AIRCRAFT WAKE TURBULENCE
     ADVISORY CIRCULAR  AC NO:  90-23E
     Date:  October 1, 1991
     Initiated  by:  AFS-430  
      
     IT'S THERE - SOMEWHERE !!!
   BUT WHERE ?
   DO I GO OVER ?  
   DO I GO UNDER ?
   BEHIND ?  ABOVE ?
   HOW FAR ???

   1.  PURPOSE.  This advisory circular (AC) is intended to alert
      pilots to the hazards of aircraft wake turbulence and recommends
      related operational procedures.

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      2.  CANCELLATION.  AC 90-23D, Aircraft Wake Turbulence, dated
      12/15/72.

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      3.  INTRODUCTION.  Every aircraft in flight generates a wake.
      Historically, when pilots encountered this wake in flight, the
      disturbance was attributed to "prop wash."  It is known, however,
      that this disturbance is caused by a pair of counter rotating
      vortices trailing from the wing tips.  The vortices from large
      aircraft pose problems to encountering aircraft.  For instance,
      the wake of these aircraft can impose rolling moments exceeding
      the roll control capability of some aircraft.  Further,
      turbulence generated within the vortices, if encountered at close
      range, can damage aircraft components and equipment and cause
      personal injuries.  The pilot must learn to envision the location
      of the vortex wake generated by large aircraft and adjust his
      flight path accordingly.

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      4.  VORTEX GENERATION.  Lift is generated by the creation of a
      pressure differential over the wing surfaces.  The lowest
      pressure occurs over the upper wing surface and the highest
      pressure under the wing.  This pressure differential triggers the
      rollup of the airflow aft of the wing resulting in swirling air
      masses trailing downstream of the wingtips.  After the rollup is
      completed, the wake consists of two counter-rotating cylindrical
      vortices (see figure 1).  Most of the energy is within a few feet
      of the center of each vortex, but pilots should avoid a region
      within about 100 feet of the vortex core.

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      5.  VORTEX STRENGTH.  The strength of the vortex is governed by
      the weight, speed, and shape of the wing of the generating
      aircraft.  The vortex characteristics of any given aircraft can
      also be changed by extension of flaps or other wing configuring
      devices.  However, as the basic factor is weight, the vortex
      strength increases proportionately with increase in aircraft
      operating weight.  Peak vortex tangential speeds up to almost 300
      feet per second have been recorded.  The greatest vortex strength
      occurs when the generating aircraft is heavy-clean-slow.  Figure
      2 shows smoke visualization of a vortex photographed during early
      smoke tower fly-by tests.

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      6.  INDUCED ROLL: In rare instances, a wake encounter could
      cause in-flight structural damage of catastrophic proportions. 
      However, the usual hazard is associated with induced rolling 
      moments which can exceed the rolling capability of the encountering
      aircraft.  In flight experiments, aircraft have been intentionally flown
      directly up trailing vortex cores of large aircraft.  It was
      shown that the capability of an aircraft to counteract the roll
      imposed by the wake vortex primarily depends on the wing span and
      counter-control responsiveness of the encountering aircraft.

      a.  Counter-control is usually effective and induced roll
      minimal in cases where the wing span and ailerons of the
      encountering aircraft extend beyond the rotational flow field of
      the vortex.  It is more difficult for aircraft with short wing
      span (relative to the generating aircraft) to counter the imposed
      roll induced by vortex flow.  Pilots of short span aircraft, even
      of the high performance type, must be especially alert to vortex
      encounters.  The wake of larger aircraft requires the respect of
      all pilots. 
   
                          
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      7.  VORTEX BEHAVIOR.  Trailing vortices have certain behavioral
      characteristics which can help a pilot visualize the wake
      location and thereby take avoidance precautions.

      a.  Vortices are generated from the moment aircraft leave
      the ground, since trailing vortices are a by-product of wing
      lift.  Prior to takeoff or landing, pilots should note the
      rotation or touchdown point of the preceding aircraft. 

      b.  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 that the vortices remain
      spaced a bit less than a wing span apart drifting with the wind,
      at altitudes greater than a wing span from the ground.  In view
      of this, if persistent vortex turbulence is encountered, a slight
      change of altitude and lateral position (preferably upwind) will
      provide a flight path clear of the turbulence.

      c.  Flight tests have shown that the vortices from larger
      (transport category) 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.
      Atmospheric turbulence hastens breakup.  Pilots should fly at or
      above the preceding aircraft's flightpath, altering course as
      necessary to avoid the area behind and below the generating
      aircraft.  However, vertical separation of 1,000 feet may be
      considered safe. 

      d.  When the vortices of 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 or 3 knots.

      e.  A crosswind will decrease the lateral movement of the
      upwind vortex and increase the movement of the downwind vortex
      (Figure 8).  Thus, a light wind with a cross-runway component of
      1 to 5 knots (depending on conditions) could result in the upwind
      vortex remaining in the touchdown zone for a period of time
      (figure 9) 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 flightpaths.
                              
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      8.  OPERATIONAL PROBLEM AREAS.  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 or parallel
      with the flightpath of the generating aircraft.  Avoid the area
      below and behind the generating aircraft, especially at low
      altitude where even a momentary wake encounter could be
      hazardous.  Pilots should be particularly alert in calm wind
      conditions and situations where the vortices could:

      a.  Remain in the touchdown area.

      b.  Drift from aircraft operating on a nearby runway.

      c.  Sink into takeoff or landing path from a crossing runway.

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

      e.  Sink into the flight path of VFR flights operating at the
           hemispheric altitudes 500 feet below.

      f.  Pilots of all aircraft should visualize the location of the vortex trail
          behind large aircraft and use proper vortex avoidance procedures
          to achieve safe operation.  It is equally important that pilots of larger
          aircraft plan or adjust their flight paths to minimize vortex exposure
          to other aircraft.
      
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      9.  VORTEX AVOIDANCE PROCEDURES.  Under certain 
      conditions, airport traffic controllers apply procedures for
      separating aircraft operating under Instrument Flight Rules. 
      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."  Whether or
      not a warning has been given, however, the pilot is expected to
      adjust his/her operations and flightpath as necessary to preclude
      serious wake encounters.  The following vortex avoidance
      procedures are recommended for the situation shown:

      a.  When landing behind a larger aircraft - same runway
      (figure 10), stay at or above the large aircraft's final approach
      flight path -- note touchdown point -- land beyond it.

      b.  When landing behind a larger aircraft - when parallel
      runway is closer than 2,500 feet (figure 11), consider possible
      vortex drift onto your runway.  If you have visual contact with
      the larger aircraft landing on the parallel runway, whenever
      possible, stay at or above the large aircraft's final approach
      flightpath -- note his touchdown point.

      c.  When landing behind a larger aircraft - crossing runway
      (figure 12), cross above the larger aircraft's flightpath.

      d.  When landing behind a departing larger aircraft - same
      runway (figure 13), note larger aircraft's rotation point -- land
      well prior to rotation point.

      e.  When landing behind a departing larger aircraft -
      crossing runway, note larger aircraft's rotation point -- if past
      the intersection -- continue the approach -- land prior to the
      intersection (figure 14).  If larger aircraft rotates prior to
      the intersection, avoid flight below the larger aircraft's
      flightpath.  Abandon the approach unless a landing is ensured
      well before reaching the intersection.

      f.  When departing behind a larger aircraft:  Note larger
      aircraft's rotation point -- rotate prior to larger aircraft's
      rotation point -- continue climb above the larger aircraft's
      climb path until turning clear of his wake (Figure 16).  Avoid
      subsequent headings which will cross below and behind
      aircraft (figure 17).  Be alert for any critical takeoff
      situation which could lead to a vortex encounter.
                      
      g.  Intersection takeoffs - same runway, be alert to
      adjacent large 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.

      h.  Departing or landing after a larger aircraft executing a
      low 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 missed approach or a touch-and-go
      landing, particularly in light quartering wind conditions.  You
      should assure that an interval of at least 2 minutes has elapsed
      before your takeoff or landing.

       i.  En route VFR - (1,000-foot altitude plus 500 feet).
      Avoid flight below and behind a larger aircraft's path.  If a
      larger aircraft is observed above on the same track (meeting or
      overtaking), adjust your position laterally, preferably upwind.

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     10.  HELICOPTERS.  A hovering helicopter generates a downwash
      from its main rotor(s) similar to the "prop wash" of a
      conventional aircraft.  However, in forward flight, this energy
      is transformed into a pair of strong, high-speed trailing
      vortices similar to wing-tip vortices of larger fixed-wing
      aircraft.  Pilots should avoid helicopter vortices since
      helicopter forward flight airspeeds are often very low which
      generate exceptionally strong vortices. 
                          
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      11.  JET ENGINE EXHAUST.  During ground operations, 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 during ground
      operations.

      a.  Engine exhaust velocities, generated by larger jet
      aircraft during ground operations and initial takeoff roll,
      dictate the desirability of lighter aircraft awaiting takeoff to
      hold well back of the runway edge at the taxiway hold line.
      Also, it is desirable to align the aircraft to face any possible
      jet engine blast effects.  Additionally, in the course of running
      up engines and taxiing on the ground, pilots of larger aircraft
      should consider the effects of their jet blasts on other
      aircraft, vehicles, and maintenance and servicing equipment. 

      b.  The Federal Aviation Administration has established
      standards for the location of runway hold lines.  For example,
      runway intersection hold short lines are established 250 feet
      from the runway centerline for precision approach runways served
      by approach category C and D aircraft.  For runways served by
      aircraft with wingspans over 171 feet, such as the B-747, taxiway
      hold lines are 280 feet from the centerline of precision approach
      runways.  These hold line distances increases slightly with an
      increase in field elevation.

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     12.  PILOT RESPONSIBILITY.  Government and industry groups
      are making concerted efforts to minimize or eliminate the hazards of
      trailing vortices.  However, the flight disciplines necessary to
      ensure vortex avoidance during visual operations must be
      exercised by the pilot.  Vortex visualization and avoidance
      procedures should be exercised by the pilot using the same degree
      of concern as in collision avoidance since vortex encounters
      frequently can be as dangerous as collisions.

      a.  Pilots are reminded that in operations conducted behind
      all aircraft, acceptance from Air Traffic Control of traffic
      information, instructions to follow an aircraft, or the
      acceptance of a visual approach clearance, is an acknowledgment
      that the pilot will ensure safe takeoff and landing intervals and
      accepts the responsibility of providing his own wake turbulence
      separation.

      b.  For VFR departures behind heavy aircraft, air traffic
      controllers are required to use at least a 2-minute separation
      interval unless a pilot has initiated a request to deviate from
      the 2-minute interval and has indicated acceptance of
      responsibility for maneuvering his aircraft so as to avoid the
      wake turbulence hazard.

                           
      Operational Tips For Light Aircraft:  
     How to Avoid Vortex Wake         
      1.  Lift Off Short of Large Aircraft Rotation Point.    
      2.  Land Well Beyond Large Aircraft Touchdown Point.   
      3.  Pass Over Flight Path of Large Aircraft, or At Least 1000' Under.          
      4.  Stay to Windward of Large Aircraft Flight Paths.       
      5.  Keep Alert, Especially on Calm Days When Vortices Persist Longest.             
           

GROUND EFFECT, HOW IT WORKS:
The wing-in-ground effect prevents the wing from creating lift robbing vortexes
because at extremely low altitude there's no room for them to form and this
generates a cushion of air beneath the aircraft.
The effect is stronger the closer the craft gets to the surface.