Commercial airlines are required to remove and overhaul aircraft landing gear about every 10 years or 18,000 cycles, depending on the airplane model, usage and applicable regulations. Below you'll find step by step of the Overhaul Process.
Step 1. The landing gear has digital pictures taken from all aspects to provide a permanent record of what is actually received.
Step 2. The landing gear is disassembled into individual components and the part numbers and serial numbers are recorded. The actual part numbers and serial numbers are compared to the Trace paperwork furnished by the customer.
Step 3. Parts are cleaned and bushings removed from those parts that have bushings. (Most landing gear parts that have bolt holes have bushings installed to
prevent wear.) All bushings are removed and discarded.
Step 4. Paint is removed utilizing a blast cabinet and the media used is plastic chips. The plastic chips remove paint but do not remove the cadmium or the parent material. The cadmium will be removed later by chemical strip where the hazardous content can be controlled.
Step 5. Parts are stress relieve baked 4 hours. Baking at 375˚ +/- 25˚ reduces residual stress from thousands of landings. Subsequent plating operations have high hydrogen content in the baths and hydrogen tends to travel within the part to any areas of stress. If hydrogen concentrates at a stress point the part can crack from hydrogen embrittlement.
Step 6. Parts are polished utilizing air tools to remove minimal corrosion.
Step 7. Parts are inspected and a detailed ICR (incoming condition report) is made and compared to the design allowances. LGS has a computer system where all parts have a detailed routing defining all operations that may be required to repair the part. The engineering department after review of the ICR selects those operations necessary to repair the part and prints a routing for that specific part and serial number.
Step 8. Parts are non-destructive tested for cracks by magnetic particle inspection or by penetrant inspection. Magnetic particle inspection for steel components puts the part in a machine where the part is magnetized, a light oil fluid that has fluorescent particles and iron dust in it is flowed over the part. A black light is utilized to show the fluorescent particles. If there is a crack the iron content of the fluid sets up in a line where the flux field of the current is broken and clearly shows a crack (the crack causes the brake in the flux field). In the case of non-magnetic parts such a aluminum the fluid used has no iron dust but does penetrate the crack if any. Part is cleaned of all surface fluid and again with a black light to highlight a crack is shown because the cleaning will not remove the fluid from a crack.
Step 9. Parts are sent on based on their specific routing to the machine shop or plating as required.
Step 10. Parts are shot-peened. Shot peen is a process all high strength parts are subjected to in new production where small iron beads fired at the parts at a high intensity. The peening pebbles the surface making a surface stress. If the part starts to crack the crack tends to follow the stress and cracks tend to be surface cracks rather than cracking into the part.
Step 11. Parts are stripped of the old cadmium, re cadmium plated and baked 23 hours at 375˚ +/- 25˚. Cadmium plating is an electroplate process bonding cadmium to the high strength steel of the part. Cadmium (much like aluminum) corrodes and the corroded cadmium provides a barrier to oxygen that is necessary for the steel to corrode. The cadmium bath is high in hydrogen content so the long bake is utilized to drive the hydrogen out of the part. This keeps the part from becoming susceptible to hydrogen embrittlement cracking upon use.
Step 12. Parts are magnetic particle inspected again. See step 6.
Step 13. New bushings are installed. The new bushings are made to the exact size for the part after all of it’s rework.
Step 14. Part is prime painted and top coat painted.
Step 15. Sealant is put at the edges of all bushings to preclude moisture from getting under the bushing. Sealing out the moisture stops the dissimilar material electric charge from setting up and causing corrosion.
Step 16. Parts are identified. And sent to stock for use or assembly.