AIM ¶ 7-4-7 — Helicopter Wake Turbulence
AIM 7-4-7 explains helicopter rotor downwash, outwash vortices, and safe distances for small aircraft near hovering or departing helicopters.
In Plain English
AIM 7-4-7 warns that helicopters generate dangerous wake turbulence in two distinct flight regimes, and small aircraft pilots must respect them.
Hover Taxi or Stationary Hover (near surface):
- Main rotor downwash strikes the ground and creates high-velocity outwash vortices.
- These vortices extend outward, upward, around, and away from the rotor in all directions.
- The hazard reaches roughly three rotor diameters from the helicopter.
- Pilots of small aircraft should avoid operating within three rotor diameters of a hovering or hover-taxiing helicopter.
Forward Flight (departing or landing):
- Helicopters in forward flight produce a pair of strong, high-speed trailing vortices similar to wing tip vortices from large fixed-wing aircraft.
- Use caution when operating behind or crossing behind landing or departing helicopters.
Why it matters: rotor wake can upset or roll a small aircraft on the ramp, taxiway, or final approach. This is recommended guidance in the AIM, but ignoring it has caused real loss-of-control accidents.
AIM Source Text
FAA AIM ¶ 7-4-77-4-7. 7-4-7. Helicopters
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.
Oral Exam Questions a DPE Might Ask
Q1How far from a hovering helicopter should a small aircraft remain, and why?
Per AIM 7-4-7, pilots of small aircraft should avoid operating within approximately three rotor diameters of any helicopter in a slow hover taxi or stationary hover, because rotor downwash hitting the surface produces high-velocity outwash vortices extending outward, upward, around, and away from the rotor in all directions.
Q2How does helicopter wake turbulence in forward flight compare to fixed-wing wake turbulence?
Per AIM 7-4-7, helicopters in forward flight (departing or landing) produce a pair of strong, high-speed trailing vortices similar to the wing tip vortices generated by larger fixed-wing aircraft, so pilots should use caution when operating behind or crossing behind them.
Q3What direction do outwash vortices from a hovering helicopter travel?
Per AIM 7-4-7, when rotor downwash hits the surface, the resulting outwash vortices circulate outward, upward, around, and away from the main rotor(s) in all directions, with behavior similar to fixed-wing wing tip vortices.
Related Paragraphs in AIM Chapter 7