I can’t tell you the number of times I’ve been told to “taxi to the pumps and we’ll get you topped off” with a light fixed-wing sitting right there and a trash can with a lid on it 20 ft. away. They don’t know what they don’t know, and they haven’t been taught. In 1996, the FAA published a report, “Flight Test Investigation of Rotorcraft Wake Vortices in Forward Flight,” based on tests performed to determine the need for rotorcraft separation standards based on the wake vortex (think “wake turbulence”) hazards of following aircraft. The tests involved four helicopters: the S-76, UH-60, CH-53, and CH-47. The FAA team used a T-34 and a Decathlon for probe aircraft. The testers learned that within 3 nm behind the helicopter, the probe aircraft experienced bank-angle upsets that exceeded 30 degrees and, in some cases, more, with some resulting in a spin. More recently, a Cessna 172 pilot
experienced helicopter wake turbulence flying behind a departing R-44, which resulted in full aileron deflection and rapid increase in vertical speed followed by a rapid decrease in vertical speed. Similarly, a Cirrus SR20 landing behind a departing UH-60 ended up cartwheeling down the runway with substantial dam- age to the aircraft and injury to the pilot. And a PC-12 fixed-wing landing behind a departing UH-60 experienced more than 30-degree bank angles. Thanks to some fast maneuvering by the pilot, and power- ful thrust, the aircraft avoided crashing. In September 2021, an experimental
Rans S-20 departed behind a landing S-76 air ambulance helicopter. The airplane reached approximately 50 to 60 ft., rolled left, then rolled right until inverted and impacted the runway, resulting in a post- crash fire and one fatality.
Taking the Lead The 1996 FAA study recommends that to avoid “hazardous” helicopter wake vorti- ces/wake turbulence, fixed-wing aircraft in trail should remain at no less than 3 nm
behind the helicopter. The report further shows that vortex decay time can take up to 3 minutes depending on the size and speed of the helicopter. But even 3 nm might not be enough. Earlier this year, I was teaching an
instrument student in a Cessna 172. An EC135 was practicing the same ILS (instru- ment landing system) approach. We were 4 nm in trail with both of us at 90 kt., and
we still felt a light wing rock from the heli- copter wake turbulence. The fly neighborly movement in the ver- tical flight industry has rightly emphasized the need to minimize the noise impact of helicopters on communities, but we must consider the effects of our rotor downwash and wake turbulence, as well. Let’s take the lead to educate the entire aviation com- munity about both phenomena.
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SEPTEMBER 2022 ROTOR 67
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