Air Safety Institute Safety Spotlight
Thunderstorm respectOn June 25, 2006, the pilot of a Piper PA-34 Seneca attempted to fly through a line of convective activity over Tafton, Pennsylvania. The extreme turbulence from a developing thunderstorm ripped the aircraft to pieces, killing the pilot and his two passengers.
The 1,700-hour, instrument-rated pilot departed Piedmont Triad International Airport in Greensboro, North Carolina, for Sanford Regional Airport in Sanford, Maine. During a weather briefing before departure, the pilot and the briefer discussed thunderstorm activity along the route.
The first two hours of the IFR flight were uneventful. At 12:33 p.m., ATC informed the pilot of an area of severe weather six miles ahead of the airplane. The controller asked if the pilot had weather radar available. He replied that he had a portable GPS receiver with weather radar, and the unit was displaying weather at his one o'clock position. It was his final radio call.
Radar data indicated that the airplane descended nearly 2,000 feet in the final 40 seconds of flight. The Seneca's last radar target was observed at an altitude of 5,300 feet, which likely was the point at which the aircraft experienced structural failure. A witness near the accident site said he heard the sound of an engine "revving up and down." This was followed by a "muffled pop," then silence.
The NTSB determined that the accident resulted from the pilot's inadvertent encounter with a thunderstorm, which led to a loss of aircraft control and in-flight breakup. The destructive force of thunderstorms cannot be overstated. In addition to extremely heavy rain, they can contain strong wind shear, large hail, and severe turbulence, each of which can damage or destroy an aircraft. Pilots may encounter these hazardous conditions up to 20 miles from large convective cells.
Onboard weather-depiction equipment is a valuable safety tool for pilots, but we must appreciate the limitations of these devices. According to its manufacturer, the yoke-mounted GPS unit in the accident airplane received updated Nexrad weather radar images about every five minutes. Much can happen in five minutes when dealing with fast-moving and rapidly developing storms.
Moreover, radar images only depict areas of precipitation, not turbulence. Attempting to "thread the needle" between those bright colored blotches on a moving map is apt to take a pilot into some violently churning air. The Aeronautical Information Manual recommends completely circumnavigating any area that contains more than 50 percent thunderstorm coverage. Or it may be wise to find a nearby airport and tie down.
Carl Peterson is an aviation technical writer with the AOPA Air Safety Foundation.
By Carl Peterson