of the vortices drifting onto your runway. Stay at or above the larger aircraft’s final approach flight path; note its touchdown point and, if safety permits, land beyond it (see figure 2.4).

Figure 2.4. Recommended Touchdown Point for Parallel Runway Operations.

2.5.2.1. Crossing Flight Paths. When landing behind a larger aircraft on a crossing runway;

cross above the larger aircraft’s flight path (see figure 2.5).

Figure 2.5. Crossing Flight Path Considerations.

2.5.3. Landing Behind Departing Aircraft – Same Runway. When landing behind a departing larger aircraft on the same runway: note larger aircraft’s rotation point and land well prior to rotation point (see figure 2.6).

Figure 2.6. Landing Behind a Departing Aircraft

2.5.4. Landing Behind Departing Aircraft – Crossing Runway. When landing behind a departing larger aircraft on a crossing runway; note larger aircraft’s rotation point. If the aircraft rotated past the intersection of the two runways, continue the approach and land prior to the intersection (see figure 2.7). If the aircraft rotated prior to the intersection, consider abandoning the approach unless able to land well before reaching the intersection (see figure 2.8).

Figure 2.7. Landing Behind a Departing Aircraft – Crossing Runway (Rotation Point Past Intersection)

Figure 2.8. Landing Behind a Departing Aircraft – Crossing Runway (Rotation Point Prior to Intersection)

2.5.5. Departing Behind a Large Aircraft. When departing behind a larger aircraft; note the other aircraft’s rotation point and, if able, rotate your aircraft prior to that point. Then, climb above the larger aircraft’s climb path until turning clear of the larger aircraft’s wake (see figure 2.1). Continue to avoid subsequent headings which will cross below and behind a larger aircraft. If unable, consider waiting 2 minutes to allow the wake induced vortices to dissipate.

2.5.5.1. When departing from a runway that intersects another, be alert for adjacent large aircraft operations, particularly upwind of your runway, and avoid headings which will cross below a larger aircraft’s flight path.

2.5.5.2. Because vortices settle and move laterally near the ground, the vortex hazard may exist along the runway and in your flight path after a larger aircraft has executed a low approach, missed approach or a touch-and-go landing, particular in light quartering wind conditions. It these cases, it is highly recommended that you wait at least 2 minutes before your takeoff or landing.

2.5.6. Flying Behind a Larger Aircraft. When operating VFR, avoid flight below and behind a larger aircraft’s flight path. If a larger aircraft is observed above on the same track (meeting or overtaking), adjust your position laterally, preferably upwind. If operating IFR, consider contacting ATC to request and offset to avoid potential wake turbulence.

2.6. Pilot Responsibility.

2.6.1. Government and industry groups are making concerted efforts to minimize the hazards of trailing vortices. However, whether operating VFR or IFR, pilots should make every effort to reduce the hazards associated with vortex encounters. Vortex visualization and avoidance procedures should be exercised by an aircrew using the same degree of concern as collision avoidance.

2.6.2. Pilots are reminded that when flying behind all aircraft, acceptance of instructions from ATC in the following situations is an acknowledgement that the pilot will ensure safe takeoff and landing intervals and accepts responsibility for providing their own wake turbulence separation:

2.6.2.1. Traffic information.

2.6.2.2. Instructions to follow an aircraft

2.6.2.3. The acceptance of a visual approach clearance.