What happened
While descending through 10,000 feet mean sea level, the Brasilia experienced a violent nose-down pitch of approximately 10 degrees, followed by a nose-up movement. The captain had the autopilot engaged at the time of the event. The sudden motion threw the lone cabin attendant in the aft end of the aircraft to the floor, resulting in one broken ankle. Following the pitch change, the captain disengaged the autopilot and hand-flew the aircraft to touchdown.
The investigation
The investigation focused on the two elevator trim actuators. The manufacturer had previously identified moisture and corrosion in several actuators submitted for repair following similar incidents. It was suspected that leaking seals allowed moisture to enter the units, which then froze at high altitudes. In many cases, these actuators returned to normal operation once the aircraft descended into warmer temperatures and the ice thawed.
Examination of the first unit revealed it was dirty, corroded, and covered in overspray. This unit leaked from the autoloc seal and shaft seals, failed the non-jamming stop test, and contained moisture within its two shaft housings. Further findings included corroded chains and pins, deteriorated packing, weak springs, a worn hub, and non-chamfered rollers on one side. Additionally, staking had warped some screws.
The second unit was also dirty and featured overspray. It exhibited leaks from the shaft seals, excessive backlash, and failed the tangential free play test. Moisture was found in both shaft housings. The investigation noted corrosion on the chains, pins, shafts, and housings, as well as deteriorated packing and rough, binding bearings. This unit also had rollers with only one chamfered side and was missing its shim.