Wake turbulence rips aircraft apart

CHI06FA154

AOPA Air Safety Foundation

Like a speedboat plying the waters of an otherwise serene lake, every aircraft in flight generates a wake. Pilots used to call the disturbance “prop wash” and attributed it to the engines. As aircraft got bigger—and the wakes grew larger and more destructive—the phenomenon was studied and the true culprit identified: counterrotating vortices trailing from the wing tips, a by-product of lift. A new term entered the air safety lexicon: wake turbulence.

This invisible hazard is easy to ignore, especially for pilots who’ve heard the “caution, wake turbulence” drumbeat over and over without incident. But those unseen swirling curlicues are a very real threat, and disaster often strikes without warning.

On June 12, 2006, while on visual approach at Kansas City International Airport in Kansas City, Mo., the pilot of a Piper Saratoga crossed below the flight path of a Boeing 737 that was landing ahead on a parallel runway. 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 Grand Glaize-Osage Beach Airport in Osage Beach, Mo., at 6:25 p.m. on an IFR flight plan in visual meteorological conditions.

At 6:57 p.m., Kansas City Approach told the pilot to expect the ILS approach to Runway 01L. About 10 minutes later, ATC instructed him to descend and maintain 4,000 feet on a heading of 280 degrees. The pilot was informed of a Boeing 737, at 2 o’clock and 6 miles from his position, southbound turning westbound and descending from 5,500 feet.

At 7:09 p.m., the Saratoga was told to turn right to a heading of 300 degrees and to expect a visual approach to Runway 01L. ATC instructed the pilot to descend and maintain 3,000 feet. Two minutes later, the pilot reported having the airport in sight. He was cleared for the visual approach to Runway 01L and instructed to contact the tower. Shortly thereafter, on the tower frequency, the pilot began a radio call that became unintelligible. The tower controller responded by clearing the aircraft to land. The pilot did not reply.

Several witnesses on the ground reported hearing fluctuating engine noises and seeing pieces of debris, including a wing, falling from the sky separate from the aircraft fuselage, which spiraled to the ground with only one wing attached. The airplane’s left wing and both sides of the stabilator were discovered about 2,000 feet from the main wreckage. NTSB investigators determined that the buckling of the wing and stabilator spars was consistent with substantial in-flight loading. There was no evidence of fatigue cracking, corrosion, or other preexisting damage.

Radar data indicated that the Saratoga crossed the flight path of the Boeing 737 twice during the visual approach. At the point of the first crossing, the accident airplane was 1,600 feet below where the airliner had been two minutes earlier. No wake was encountered. The second time the Saratoga crossed the jet’s flight path, the accident aircraft was 600 feet below where the Boeing 737 had been two minutes earlier. The Saratoga’s airspeed was 183 knots—more than 50 knots above its design maneuvering speed and just 6 knots shy of its never-exceed speed. Radar contact was lost nine seconds later.

The NTSB determined that the accident’s probable cause was the pilot’s improperly planned approach that resulted in the encounter with wake turbulence while the airplane’s airspeed exceeded maneuvering speed. The encounter caused the subsequent loss of aircraft control and the in-flight separation of the left and right sides of the stabilator and the left wing.

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. The AIM further advises that when landing behind a larger aircraft (including one on a parallel runway within 2,500 feet), 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. Hitting a roiling wake at near-maximum airspeed can only end one way—in pieces.