In forward flight, the downwash from the main rotor of a helicopter is transformed into a pair of trailing vortices comparable to the wing tip vortices generated by a fixed wing aircraft and there is some evidence that these vortices are more intense than those from a similar-weight fixed-wing aircraft. It is therefore recommended that helicopters should be operated well clear of light aircraft when hovering or whilst air taxiing.
Two blade main rotor systems, typical of lighter helicopters, produce stronger wake than rotor systems with more blades.
The strength of the vortex depends not only on the blade geometry and loading, but also on the aircraft's operational state (i.e., hovering, climbing, descending, or manoeuvring) but is more pronounced in forward flight, especially when pitch is being applied, such as at take off or landing
When the descent velocity of a rotor approximately matches its wake's velocity, the helical wake tends to roll up into a thick vortex ring that remains near the rotor plane and interferes with the rotor's inflow. Vortex Ring may be considered as the helicopter equivalent of aerodynamic stall and is generally of greater danger to the helicopter than to other aircraft in the vicinity becuase other aircraft are unlikely to fly a profile in proximity to such wake turbulence of vertical, rather than horizontal, extent.
Note that downwash, although not a vortex issue, must always be considered if planning to pass above a light aircraft.
Typical helicopter wake vortices Source: AAIB Bulletin: 7/2010 EW/C2009/09/07
Helicopters are also susceptible to the effects of wake turbulence from large fixed wing aircraft, and ICAO separation requirements apply equally to helicopters as well as fixed wing aircraft. No specific separation minima are considered necessary for helicopters because of the way they operate in relation to streams of in trail fixed wing aircraft. Separation can be applied as a minimum distance or time, dependent on criteria such as aircraft weight and weather conditions. Separation minima are not usually applied to helicopters unless they are operating IFR and in streams of fixed wing aircraft.
A hovering helicopter generates a downwash from its main rotor(s). Pilots should avoid taxiing or flying within a distance of three rotor diameters of a helicopter hovering or in a slow hover taxi, as the downwash can contain high wind speeds.
Source: FAA AC 90-23G
In forward flight, the wake 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 strong wake turbulence.
Source: FAA AC 90-23G
Accidents and Incidents
- P28A / S76, Humberside UK 2009: On 26 September 2009, a privately operated Piper PA28-140 with only the pilot on board was about to touch down on Runway 26 at Humberside Airport, after a day VMC approach when the aircraft rolled uncontrollably to the right in the flare and struck the ground. The aircraft came to rest inverted beside the runway and suffered significant damage but there was no fire. The pilot sustained serious injuries. The Investigation concluded that:
“The uncontrollable right roll experienced by the pilot of (the accident aircraft) was probably the result of the aircraft flying through the wake turbulence generated by the preceding Sikorsky S76.”