What is the proper procedure to avoid wake turbulence

Every aircraft generates a wake while in flight
Wake turbulence is a function of an aircraft producing lift, creating a vortex that forms two counter-rotating vorticies trailing behind the aircraft
Wake turbulence from the generating aircraft can affect encountering aircraft due to the strength and behavior of the vortices
- This can impose rolling moments exceeding the roll-control authority of encountering aircraft, causing possible injury to occupants and damage to aircraft
In a slow hover taxi or stationary hover near the surface, helicopter main rotor(s) generate downwash producing high velocity outwash vortices to a distance approximately three times the diameter of the rotor
Pilots must learn to envision the location of the vortex wake generated by larger aircraft and adjust the flight path accordingly

Wake Vortex Generation
The creation of a pressure differential over the wing surface generates lift
The lowest pressure occurs over the upper wing surface and the highest pressure under the wing
This pressure differential triggers the roll up of the airflow at the rear of the wing resulting in swirling air masses trailing downstream of the wingtips
After the roll up is completed, the wake consists of two counter-rotating cylindrical vortices [Figure 1]
The wake vortex is formed with most of the energy concentrated within a few feet of the vortex core
Most of the energy is within a few feet of the center of each vortex, but pilots should avoid a region within about 100' of the vortex core
More aircraft are being manufactured or retrofitted with winglets to increase fuel efficiency (by improving the lift-to-drag ratio)
- Studies have shown however, that winglets have a negligible effect on wake turbulence generation, particularly with the slower speeds involved during departures and arrivals

Wake Encounter Counter Control
The strength of the vortex is governed by the weight, speed, wingspan, and shape of the generating aircraft's wing [Figure 2]
- The vortex strength from an aircraft increases proportionately to an increase in operating weight or a decrease in aircraft speed
Characteristics change with extension of flaps or other wing configuring devices
Peak vortex tangential speeds exceeding 300' per second have been recorded
The greatest vortex strength is created when:
These effects are amplified with an aircraft under high wing-loading
With the exception of gear and flaps down, which actually tend to disrupt wake turbulence, you can see it is mostly the terminal area when you are low to the ground that you may expect to see this phenomena
Wake Ends/Wake Begins
Wake Ends/Wake Begins
- In rare instances, a wake encounter could cause catastrophic inflight structural damage to an aircraft
  - However, the usual hazard is associated with induced rolling moments that can exceed the roll-control authority of the encountering aircraft
  - This is especially dangerous during takeoff and landing when there is little altitude for recovery
- During inflight testing, aircraft intentionally flew directly up trailing vortex cores of larger aircraft
  - These tests demonstrated that the ability of aircraft to counteract the roll imposed by wake vortex depends primarily on the wingspan and counter-control responsiveness of the encountering aircraft
  - These tests also demonstrated the difficulty of an aircraft to remain within a wake vortex
  - The natural tendency is for the circulation to eject aircraft from the vortex
  - Counter-control is usually effective and induced roll minimal in cases where the wingspan and ailerons of the encountering aircraft extend beyond the rotational flow field of the vortex
  - It is more difficult for aircraft with short wingspan (relative to the generating aircraft) to counter the imposed roll induced by vortex flow
- The wake of larger aircraft requires the respect of all pilots
- Pilots of short span aircraft, even of the high performance type, must be especially alert to vortex encounters

Vortices are generated from the moment the aircraft leaves the ground as a by product of lift [Figure 3]
- Prior to takeoff or touchdown pilots should note the rotation or touchdown point of the preceding aircraft
Circulation is outward, upward and around the wing tips
- Vortices remain spaced a bit less than a wingspan apart, drifting with the wind, at altitudes greater than a wingspan from the ground
- In view of this, if persistent vortex turbulence is encountered, a slight change of altitude (upward) and lateral position (upwind) should provide a flight path clear of the turbulence
Those from larger 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 [Figure 4]
- When present, atmospheric turbulence hastens breakup
- Pilots should fly at or above the preceding aircraft's flight path, altering course as necessary to avoid the area directly behind and below the generating aircraft
- However, vertical separation of 1,000 feet may be considered safe
When vortices sink close to the ground (within 1-200') they tend to move laterally over the ground at a speed of 2 or 3 knots [Figure 5/6/7]
Vortex Flow Field
Pilots should be alert at all times for possible wake vortex encounters when conducting approach and landing operations
- The pilot is ultimately responsible for maintaining an appropriate interval, and should consider all available information in positioning the aircraft in the terminal area, to avoid the wake turbulence created by a preceding aircraft
- Test data shows that vortices can rise with the air mass in which they are embedded
- The effects of wind shear can cause vortex flow field "tilting"
- In addition, ambient thermal lifting and orographic effects (rising terrain or tree lines) can cause a vortex flow field to rise and possibly bounce
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 could result in the upwind vortex remaining in the touchdown zone for a period of time and hasten the drift of the downwind vortex toward another runway
A tailwind condition can move the vortices forward into the touchdown zone
The light quartering tailwind requires maximum caution
Vortex Movement in Ground Effect - Tailwind

The probability of induced roll increases when the encountering aircraft's heading is generally aligned with the flight path of the generating aircraft
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 could result in the upwind vortex remaining in the touchdown zone for a period of time 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 QUARTERING TAILWIND REQUIRES MAXIMUM CAUTION
- Pilots should be alert to large aircraft upwind from their approach and takeoff flight paths
Pilots should be particularly alert in calm wind conditions and situations where the vortices could:
- Remain in the touchdown area
- Drift from aircraft operating on a nearby runway
- Sink into the takeoff or landing path from a crossing runway
- Sink into the traffic pattern from other airport operations
- Sink into the flight path of VFR aircraft operating on the hemispheric altitude 500' below
AVOID THE AREA BELOW AND BEHIND THE GENERATING AIRCRAFT, ESPECIALLY AT LOW ALTITUDE WHERE EVEN A MOMENTARY WAKE ENCOUNTER COULD BE HAZARDOUS

Vortex Movement Near Ground - No Wind

Vortex Movement Near Ground - with Cross Winds

Under certain conditions, airport traffic controllers apply procedures for separating IFR aircraft
If a pilot accepts a clearance to visually follow a preceding aircraft, the pilot accepts responsibility for separation and wake turbulence avoidance
The controllers will also provide to VFR 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"
After being told "caution, wake turbulence" the controller generally do not provide additional information
Whether or not a warning or information has been given, however, the pilot is expected to adjust aircraft operations and flight path as necessary to preclude serious wake encounters

- Stay at or above the larger aircraft's final approach flight path-note its touchdown point-land beyond it
  - Incident data shows that the greatest potential for a wake vortex incident occurs when a light aircraft is turning from base to final behind a heavy aircraft flying a straight-in approach
  - Use extreme caution to intercept final above or well behind the heavier aircraft
  - When a visual approach is issued and accepted to visually follow a preceding aircraft, the pilot is required to establish a safe landing interval behind the aircraft s/he was instructed to follow
  - Pilots must not decrease the separation that existed when the visual approach was issued unless they can remain on or above the flight path of the preceding aircraft
- Consider possible drift to your runway
- Stay at or above the larger aircraft's final approach flight path- note its touchdown point
- Cross above the larger aircraft's flight path
- Note the larger aircraft's rotation point and land well prior to rotation point
- Note the larger aircraft's rotation point and if past the intersection, continue the approach to land prior to the intersection
- If larger aircraft rotates prior to the intersection, avoid flight below the larger aircraft's flight path
- Abandon the approach unless a landing is ensured well before reaching the intersection
- Note the larger aircraft's rotation point and rotate prior to the larger aircraft's rotation point
- Continue climbing above the larger aircraft's climb path until turning clear of the larger aircraft's wake
- Avoid subsequent headings which will cross below and behind a larger aircraft
- Be alert for any critical takeoff situation which could lead to a vortex encounter
- Be alert to adjacent larger 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
- 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
- If you can, climb above the preceding aircraft's flight path
- If you can't out climb it, deviate slightly upwind, and climb parallel to the preceding aircraft's course
- Avoid headings that cause you to cross behind and below the preceding aircraft
- You should ensure that an interval of at least 2 minutes has elapsed before your takeoff or landing
- Avoid flight below and behind a large aircraft's path
- If you must cross under, do so at least 1000' below
- If a larger aircraft is observed above on the same track (meeting or overtaking) adjust your position laterally, preferably upwind

Any uncommanded aircraft movements (i.e., wing rocking) may be caused by wake
This is why maintaining situational awareness is so critical. Ordinary turbulence is not unusual, particularly in the approach phase
A pilot who suspects wake turbulence is affecting his or her aircraft should get away from the wake, execute a missed approach or go-around and be prepared for a stronger wake encounter
The onset of wake can be insidious and even surprisingly gentle
There have been serious accidents where pilots have attempted to salvage a landing after encountering moderate wake only to encounter severe wake vortices
Pilots should not depend on any aerodynamic warning, but if the onset of wake is occurring, immediate evasive action is a MUST!

While the behavioral characteristics are similar to a fixed wind aircraft, circulation is outward, upward, around, and away from the main rotor(s) in all directions
- In fact, helicopter wakes may be of significantly greater strength than those from a fixed wing aircraft of the same weight
Pilots of small aircraft 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 fixed wing aircraft

The strongest wake can occur when the helicopter is operating at lower speeds (20 - 50 knots)
Some mid-size or executive class helicopters produce wake as strong as that of heavier helicopters
- This is because two blade main rotor systems, typical of lighter helicopters, produce stronger wake than rotor systems with more blades

Pilots of small aircraft should use caution when operating behind or crossing behind landing and departing helicopters

- Controllers, while providing radar vector service, are responsible for applying the wake-turbulence longitudinal separation distances between IFR aircraft and wake-turbulence advisories to VFR aircraft
- Air traffic controllers are responsible for providing cautionary wake-turbulence information to assist pilots prior to their assuming visual responsibility for avoidance
- Controllers must issue wake-turbulence cautionary advisories and the position, altitude if known, and direction of flight of heavy jets or B-757s to:
  - VFR aircraft not being radar vectored, but which are behind heavy jets or B757s
  - VFR arriving aircraft that have previously been radar vectored and the vectoring has been discontinued
  - IFR aircraft that accept a visual approach or visual separation
- Air traffic controllers should also issue cautionary information to any aircraft if, in their opinion, wake turbulence may have an adverse effect on it
- Tower controllers are responsible for runway separation for aircraft arriving or departing the airport
  - Tower controllers do not provide visual wake-turbulence separation to arrival aircraft; that is the pilot’s responsibility
- Air traffic controllers are responsible for applying appropriate wake-turbulence separation criteria for departing aircraft

Because of the possible effects of wake turbulence, controllers are required to apply no less than minimum required separation to all aircraft operating behind a Super or Heavy, and to Small aircraft operating behind a B757, when aircraft are IFR; VFR and receiving Class B, Class C, or TRSA airspace services; or VFR and being radar sequenced
- Separation is applied to aircraft operating directly behind a super or heavy at the same altitude or less than 1,000 feet below, and to small aircraft operating directly behind a B757 at the same altitude or less than 500 feet below:
  - Heavy behind super - 6 miles
  - Large behind super - 7 miles
  - Small behind super - 8 miles
  - Heavy behind heavy - 4 miles
  - Small/large behind heavy - 5 miles
  - Small behind B757 - 4 miles
- Also, separation, measured at the time the preceding aircraft is over the landing threshold, is provided to small aircraft:
  - Small landing behind heavy - 6 miles
  - Small landing behind large, non-B757 - 4 miles
- Note that these terms are found in the Pilot/Controller Glossary Term - Aircraft Classes
Additionally, appropriate time or distance intervals are provided to departing aircraft when the departure will be from the same threshold, a parallel runway separated by less than 2,500 feet with less than 500 feet threshold stagger, or on a crossing runway and projected flight paths will cross:
- Three minutes or the appropriate radar separation when takeoff will be behind a super aircraft;
- Two minutes or the appropriate radar separation when takeoff will be behind a heavy aircraft
- Two minutes or the appropriate radar separation when a small aircraft will takeoff behind a B757
- NOTE: Controllers may not reduce or waive these intervals
A 3-minute interval will be provided when a small aircraft will takeoff:
- From an intersection on the same runway (same or opposite direction) behind a departing B757, or
- In the opposite direction on the same runway behind a B757 takeoff or low/missed approach
  - NOTE: This 3-minute interval may not be waived upon specific pilot request
A 4-minute interval will be provided for all aircraft taking off behind a super aircraft, and a 3-minute interval will be provided for all aircraft taking off behind a heavy aircraft when the operations are as described in subparagraphs b1 and b2 above, and are conducted on either the same runway or parallel runways separated by less than 2,500 feet. Controllers may not reduce or waive this interval
Pilots may request additional separation (i.e., 2 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
NOTE: 14 CFR Section 91.3(a) states: "The pilot-in-command of an aircraft is directly responsible for and is the final authority as to the operation of that aircraft"
Controllers may anticipate separation and need not withhold a takeoff clearance for an aircraft departing behind a large, heavy, or super aircraft if there is reasonable assurance the required separation will exist when the departing aircraft starts takeoff roll
With the advent of new wake turbulence separation methodologies known as Wake Turbulence Recategorization, some of the requirements listed above may vary at facilities authorized to operate in accordance with Wake Turbulence Recategorization directives
Note that ultimately, when operating under VFR, it is up to the pilot, and not ATC, to provide this separation

Wake turbulence can still exist at RVSM altitudes, but will generally be moderate or less in magnitude
Pilots should remain alert when operating:
- In the vicinity of aircraft climbing or descending through their altitude
- Approximately 10-30 miles after passing 1,000' below opposite direction traffic
- Approximately 10-30 miles behind and 1,000' below same direction traffic
Pilots may request a vector or different altitude if believed to be at risk for wake turbulence
Turbulence events should be reported using the NASA Aviation Safety Reporting System (ASRS) on the FAA RVSM Documentation web page under Safety Reporting

Pilots should be aware of the potential for wake turbulence encounters in RVSM airspace. Experience gained since 1997 has shown that such encounters in RVSM airspace are generally moderate or less in magnitude
Prior to DRVSM implementation, the FAA established provisions for pilots to report wake turbulence events in RVSM airspace using the NASA Aviation Safety Reporting System (ASRS). A "Safety Reporting" section established on the FAA RVSM Documentation webpage provides contacts, forms, and reporting procedures
To date, wake turbulence has not been reported as a significant factor in DRVSM operations. European authorities also found that reports of wake turbulence encounters did not increase significantly after RVSM implementation (eight versus seven reports in a ten-month period). In addition, they found that reported wake turbulence was generally similar to moderate clear air turbulence
- - In the vicinity of aircraft climbing or descending through their altitude
  - Approximately 10-30 miles after passing 1,000' below opposite-direction traffic
  - Approximately 10-30 miles behind and 1,000' below same-direction traffic
- Pilots encountering or anticipating wake turbulence in DRVSM airspace have the option of requesting a vector, FL change, or if capable, a lateral offset
  - Offsets of approximately a wing span upwind generally can move the aircraft out of the immediate vicinity of another aircraft's wake vortex
  - In domestic U.S. airspace, pilots must request clearance to fly a lateral offset. Strategic lateral offsets flown in oceanic airspace do not apply

Some accidents have occurred even though the pilot of the trailing aircraft had carefully noted that the aircraft in front was at a considerably lower altitude. Unfortunately, this does not ensure that the flight path of the lead aircraft will be below that of the trailing aircraft
A wake encounter can be catastrophic. In 1972 at Fort Worth a DC-9 got too close to a DC-10 (two miles back), rolled, caught a wingtip, and cartwheeled coming to rest in an inverted position on the runway. All aboard were killed. Serious and even fatal GA accidents induced by wake vortices are not uncommon. However, 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 with the flight path of the generating aircraft
A common scenario for a wake encounter is in terminal airspace after accepting clearance for a visual approach behind landing traffic. Pilots must be cognizant of their position relative to the traffic and use all means of vertical guidance to ensure they do not fly below the flight path of the wake generating aircraft

A wake turbulence encounter can range from negligible to catastrophic
The impact of the encounter depends on the weight, wingspan, size of the generating aircraft, distance from the generating aircraft, and point of vortex encounter
Pilots, in all phases of flight, must remain vigilant of possible wake effects created by other aircraft
- Studies have shown that atmospheric turbulence hastens wake breakup, while other atmospheric conditions can transport wake horizontally and vertically
- While ATC has required warnings they must provide, the pilot has the ultimate responsibility for ensuring appropriate separations and positioning of the aircraft in the terminal area to avoid the wake turbulence created by a preceding aircraft
Offsets of approximately a wing span upwind generally can move the aircraft out of the immediate vicinity of another aircraft's wake vortex
- In domestic U.S. airspace, pilots must request clearance to fly a lateral offset. Strategic lateral offsets flown in oceanic airspace do not apply
Your biggest hazard from wake turbulence will be induced roll
- In rare instances a wake encounter could cause inflight structural damage of catastrophic proportions
Pilots should attempt to visualize the vortex trail of aircraft whose projected flight path they may encounter. When possible, pilots of larger aircraft should adjust their flight paths to minimize vortex exposure to other aircraft
- If unable, or to increase safety margin, allowing time for the vortex to dissipate reduces risk
While it may be ATC's job, it is the pilot's responsibility for wake turbulence separation as Pilot-In-Command
Recognize that every single aircraft generates wake turbulence
- Remember the effects of wake turbulence will vary based on the big three: heavy, clean, and slow
Based on extensive analysis of wake vortex behavior, new procedures and separation standards are being developed and implemented in the US and throughout the world
- Wake research involves the wake generating aircraft as well as the wake toleration of the trailing aircraft
The FAA and ICAO are leading initiatives, in terminal environments, to implement these next-generation wake turbulence procedures and separation standards
- The FAA has undertaken an effort to recategorize the existing fleet of aircraft and modify associated wake turbulence separation minima. This initiative is termed Wake Turbulence Recategorization (RECAT), and changes the current weight-based classes (Super, Heavy, B757, Large, Small+, and Small) to a wake-based categorical system that utilizes the aircraft matrices of weight, wingspan, and approach speed. RECAT is currently in use at a limited number of airports in the National Airspace System
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