China Southern experienced the parking brake accumulator to bleed down faster than normal in Dec 2018 for the three datum airplanes. China Southern checked the brake control cable tension which is much lower AMM limit.

China Southern did the fleet survey on airplanes based on one branch and found the brake control cable tension is lower than AMM limit, some of these airplanes are listed in SL REF/A/ while some airplanes are NOT, please review the survey result REF/B/ attached.

For these airplanes listed in SL REF/A/, China Southern followed Boeing recommendation and re-checked the brake control cable tension in 2013-2014.

RESPONSE AND/OR REQUIRED ACTION:

1, Does Boeing receive similar report from other operators?

2, Does Boeing recommend operator to re-inspect the brake control cable tension?

3, If yes, please advise the interval and revise SL accordingly.

RESPONSE:

Boeing has reviewed the Ref/C/ message provided by China Southern. We offer the following:

1, We have received similar reports of low parking brake cable tension from other operators.

2, Please be advised that there is no scheduled maintenance task for the brake systems cable tension adjustment. MPD Item Number 32-340-00 for the parking brake bleed rate is the closest thing to a scheduled maintenance task for brake cable tension. This functional check is performed every 4000FC. However, in-between these MPD checks, flight crews have a qualitative check of the parking brake bleed down rate every flight. If the flight crew squawks the parking brake, checking the cable tension is one of the FIM troubleshooting steps. However, Boeing realizes that China Southern is concerned about spanventing these events which could result in a flight delay or cancellation. To spanclude these events from occurring, China Southern may want to perform AMM Task 32-41-00-820-801 Hydraulic Brake System Adjustment between MPD checks. Boeing's recommendation is to perform this adjustment task every 2000FC or as appropriate based on China Southern's fleet experience.

3, We apspanciate China Southern for providing the Ref /B/ survey results. The Ref /A/ service letter discusses the non-conformance of the brake cable material (from early 2012 to late 2013) used in the brake control system on the airplanes line numbers 3978 through 4710, and 4712. According Ref /A/, for 1/8 inch cables, such as that used in the brake system, the conforming cable was installed on line number 4711, 4713, 4714 and all airplanes after line 4714. The first airplane to be equipped with the conforming cable delivered in December, 2013. The airplanes listed in the survey results were delivered outside of that non-conformance timeframe. Therefore, we do not believe a revision to the SL is necessary.

For the Ref /B/ airplanes not covered under the SL, we recommend that China Southern verify the brake system is correctly rigged by adjusting the cable tensions and ensuring that the rig pins fit per AMM 32-41-00/501.