Event Description
During a scheduled descent, the captain attempted to apply nose-up stabilizer trim but received no response. The nose-down trim function remained fully operable throughout the flight. The flight crew did not attempt to engage the alternate trim system. They declared a jammed stabilizer to air traffic control and subsequently landed the aircraft with the stabilizer set in a nose-down trim position. The landing was completed without further incident.
Investigation Findings
After the flight, the horizontal stabilizer control unit was disassembled for examination. The inspection revealed that the gearbox contained an inadequate amount of lubricant and exhibited extensive corrosion. The corrosion was noted on multiple internal components of the gearbox.
Maintenance Program Observations
The investigation reviewed the maintenance procedures for the gearbox. The airframe manufacturer’s published procedure for replenishing the gearbox lubricant was found to have a design that could lead to underservicing, meaning that the gearbox might not receive the proper amount of lubricant even if the procedure was followed correctly. Additionally, the operator’s scheduled gearbox service interval was determined to be 250% of the manufacturer’s recommended interval, meaning the gearbox was serviced less frequently than advised.