What happened
Following a five-hour flight, the crew of a Westwind jet began descending toward the destination airport. During the descent, air traffic control issued specific altitude instructions, first directing the aircraft to cross a V/OR at 18,000 feet and subsequently instructing the crew to pass through an intersection at 6,000 feet. To comply with these instructions, the aircraft required a descent of 12,000 feet within a distance of 36 nautical miles.
After acknowledging the clearance, the aircraft ceased all radio communication. The plane subsequently struck treetops and crashed into a forested area. This flight marked the initial operation of the aircraft following recent maintenance procedures, which had included the disassembly and reassembly of the horizontal stabilizer trim actuator.
Findings
Investigations into the wreckage and the components of the horizontal stabilizer trim actuator revealed that the device had been improperly assembled prior to the flight. Specifically, components within the actuator had separated, showing no signs of damage at their attachment points.
Evidence from a separate ground test on a stationary aircraft demonstrated that a similar assembly error could lead to mechanical failure. During that test, when the pitch trim was moved toward a nose-down position, the jackscrews began backing out of the rod end caps. This process eventually caused the jackscrews to disconnect from the caps, resulting in the horizontal stabilizer becoming disconnected from the actuator mechanism.