August 2015 Issue

Rotor Wash

Dealing with prop wash or jet blast is relatively straightforward: It is directed behind the aircraft. Wingtip vortices are a bit more complicated, but still they are easy enough to visualize. Helicopter rotorwash can almost be seen as a hybrid blend of the two. A recent accident at a Colorado airport implicated the rotor wash from a Blackhawk helicopter in the pattern with a Cirrus. It did not end well for the Cirrus, which dragged a wing tip and cartwheeled while in the landing flare. The drift of rotor wash from the recently departed Blackhawk is suspected as a contributing factor.

According to the Aeronautical Information Manual (AIM), “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.”

The difference is that helicopter turbulence is a lot more complicated to visualize. When a helicopter shifts from hover to taxi, what happens to the vortices? How do they move and drift? Section 7 of the AIM offers guidelines for dealing with rotor wash: “Pilots of small aircraft should avoid operating within three rotor diameters of any helicopter in a slow hover taxi or stationary hover.” In other words, keep your distance.