Airline tires take a beating. When a tire reaches its service limit, maintenance technicians remove the entire wheel and replace it; the same for brakes. So, what happens to the old wheels, tires, and brakes? They certainly aren’t thrown away. Technicians send old wheels and brakes to a Wheel & Brake Shop for overhaul.
This is the second of two articles about aircraft wheels. Be sure to read the first article: Aircraft Wheels for more information about airliner wheels and tires!
Most airlines have their own wheel and brake shops — large carriers have several shops located around their systems. Smaller airlines may use a vendor for tire and brake services.
The UPS Airlines Wheel and Brake Shop in Louisville, KY provides wheel and brake services for its North and South America networks. I had the pleasure of visiting the shop to see what they do. It’s one of the most fascinating behind-the-scenes tours I’ve experienced.
Located in a nondescript warehouse, the facility operates a 9 hour shift, 5 days a week. It’s a busy shift. In 2018, the shop overhauled over 8,000 wheels.
Dozens of worn wheel assemblies arrive by air and ground each morning from as far away as Anchorage, Alaska and Santiago, Chile. Each wheel begins its visit at the disassembly area.
Wheels are first depressurized of any remaining nitrogen. Heat generated from takeoffs and landings fuse the tire beads to the wheel rims. The Bead Breaker is a hydraulic press that squeezes the 2 sides of the tire together to separate the tire beads from the rim. They become unstuck with a satisfying pop.
Two technicians raise the wheel to a comfortable work height and remove 20 or so nuts and bolts that hold the wheel hubs together. Wheels are separated and removed from the tire.
Farewell Old Tires!
Worn out tires are shipped to the manufacturer to be retread or recycled. A tire carcass (also called a casing) can be retread as many as 16 times. Airlines often impose lower retread limits to enhance safety. UPS retreads their main tires between 3 and 9 times, depending on aircraft and tire type. UPS tracks tire carcasses carefully to assure it receives the very same tires back from the retread facility.
Take a Bath
After disassembly, technicians place wheel halves and their hardware in an automated parts washing system. The washer looks like a large restaurant dishwasher and uses environmentally friendly solvents. Parts exiting the wash are sparkling clean and look nearly new.
When a wheel visits the shop, it receives a minor or major inspection as required by the manufacturer.
Most visits require wheel components to have a minor inspection. After cleaning, technicians visually inspect all parts for visible wear or cracks.
Wheel parts undergo a major inspection at regular intervals. After cleaning, technicians subject all parts (wheel hubs, nuts, and bolts) to eddy current and ultrasonic testing. The test equipment can detect hidden and microscopic deformities and cracks in the metal. Any component that is less than perfect is scrapped and the metal recycled.
Nothing is left to chance. Even the bolts holding the wheels halves together are ultrasonically tested for defects.
Alodine Coating Station
The Alodine Coating Station gives aluminum alloy wheels a protective coating to inhibit corrosion. Alodine coating is also known as chromate conversion coating – the metal surface is chemically transformed into a protective layer. I’m no expert on this stuff; I learned a bit about it here.
Wheel and Tire Assembly
After cleaning and inspection, it’s time to reassemble the wheels with fresh tires.
A TPMS (Tire Pressure Monitoring System) sensor (MD-11 and 747-8 main tires) and over pressure relief valve are installed in the outer wheel rim. Three fusible thermal plugs are screwed into holes on the inside of the wheel rim.
Learn more about these safety devices in the previous article: Aircraft Wheels
After the new tire is pushed onto the inboard hub, the assembly is laid on its side so the smaller outer hub can be positioned over the bolts. Techs hand thread the nuts, then snug them up with a pneumatic wrench. A large manual torque wrench is used to achieve final torque to factory specifications.
The following series of photos show the assembly process for a 747-8 main wheel.
The wheel and brake shop has a large liquid nitrogen tank outside the facility. Nitrogen pressure is reduced to 300 psi before entering the building and distributed to three tire filling cages.
Each wheel is rolled into a filling cage, nitrogen hose connected to the valve stem, and the cage is closed. A digital tire inflator accurately fills the tire to the proper pressure.
Why use a cage? Aircraft tires are pressurized to over 200 psi. If a mistake was made during assembly, or if the tire inflation system malfunctions, the results could be catastrophic.
The digital inflator pressurizes the tires slowly. It takes 30 minutes to an hour for a main wheel to reach full pressure.
Shop technicians disassemble, clean, inspect, and rebuild wheel bearings. After new bearings are installed in pressurized wheels, protective shipping covers are attached to protect the bearings and rims from damage.
Brakes Are Overhauled
Line technicians inspect aircraft brakes every day. Brake assemblies on a UPS 767 last about 1900 cycles, so they aren’t changed nearly as often as tires.
When a brake assembly reaches its service limit, a technician removes the wheel, pulls off the brake, and replaces it with a new one. Like wheels, fresh brake assemblies are kept on hand at scheduled UPS destinations.
Worn brake assemblies from UPS aircraft make their way to the UPS Tire & Brake Shop in Louisville. The brakes go through a similar process as wheels. Old assemblies are taken apart and cleaned. Parts that are warn beyond limits are discarded and recycled. Serviceable parts undergo thorough visual and metallurgical testing.
Wheel & Brake Shop techs reassemble the inspected brake assemblies and ship them back to UPS aircraft destinations worldwide where aircraft mechanics will install them as needed.
The Circle of Life (for wheels & brakes)
The journey begins all over again at airports throughout the UPS system. The photo above shows wheels worn to service limits in Anchorage, Alaska, awaiting shipment back to the Wheel & Brake Shop in Louisville.
I’d to give a very special thank you to the staff of the UPS Airlines Wheel & Brake Shop and UPS Airlines Public Relations for granting me access to this fascinating place!