Corrosion is one of the major threats to landing gear safety and reliability. The time between overhaul, various climates and conditions that aircraft are operated in, and the frequency of lubrication and pressure washing of the landing gear all may result in corrosion damage to various parts. Corrosion weakens critical structural members of the landing gear, concentrating points of stress that can lead to failure. Corroded surfaces in sealing areas cause hydraulic and nitrogen leaks and interfere with the smooth and proper operation of mechanisms. In some cases, service bulletins, manufacturer modifications and upgrades that improve sealing efficiency can be incorporated; such as the replacement of magnesium parts and the addition of improved protective coatings.
A list of all discrepancies noted during the disassembly and evaluation is generated and recorded in the work order. This list is developed into a proposed work-scope and a quote is given to the owner/operator for repairs and parts needed to restore the landing gear to an overhauled condition. Upon the customer approval, work continues on the landing gear. Parts are ordered from the stockroom. Any necessary repairs are initiated and all maintenance is performed using the manufacturer’s maintenance instructions and FAA Approved Repairs.
Bearings are inspected and can be reconditioned and reused in some cases. Bushings are normally worn and must be replaced. The bushing bores are inspected and reworked if needed. Bushings are sized to fit their mating pins or part for a perfect fit. Replacement of bushings and bearings is precision, close tolerance work. In many cases, these parts carry the weight of the aircraft and the shock of landing. Many bushing and bearing installations are interference, or pinch fit. Sometimes these parts are assembled by differential temperature method. This involves heating the bore, observing a temperature that does not affect the base metal temper or heat treat, and chilling the part by immersing it in liquid nitrogen or a slurry of dry ice and alcohol. Many times, the manufacturer specifies that these parts are installed with some type of protective coating between them and the bore. In all cases, great care must be taken when assembling close tolerance parts to avoid damage to the assembly.
Platings and coatings
Disassembly of landing gear
Chrome-plated surfaces are inspected and replated with the correct specification process if needed. All protective processes such as cadmium plating, black oxide coatings, primer and paint are renewed, even when this is not required by the manufacturer’s instructions. Primer and paint, when specified, is always applied prior to assembly for complete protection against corrosion.
The landing gear assembly procedure is inspected, both in process and during final assembly, by the Inspection Department. Special tools and assembly fixtures are often used during the assembly process to ensure the correct fit and operation of parts. Wiring harnesses for position indicating, weight on wells, brake anti-skid and nose-wheel steering are completely overhauled and thoroughly tested for proper operation upon installation. Wheel brake and nose-steering and rigid hydraulic lines are inspected and tested. Flexible lines are replaced. All the landing gear hydraulic system components are tested at final assembly.
The shock absorber and other hydraulic components are serviced with hydraulic oil and dry nitrogen (always use dry nitrogen to inhibit corrosion of internal components). The landing gear/shock absorber is put in a special hydraulic press to compress the shock for testing proposes for proper operation, internal friction, correct service pressures and leakage. After the testing procedure, the shock absorber is deflated to a reduced pressure for shipping.
The gear receives final sealing as required and paint touch up. Data plates are attached at final inspection. The work order is completed, signed off by the technicians and inspectors and an 8130-3 is attached to the gear assembly.
This has been a brief look at what goes on during a landing gear overhaul. It is a process that helps ensure the continued safe operation of the landing gear until the next TBO.
Thomas Davis is Chief Inspector for Dallas,
Texas-based TXI Aviation, a Part 145 Repair Station certified for landing gear overhaul. For more information, you can contact TXI at (972) 647-7300 or visit their website at www.txiaviation.com.
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