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The wind conditions which would most likely cause a helicopter's downwash to create a wake turbulence issue for a taxiing or departing aircraft are:
  • A
    Moderate winds with the helicopter crosswind of taxiing or departing aircraft.
  • B
    Moderate winds with the helicopter upwind of taxiing or departing aircraft.
  • C
    Lights winds with the helicopter downwind of taxiing or departing aircraft.
  • D
    Light winds with the helicopter upwind of taxiing or departing aircraft.

Refer to figure.
WAKE TURBULENCE
Whenever an airplane generates lift, air spills over the wing tips from the high pressure areas below the wings to the low pressure areas above them. This flow causes rapidly rotating whirlpools of air called wingtip vortices. An aircraft generates vortices from the moment it rotates on take-off to touchdown.

Although wake turbulence settles, it persists in the air for several minutes, depending on wind conditions. In calm winds, wing tip vortices separate outwards on each side of the runway. In light winds of 3 to 7 knots, the vortices can stay in the touchdown area, sink into your take-off or landing path, or drift over a parallel runway.

General Guides:

  • To avoid turbulence when landing behind a large aircraft, stay above the large airplane’s glide path and land beyond its touchdown point.

  • If a large airplane has just taken-off as you approach, touch down well before the large aircraft’s lift-off point.

  • When departing after a large aircraft has landed, lift-off beyond its touchdown location.

  • When taking-off behind a large aircraft, lift-off before the large aircraft’s rotation point and climb out above or upwind of its flight path.

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