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Go beyond the book when teaching wake turbulenceGo beyond the book when teaching wake turbulence

Twice in the past three weeks, relatives of pilots who suffered wake-turbulence upsets have called the Air Safety Institute. They didn’t know each other, but shared a concern that the risk isn’t sufficiently appreciated by the community—meaning not only the light-aircraft pilots who are primarily responsible for avoiding it, but those operating machines powerful enough to generate dangerous rotor wash or wingtip vortices, not to mention air traffic control.

The first was a pilot himself (and AOPA member). His wife, a student pilot, sustained multiple fractures during a dual lesson when a military helicopter overflew their Cherokee 140 on short final. Rotor wash rolled the airplane 90 degrees before any control authority was regained, and it struck a wingtip. The resulting impact destroyed the airplane; the instructor escaped with relatively minor injuries. This took place at a nontowered airport in North Carolina where resident fixed-wing pilots have complained of other conflicts with Black Hawks and Apaches conducting training missions. Conversations with the safety officer of the unit involved have so far failed to produce a lasting resolution.

It sounded like a one-off, but a quick search revealed that it was one of three similar accidents that occurred nationwide on the same day. In Alabama, a Cessna 172 on short final was rolled by rotor wash under nearly identical circumstances, while in Salt Lake City a high-wing homebuilt taildragger was blown off the runway during the landing roll as five Blackhawks hovered over the taxiway. The Skyhawk was destroyed by a post-crash fire and its pilot was seriously hurt. The homebuilt ground-looped, causing minor injuries to its pilot.

Bad as these were, the second call was far worse. The caller’s husband had been a passenger in a Piper Arrow making the jaunt from Racine, Wis., to Oshkosh in July. The pilot was a 33,000-hour airline transport pilot and CFI. Climbing through 1,800 feet seven minutes after takeoff with VFR flight following, he acknowledged ATC’s alert for an MD-80 airliner on final approach to Milwaukee. The Arrow pilot offered to “go lower,” which the controller declined, and then called traffic in sight. The controller instructed him to “pass behind that traffic and then you can proceed northbound as requested.” The Piper passed less than a mile and a half behind the airliner, crossing its path 39 seconds after the MD-80 had passed the same point at the same altitude—and almost immediately broke up in flight. Radar showed four closely spaced primary returns; search-and-rescue efforts only succeeded in recovering about half of the airframe. The fuselage was broken into three pieces, the left wing separated entirely, and only the inboard portion of the right wing remained attached.

The National Transportation Safety Board noted that the controller never issued a wake turbulence advisory, but the pilot’s experience might have led him to anticipate the risk. His offer to “go lower” suggests that it had been a long time since he’d thought about the mechanics of wingtip vortices, which propagate laterally and downward but do not rise. Perhaps he also placed a little too much reliance in the rule of thumb that holds that they’re most powerful when the aircraft generating them is heavy, clean, and slow. The landing MD-80 presumably wasn’t in a clean configuration, but “heavy” and “slow” proved more than enough.

Wake turbulence and its cousins are almost unique among environmental hazards in being simultaneously invisible, unpredictable, isolated, and severe. Thunderstorms can usually be seen visually or via radar; high winds give plenty of warning of the turbulence they cause, while clear-air turbulence is rare at the altitudes flown by most light aircraft. Icing requires visible moisture and cool temperatures. But while we know that helicopters generate downwash and airplanes create vortices, we can only guess at where the wind will push them and how quickly they’ll dissipate as they descend. The only sure ways to stay clear are to remain above the flight path of the other aircraft or maintain enough lateral separation to be sure they’ll have room to settle—both difficult to manage when that 22,000-pound, 3,800-horsepower combat helicopter suddenly overflies your approach path. Greater awareness of the effect they can have on lighter traffic on the part of both the pilots operating the heavy iron and those trying to share the same airspace would help ratchet down the risk. If your home field also sees a lot of military traffic, it’s a profitable subject to bring up with your instructors.

ASI Staff

David Jack Kenny

David Jack Kenny is a freelance aviation writer.

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