Landing Gear Overhaul
A behind the scenes look at some of the steps involved in this inspection
By Thomas Davis
Photos by Joe Escobar
There is an old adage in aviation: "Take offs are optional and landings are mandatory." Landing gear supports the aircraft at rest and during the landing where the most pressure and stresses are imposed. The shock absorber does what it is designed to do, absorbs the initial shock of the landing and transmits the shock forces in reduced levels to the rest of the landing gear and to the airframe. In order to assure safe operation of the landing gear, most aircraft manufacturers have specific overhaul limits established, as well as life limits imposed on the landing gear assemblies, or on parts within the assembly. The following article is based on some of the procedures used by Dallas, Texas-based TXI Aviation when performing landing gear overhauls.
Components receive NDT inspections as required.
Upon arrival, a visual inspection is conducted for obvious damage and missing parts. A complete inventory of the landing gear assemblies is performed. The customer is notified immediately of any missing or surplus parts. Taking care of missing parts promptly keeps the work on schedule and avoids unnecessary cost to the client. Returning any surplus material to the customer right away is important because many times landing gear work is done in conjunction with other airframe work which might involve these parts.
Once a work order is opened and the Incoming/Outgoing check sheet is begun, the gear is disassembled. The parts are then cleaned and prepared for inspection. Landing gears are subject to rough service and are normally pretty dirty when received for overhaul. Dirt, asphalt, salt deposits, brake dust, grease, and oil, in addition to paint and primer, must be removed. A variety of approved cleaning methods are employed including solvents, steam, soap and other cleaning agents, and blasting. When using media blasting, plastic blast media is used on non-ferrous material such as aluminum and magnesium, and glass blast media is used on ferrous material such as steel. Ferrous and non-ferrous materials and their blast media are kept separated to prevent contamination and dissimilar metal corrosion. Once the parts are clean, they are ready for inspection.
The evaluation process involves inspecting the landing gear per the manufacturer’s overhaul or inspection requirements. The general objective is to inspect the gear for wear, corrosion and damage. Fits and clearances of major parts are measured for wear tolerances. Dimensional inspections are performed using precision measuring equipment and tolerances can be as fine as .0001 inch. When applicable, a geometrical check of the gear’s major parts is performed to assure proper alignment and confirm the landing gear was not bent or overstressed.
Parts that require non-destructive testing (NDT) are sent to the NDT department for the inspections required. Magnetic Particle, Dye Penetrant or Eddy Current inspection of all parts requiring NDT are accomplished. Parts with indications of cracks or defects are noted in the work order for further evaluation, repair or replacement.
During the evaluation phase, FAA life-limited parts, Airworthiness Directives and Manufacturer Service Bulletins are researched to ensure compliance. For maximum reliability, manufacturer service bulletins and other improvements to aircraft landing gear should always be incorporated per the aircraft and component manufacturer’s current instructions. Many of these bulletins are interrelated with each other as well as with the overhaul manuals and aircraft maintenance schedule.
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.