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Air Safety Institute Safety Spotlight

Killer wake

Every aircraft in flight generates a wake, caused by counter-rotating vortices trailing from the wing tips, a byproduct of lift. The unseen vortices known as wake turbulence can roil with tornadic forces (up to 200 miles per hour), and disaster can strike without warning.

On June 12, 2006, the pilot of a Piper Saratoga crossed below the flight path of a Boeing 737 that was landing ahead on a parallel runway at Kansas City International Airport. The Saratoga encountered wake turbulence so violent that it tore apart the aircraft in flight. The pilot and his passenger were killed.

The pilot had departed Osage Beach, Missouri, on an IFR flight plan in visual meteorological conditions. At 7:07 p.m., ATC informed the Saratoga pilot of a Boeing 737, six miles from his position, descending from 5,500 feet. Two minutes later, ATC instructed him to descend and maintain 3,000 feet; when the field was in sight, he was cleared for a visual approach to Runway 1 Left. Shortly thereafter, the pilot began a radio call that became unintelligible. Radar data indicated that the Saratoga had crossed the flight path of the Boeing 737 about 600 feet below where the jet had been two minutes earlier. The Saratoga's airspeed was 183 knots--more than 50 knots above its design maneuvering speed and just six knots shy of its never-exceed speed. Radar contact was lost nine seconds later. The NTSB concluded that the accident was caused by the pilot's improperly planned approach, which resulted in an encounter with wake turbulence while the airplane's airspeed exceeded maneuvering speed, leading to a loss of aircraft control and in-flight breakup.

According to the Aeronautical Information Manual (AIM), once a pilot has received traffic information and instructions to follow an aircraft and has accepted a visual approach clearance, it is the pilot's responsibility to ensure safe takeoff and landing intervals and a flight path that will steer the airplane clear of potential wake turbulence. When landing behind a larger aircraft, a pilot should stay at or above the larger aircraft's final approach flight path, note its touchdown point, and then land beyond it. As pilots, it's critical for us to understand the mechanics of wake turbulence and know how to avoid it. If we suspect we might encounter wake turbulence--or any kind of choppy air--staying at or below design maneuvering speed is key.

Carl Peterson is an aviation technical writer with the AOPA Air Safety Foundation. He has been flying since 1989.

By Carl Peterson

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