Nose-Gear Shimmy, Low Torque-Link Bolt Torque, and Lug Crack
After a heavy maintenance visit, a 737-800 has a nose gear shimmy complaint. During a taxi test, shimmy occurs at 18 knots, and a 6 mm crack is found at the upper lug of the nose gear torque link fork. The torque link bolt was found 30% below specified torque. The maintenance manual allows blending of the lug if crack length is less than 5 mm and depth less than 0.5 mm, followed by fluorescent penetrant inspection. The operator wants to release the aircraft for a 2-hour flight with 160 passengers. The steering system shows no fault messages, but the shimmy dampener has no recorded servicing for 900 flight hours. The AME must decide.
1. Evaluate the crack against the maintenance manual allowance. Can the lug be blended and the aircraft released?
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The crack is 6 mm, and the manual allows blending only when the crack is less than 5 mm and the depth is less than 0.5 mm. The length alone exceeds the limit, so blending is not permitted. The depth is not stated, but that does not help; a crack that is 1 mm over the length limit is outside the approved data. The 30% below specified torque is significant: a loose torque link bolt allows relative movement, fretting, and crack initiation at the lug. The shimmy at 18 knots shows the nose gear is not stable, and shimmy adds cyclic loading that can grow the crack. Releasing the aircraft for a 2-hour passenger flight would be unsupported. The lug or torque link assembly should be replaced or repaired under approved data, and the cause of the low torque and shimmy should be corrected before return to service.
2. What is the likely relationship between the low torque, shimmy, and crack, and what inspections would you perform?
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The low torque is the likely root cause. Without correct clamp-up, the torque link joint moves, the lug frets, and a crack starts at the highest stress point. The crack then reduces the stiffness of the nose gear assembly, which can produce or worsen shimmy. The shimmy dampener with no servicing for 900 flight hours is another contributor and should be serviced or replaced. I would inspect both torque links and lugs with fluorescent penetrant and eddy current, check the bolt holes for elongation and the bolts for wear, check the nose wheel for balance and tire damage, and check the steering actuator and shimmy dampener. After repair, torque to the specified value, rig the steering, and perform a taxi test through the shimmy speed range.
3. If a ferry is necessary, what conditions and approvals would you require?
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A ferry with a 6 mm crack in a nose gear torque link lug would require a special flight permit and an engineering assessment specific to that crack. I would require the gear to remain down and locked, no retraction, no passengers, low taxi and flight speeds, and a route to a nearby maintenance base. The aircraft should be inspected before and after taxi and after landing. If the crack cannot be shown to be stable for the flight, or if the shimmy is uncontrolled, I would refuse the ferry and repair on site. In practice, replacing the torque link or lug at the current station is the lower-risk option; a ferry is not a substitute for an approved repair.