AIM Text
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. When rotor downwash hits the surface, the resulting outwash vortices have behavioral characteristics similar to wing tip vortices produced by fixed wing aircraft. However, the vortex circulation is outward, upward, around, and away from the main rotor(s) in all directions. 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 wing tip vortices of larger fixed wing aircraft. Pilots of small aircraft should use caution when operating behind or crossing behind landing and departing helicopters.
Source: FAA Aeronautical Information Manual · current edition · paragraph 7-4-7.
The short answer
This AIM section is FAA advisory guidance on helicopter-generated wake. In a slow hover taxi or stationary hover, main rotors produce high-velocity outwash vortices out to about three times the rotor diameter. It advises small-aircraft pilots to avoid operating within three rotor diameters of a hovering helicopter and to use caution behind landing or departing helicopters.
Research Notes
Common Questions
How close can a small aircraft safely operate near a hovering helicopter?
The AIM advises pilots of small aircraft to avoid operating within three rotor diameters of any helicopter in a slow hover taxi or stationary hover, because outwash vortices extend to roughly three times the rotor diameter.
Do helicopters produce wake vortices in forward flight?
Yes. Per this advisory section, in forward flight departing or landing helicopters produce a pair of strong, high-speed trailing vortices similar to the wing tip vortices of larger fixed wing aircraft, so pilots should use caution when operating behind or crossing behind them.
How does helicopter hover downwash behave near the surface?
The AIM explains that when rotor downwash hits the surface, the resulting outwash vortices behave like fixed-wing wing tip vortices, but the circulation is outward, upward, around, and away from the main rotor in all directions.