What happened
While approaching the airport, the pilot was in receipt of landing instructions when radio transmissions were abruptly terminated. Witnesses on the ground observed the aircraft perform an abrupt nose-over and descend into the water at a high rate of speed. The impact caused the aircraft to fragment, and it sank immediately.
The investigation
The aircraft had recently undergone maintenance that included the removal and installation of its wings, as well as the installation of an autopilot and electric trim system. Following this maintenance, the aircraft was undergoing flight testing. During this test, the pilot experienced a loss of control characterized by a pitch-down motion consistent with witness accounts of the accident.
An examination of the upper stabilator pulley assembly revealed that the control cable had been rubbing against the side of the pulley. This friction caused the pulley to fracture. The cable subsequently became wedged within the pulley, which then became wedged against the side of its support bracket. This mechanical failure resulted in restricted movement of the stabilator.