Avcon Industries

Learjet 25

During a test flight, the airplane encountered an elevator system oscillation while in a high speed dive outside the normal operating envelope. The 17 second oscillation was recorded on the cockpit voice recorder and had an average frequency of 28 Hz. The aft elevator sector clevis (p/n 2331510-32) fractured due to reverse bending fatigue caused by vibration, resulting in a complete loss of elevator control. The flight crew reported that pitch control was established by using horizontal stabilizer pitch trim. The flightcrew stated that during final approach to runway 17 (13,337 feet by 200 feet, dry/asphalt) the aircraft's nose began to drop and that the flying pilot was unable to raise the nose using a combination of horizontal stabilizer trim and engine power. The aircraft landed short of the runway, striking an airport perimeter fence and a berm. The surface winds were from the south at 23 knots, gusting to 32 knots.