Feedback Issue 2/2009

Hangar Noise

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Elevator Trim Actuators – Freezing Moisture

This SDR stated that the pilot noted that the elevator trim was difficult to move during cruise flight. Upon later inspection by maintenance personnel, the actuators (2) were found corroded internally and the grease was found to be contaminated and hardened. Hawker Beechcraft Corporation (HBC) had already issued Service Bulletin (SB) 27-3840 R1 to regrease these actuators with an improved lubricant. This SB had not yet been carried out. Both the L/H and R/H actuators were replaced and the aircraft was returned to service.

Maintenance personnel stated that the elevator trim actuators have to be removed in order to lubricate. There are grease fittings at the rear of the unit but only for the screw jack portion and not the gears. The actuator AFT cap, which was corroded, is made from magnesium and it appears that the corrosion begins at the area closest to the steel follow-up rod and the mating surfaces of the aluminum cap and body. The close proximity of these dissimilar metals may be promoting this corrosion problem.

Additionally, the lubricant gets contaminated with moisture/condensation causing the lubricant to become partially frozen at altitude. This causes the elevator actuator to react slower than normal to travel inputs from the pilot.

A Transport Canada Civil Aviation (TCCA) investigation with the Federal Aviation Administration (FAA) and the type certificate holder revealed the following information:

HBC are aware of previous reports of auto pilot disconnects in the pitch attitude that led to discovery of a freezing issue, thought to be caused by actuators being overgreased. This led to the issuance of HBC SB 27-3840 in June 2007. The SB was designated “optional” to permit the operator to change out the lubricant of the actuator with new grease. HBC also stated that the new lubricant should not be mixed although the supplier states that the two lubricants are compatible.

The actuator consists of two sections. There is a gearbox section with a light coating of lubricant on the gears. This section of the actuator has a drain hole to allow moisture to exit. The other section is greased by filling the screw and nut section with lubricant via a grease nipple. The gearbox section has a removable cover to inspect for lubricant overflowing through the actuator bearing into the gearbox section. Excess grease in the gearbox section may cover the drain hole and trap moisture in this area. This leads to frozen moisture in the gearbox and corrosion of the magnesium cap. The frozen moisture in the gearbox can cause high friction of the actuator and can also freeze between the gears and lead to high friction and jams.

HBC stated that the change of lubricant and lubrication procedures appear to have helped prevent the freezing condition, however the new lubricant is still somewhat affected by both moisture and cold weather, especially at high altitudes. A change in the aircraft maintenance manual will soon be introduced to provide instructions on how to properly lubricate the screw section of the actuator and on how to check the gearbox for overflow of lubricant.

To prevent future occurrences of this nature, TCCA highly recommends that HBC SB 27-3840 be complied with at the earliest opportunity.