aircraft have shown that vortices remain spaced a bit less than  a wingspan apart, drifting with the wind, at altitudes greater  than a wingspan from the ground. Tests have also shown that  the vortices 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.  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 2–3 knots. A crosswind decreases  the lateral movement of the upwind vortex and increases  the movement of the downwind vortex. A light quartering  tailwind presents the worst case scenario as the wake  vortices could be all present along a significant portion of  the final approach and extended centerline and not just in the  touchdown zone as typically expected.  The following procedures are in place to assist pilots in vortex  avoidance in the given scenario.  Title 14 of the CFR part 91 has established right-of-way  rules, minimum safe altitudes, and VFR cruising altitudes  to enhance flight safety. The pilot can contribute to collision  avoidance by being alert and scanning for other aircraft. This  is particularly important in the vicinity of an airport.  Effective scanning is accomplished with a series of short,  regularly spaced eye movements that bring successive areas of  the sky into the central visual field. Each movement should not  exceed 10°, and each should be observed for at least 1 second  to enable detection. Although back and forth eye movements  seem preferred by most pilots, each pilot should develop a  scanning pattern that is most comfortable and then adhere to  it to assure optimum scanning. Even if entitled to the right-of­ way, a pilot should yield if another aircraft seems too close.  The following procedures and considerations are in place to  assist pilots in collision avoidance under various situations:  Clearing Procedures  Vortex Avoidance Procedures  • Landing behind a larger aircraft on the same runway—  stay at or above the larger aircraft’s approach  flight path and land beyond its touchdown point.  [Figure 14-47A]  • Landing behind a larger aircraft on a parallel runway  closer than 2,500 feet—consider the possibility of drift  and stay at or above the larger aircraft’s final approach  flight path and note its touchdown point.  [Figure 14-47B]  • Landing behind a larger aircraft on crossing runway—  cross above the larger aircraft’s flight path.  • Landing behind a departing aircraft on the same  runway—land prior to the departing aircraft’s  rotating point.  • Landing behind a larger aircraft on a crossing  runway—note the aircraft’s rotation point and, if that  point is past the intersection, continue and land prior  to the intersection. If the larger aircraft rotates prior  to the intersection, avoid flight below its flight path.  Abandon the approach unless a landing is ensured well  before reaching the intersection.  [Figure 14-47C]  • Departing behind a large aircraft—rotate prior to the  large aircraft’s rotation point and climb above its climb  path until turning clear of the wake.  • For intersection takeoffs on the same runway—  be alert to adjacent larger aircraft operations,  particularly upwind of the runway of intended use.  If an intersection takeoff clearance is received, avoid  headings that cross below the larger aircraft’s path.  • If departing or landing after a large aircraft executing  a low approach, missed approach, or touch-and-go  • Before takeoff—prior to taxiing onto a runway or  landing area in preparation for takeoff, pilots should  scan the approach area for possible landing traffic,  executing appropriate maneuvers to provide a clear  view of the approach areas.  • Climbs and descents—during climbs and descents in  flight conditions that permit visual detection of other  traffic, pilots should execute gentle banks left and right  at a frequency that permits continuous visual scanning  of the airspace.  • Straight and level—during sustained periods of  straight-and-level flight, a pilot should execute  appropriate clearing procedures at periodic intervals.  • Traffic patterns—entries into traffic patterns while  descending should be avoided.  • Traffic at VOR sites—due to converging traffic,  sustained vigilance should be maintained in the  vicinity of VORs and intersections.  • Training operations—vigilance should be maintained  and clearing turns should be made prior to a practice  14-28  • En route, it is advisable to avoid a path below and  behind a large aircraft, and if a large aircraft is  observed above on the same track, change the aircraft  position laterally and preferably upwind.  Collision Avoidance  landing (since vortices settle and move laterally  near the ground, the vortex hazard may exist along  the runway and in the flight path, particularly in a  quartering tailwind), it is prudent to wait at least 2  minutes prior to a takeoff or landing. 