Lubrication of the horizontal stabilizer trim actuator (HSTA)

Effectivity: Challenger 300 series aircraft 

ATA: 27 

By: Patrick Bernèche – Technical Services   

Bombardier has recently received two reports from operators who have spare horizontal stabilizer trim actuator (HSTA) units with no oil visible in the HSTA 90-degree angle filler port. In one case, the HSTA was later confirmed properly serviced at the vendor. In the other case, a minimal amount of oil, well within the allowable limits of AMM TASK 12−22−00−640−809 (Lubrication of the Horizontal Stabilizer Trim Actuator), was added prior to its installation.  

The assessments of both cases concluded that both units were properly serviced by the vendor.  

It is not necessary to check the oil level of spare HSTAs received from Bombardier prior to installation. The HSTA oil level is verified by the vendor before shipment to Bombardier. 

In some cases, the fluid level may not be visible through the HSTA 90-degree angle filler port. This condition is not to be interpreted as the unit is missing oil. During the vendor servicing, air can become trapped inside the HSTA housing and cause the above-mentioned condition.  

In case of doubt, the AMM TASK 12−22−00−640−809 allows up to 135 ml of oil to be added into the HSTA. It also provides instruction to lightly tap the housing of the HSTA with the two fill port plugs removed to allow any trapped air to escape.  

This “added oil method” is used to validate if the HSTA oil level is in the acceptable range. If more than 135 ml of oil is required, the HSTA must be returned to the vendor for inspection. There is no oil sight glass or dip stick to check the HSTA oil level. The “added oil method” provided by the AMM task 12-22-00-640-809 is the only approved procedure for checking the HSTA oil level. 

Bombardier would like to remind all maintenance personnel that it is not acceptable to insert any object into the HSTA filler hole to check the oil level. This can cause HSTA internal damage or oil contamination.  

If you have any questions about HSTA lubrication, please contact your Bombardier Field Service Representative (FSR) or the Customer Response Centre (CRC) team 24/7.