Flap Transmission Universal Joint Bolt Failure
It was during a standard ground operation of a Boeing 737-200 aeroplane when a flap retraction discrepancy was discovered with the right wing outboard flap panel. This was detected as the outboard flap panel would not fully retract past the #1 position. Further maintenance investigations revealed that a #7 flap transmission universal joint bolt was missing its’ adjoining locking-nut, as seen in figures 1 and 2. The bolt thread to pin shank had sheared allowing it to migrate out from its yoke. The defective bolt was replaced, a dual inspection and functional checked was performed and the aeroplane was returned to service.
Through the evaluation of the operator, it was suspected that the cause of the universal joint bolt failure may have been attributed to stress corrosion imparted by a possible over-torque of the locking-nut and/or aggravated due to the aeroplanes previous operational environment. As a subsequent precautionary measure, the operator removed all flap transmission universal joint bolts (quantity 4, part number (P/N) 65-76606-1 and quantity 4, P/N 65-76607-1) for a magnetic particle inspection (MPI) resulting in a second bolt being indentified with a hair line crack, as seen in figure 3.
Further precautionary measures were taken from the operator through a campaign to inspect their remaining B737 aeroplane for any similar potential defects.
As an added measure of safety by the operator, three additional actions were taken to prevent flap transmission universal joint bolt failures as follows;
- “high lite” the effected attaching bolt hardware with yellow paint to ensure any loss of retention is readily detectable.
- include a specific task within the pre-flight inspection for the presence of the locking nut on the bolt of the universal joint assemblies.
- place the flap transmissions assembly on an in house soft time overhaul limit of 9000 hours or 10 years, whichever occurs first.
Through this proactive approach by the maintainer, the company’s mandate pertaining to safety was met.
A review of Boeing’s available maintenance documents in relation to this event identified the issuance of Service Bulletin 737-27-1265 in March 2005 which recommends the replacement of the universal joint bolts with a newly designed bolt.
The bolt material was changed to address stress corrosion and a self-locking castellated nut with cotter pin was added to address the potential backing-off and loss of the bolt.
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