Wake Turbulence



Wake Vortex Generation
Figure 1: Wake Vortex Generation

Introduction:

  • Wake turbulence is a phenomena in which a vortex is created by another aircraft's wings which translates into hazardous turbulent air
    • Wake turbulence is a biproduct of lift
    • Pilots must learn to envision the location of the vortex wake generated by larger (transport category) aircraft and adjust the flight path accordingly
  • Every aircraft generates a wake while in flight caused by counter-rotating vortices trailing from the wing tips
  • Vortices from larger aircraft pose problems to encountering aircraft including rolling moments exceeding the roll-control authority, damaging aircraft and equipment

  • During ground operations and during takeoff, jet engine blast (thrust stream turbulence) can cause damage and upsets if encountered at close range
  • Exhaust velocity versus distance studies at various thrust levels have shown a need for light aircraft to maintain an adequate separation behind large turbojet aircraft
  • Pilots of larger aircraft should be particularly careful to consider the effects of their "jet blast" on other aircraft, vehicles, and maintenance equipment during ground operations

Vortex Formation:

  • Lift is generated by the creation of a pressure differential over the wing surface
  • 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 aft of the wing resulting in swirling air masses trailing downstream of the wingtips
  • Wake Encounter Counter Control
    Figure 2: Wake Encounter Counter Control
  • After the roll up is completed, the wake consists of two counter-rotating cylindrical vortices [Figure 2]
  • 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

Vortex Strength:

  • Governed by weight, speed, and shape of the generating wing
  • Characteristics change with wing configuration devices or speed however as the basic factor is weight, the vortex strength increases proportionally
  • Peak vortex tangential speeds exceeding 300' per second have been recorded
  • The greatest vortex strength is created when:
    • Heavy
    • Clean
    • Slow
  • 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
Figure 3: Wake Ends/Wake Begins

Induced Roll:

  • The usual hazard is associated with induced rolling moments which 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
  • Flight tests show that the capability of an aircraft to counteract the roll imposed by the wake vortex primarily depends on the wingspan and counter-control responsiveness of the encountering aircraft
    • 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

Behavior:

  • 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
  • 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
  • At altitude, vortices sink at a rate of 300 to 500' per minute and stabilize about 500 to 900' below the flight level of the generating aircraft
  • Pilots should fly upwind, at or above the generating aircraft's altitude, creating about 1,000' of vertical separation to be 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
Figure 4: Vortex Flow Field

  • While widely rumored that wake turbulence can bounce, this is not substantiated
    • While bouncing may not occur, test data have shown that vortices can rise with the air mass in which they are embedded
    • Wind shear, particularly, can cause vortex flow field "tilting"
    • Also, ambient thermal lifting and orographic effects (rising terrain or tree lines) can cause a vortex flow field to rise
  • 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 awhile
  • 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
Figure 5: Vortex Movement in Ground Effect - Tailwind

Operations Problem Areas:

  • 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
Figure 6: Vortex Movement Near Ground - No Wind

Vortex Movement Near Ground - with Cross Winds
Figure 7: Vortex Movement Near Ground - with Cross Winds

Avoidance Procedures:

  • 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
    • When in doubt, ask

  • Landing behind a larger aircraft - same runway:
    • 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
  • Landing behind a larger aircraft - when parallel runway is closer than 2,500':
    • Consider possible drift to your runway
    • Stay at or above the larger aircraft's final approach flight path- note its touchdown point
  • Landing behind a larger aircraft - crossing runway:
    • Cross above the larger aircraft's flight path
  • Landing behind a departing larger aircraft - same runway:
    • Note the larger aircraft's rotation point and land well prior to rotation point
  • Landing behind a departing larger aircraft - crossing runway:
    • 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
  • Departing behind a larger aircraft:
    • 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
  • Intersection takeoffs - same runway:
    • 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
  • Departing or landing after a larger aircraft executing a low approach, missed approach, or touch-and-go landing:
    • 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
  • En route VFR (thousand-foot altitude plus 500'):
    • 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

Warning Signs:

  • 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!

Helicopters:

  • 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
  • In a slow hover taxi or stationary hover near the surface, helicopters generate down wash producing high velocity out-wash vortices to a distance approximately 3 times the diameter of the rotor with similar characteristics as fixed wing aircraft

Pilot Responsibility:

  • Although ATC is there to help, the flight disciplines necessary to ensure vortex avoidance during VFR operations must be exercised by the pilot, especially when under VFR
  • Pilots are reminded that in operations conducted behind all aircraft, acceptance of instructions from ATC in the following situations is an acknowledgment that the pilot will ensure safe takeoff and landing intervals and accepts the responsibility for providing wake turbulence separation
    • Traffic information
    • Instructions to follow an aircraft; and
    • The acceptance of a visual approach clearance
  • For operations conducted behind heavy aircraft, ATC will specify the word "heavy" when this information is known. Pilots of heavy aircraft should always use the word "heavy" in radio communications
  • Heavy and large jet aircraft operators should use the following procedures during an approach to landing. These procedures establish a dependable baseline from which pilots of in-trail, lighter aircraft may reasonably expect to make effective flight path adjustments to avoid serious wake vortex turbulence
    • Pilots of aircraft that produce strong wake vortices should make every attempt to fly on the established glidepath, not above it; or, if glidepath guidance is not available, to fly as closely as possible to a "3-1" glidepath, not above it
      • Fly 3,000' at 10 miles from touchdown, 1,500' at 5 miles, 1,200' at 4 miles, and so on to touchdown
    • Pilots of aircraft that produce strong wake vortices should fly as closely as possible to the approach course centerline or to the extended centerline of the runway of intended landing as appropriate to conditions
  • Pilots operating lighter aircraft on visual approaches in-trail to aircraft producing strong wake vortices should use the following procedures to assist in avoiding wake turbulence. These procedures apply only to those aircraft that are on visual approaches
    • Pilots of lighter aircraft should fly on or above the glidepath. Glidepath reference may be furnished by an ILS, by a visual approach slope system, by other ground-based approach slope guidance systems, or by other means. In the absence of visible glidepath guidance, pilots may very nearly duplicate a 3-degree glideslope by adhering to the "3 to 1" glidepath principle
      • Fly 3,000' at 10 miles from touchdown, 1,500' at 5 miles, 1,200' at 4 miles, and so on to touchdown
    • If the pilot of the lighter following aircraft has visual contact with the preceding heavier aircraft and also with the runway, the pilot may further adjust for possible wake vortex turbulence by the following practices:
      • Pick a point of landing no less than 1,000' from the arrival end of the runway
      • Establish a line-of-sight to that landing point that is above and in front of the heavier preceding aircraft
      • When possible, note the point of landing of the heavier preceding aircraft and adjust point of intended landing as necessary
        • A puff of smoke may appear at 1,000' markings of the runway, showing that touchdown was that point; therefore, adjust point of intended landing to the 1,500' markings
      • Maintain the line-of-sight to the point of intended landing above and ahead of the heavier preceding aircraft; maintain it to touchdown
      • Land beyond the point of landing of the preceding heavier aircraft
    • During visual approaches pilots may ask ATC for updates on separation and ground speed with respect to heavier preceding aircraft, especially when there is any question of safe separation from wake turbulence

Separation:

  • Because of the possible effects of wake turbulence, controllers are required to apply no less than specified minimum separation for aircraft operating behind a heavy jet and, in certain instances, behind large non-heavy aircraft (i.e., B757 aircraft)
    • Separation is applied to aircraft operating directly behind a heavy/B757 jet at the same altitude or less than 1,000' below:
      • Heavy jet behind heavy jet - 4 miles
      • Large/heavy behind B757 - 4 miles
      • Small behind B757 - 5 miles
      • Small/large aircraft behind heavy jet - 5 miles
    • Also, separation, measured at the time the preceding aircraft is over the landing threshold, is provided to small aircraft:
      • Small aircraft landing behind heavy jet - 6 miles
      • Small aircraft landing behind B757 - 5 miles
      • Small aircraft landing behind large aircraft- 4 miles
    • Additionally, appropriate time or distance intervals are provided to departing aircraft:
      • Two minutes or the appropriate 4 or 5 mile radar separation when takeoff behind a heavy/B757 jet will be:
        • From the same threshold
        • On a crossing runway and projected flight paths will cross
        • From the threshold of a parallel runway when staggered ahead of that of the adjacent runway by less than 500' and when the runways are separated by less than 2,500'

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 large aircraft
    • In the opposite direction on the same runway behind a large aircraft takeoff or low/missed approach

NOTE:
This 3-minute interval may be waived upon specific pilot request

  • A 3-minute interval will be provided for all aircraft taking off when the operations are as described in subparagraph b1 and 2 above, the preceding aircraft is a heavy/B757 jet, and the operations are on either the same runway or parallel runways separated by less than 2,500'. 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 aircraft if there is reasonable assurance the required separation will exist when the departing aircraft starts takeoff roll
  • Note that ultimately, when operating under VFR, it is up to the pilot, and not ATC, to provide this separation

Reduced Vertical Separation Minimum (RVSM):

  • Wake turbulence can still exist at RVSM altitudes but will generally be moderate or less in magnitude
  • Turbulence events can be reported using the NASA Aviation Safety Reporting System (ASRS) on the FAA RVSM Documentation web page under Safety Reporting
  • 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
    • If you find yourself in this these situations, request a vector or different altitude
    • The FAA tracks all reports on their website


Case Study:

  • 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

Conclusion:

  • 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 of short span aircraft, even of the high performance type, must be especially alert to vortex encounters
  • While it may be ATC's job, it is the pilot's responsiblity for wake turbulence separation as Pilot-In-Command

Reference:



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