The failure of a critical navigational aid was blamed by the National Transportation Safety Board yesterday for causing the business jet explosion and crash over McLean on April 28, 1977, that killed all four people on board the plane and destroyed a house on the ground.
The crash - preceded by a brilliant fireball in the sky that was seen for miles on a rainy night - apparently happened when the pilot tried to make a correction that was dictated by the faulty navigational aid.
The process of making that correction, the board said, "led to a loss of control and overstress of the aircraft structure." In other words, the plane, a Hawker-Siddeley 125, was ripped apart by forces exceeding its limitations.
The plane took off from Washington National Airport at 8:34 p.m. that spring evening and climbed to 9,200 feet. It came apart over the McLean Hunt subdivision just northwest of Tysons Corner. Parts of the plane, from small rivets to huge pieces of wing, rained over a wide area.
The home of Dennis Clarke, 1171 Old Stable Rd., was demolished and another residence was badly damaged. Clarke, his wife and four children fled from the house as parts of the plane came crashing through. Two cars were also damaged. There were no injuries on the ground.
The plane was owned by the Southern Co., an Atlanta-based holding company for electric utilities, and was en route to Birmingham. Two Southern Co. executives, the pilot and the copilot were killed.
The faulty navigational device was the "attitude indication system," which tells the pilot whether he is in level flight. The pilot was on instruments, it was raining and it was night. Given those conditions, the attitude indicator is the only clue the pilot has as to the "attitude," or position of the aircraft in relation to the horizon.
The board found that the pilot's and copilot's indicators showed differing aircraft positions, indicating they stopped operating at different times.
"If the attitude indicator tipped, the pilot would tend to correct it," a knowledgeable aviation source said. "When you correct, it is easy to exceed the design limits of the airframe." The left wing ruptured and the plane came apart.
Exactly what the pilot did or said is unknown, because business jets, unlike commercial carriers, are not required to carry cockpit voice recorders. The board has been pushing the Federal Aviation Administration to require such devices.
Turbulence, sabotage or engine malfunctions were ruled out as possible causes.