Fortunately, many landing gear are over-engineered for the most part and few of these cracks and wear problems manifest themselves in the field. These problems are then discovered at overhaul and repaired, or components are replaced and rebuilt.
The overhaul process
According to Gramzinski, when their technicians receive a gear assembly in the shop for overhaul, the first thing they do is take special precautions to be sure the strut is not pressurized before disconnecting the torque link. In fact, Gramzinski says they have made it policy to tell their customers to depressurize the gear when they remove them from the aircraft. This is for safety and regulatory reasons.
Gramzinski reminds us that it's not really legal to ship a pressurized vessel under pressure, so you open yourself to liability for shipping hazardous materials if you don't depressurize the cylinder. Once satisfied, the strut is deflated and the technicians at PAA conduct a complete teardown and inspection. This requires draining the strut's oil, which is usually quite dirty and contaminated and turned to a sludge. "We also comply with all ADs and service bulletins on all gear."
"The most common is the nose gear on old style King Airs that have a welded strut assembly. These welds must be inspected with magnetic particle inspection for cracks. There is also a service bulletin on the early King Air torque links. These links must also be mag particle inspected for cracks," he says.
Gramzinski says many shops and FBOs overhaul landing gear, but they are seldom in complete compliance with overhaul requirements. "The unwitting shops will open up the gear and throw a couple of seals and O-rings in and throw it back together. I call that servicing — not overhauling. Overhauling is reestablishing new limits, bringing all components to the NDI lab and checking them for cracks. The NDI has to be done correctly as well. For instance, there are areas of the cylinder that when dye penetrant inspected won't show anything — then we put them on the magnetic particle inspection equipment and the cracks show quite well.
"One common mistake made by many shops is not measuring the center bearing internally of the nose landing gear cylinder on the 1900 and King Air. Many shops simply don't have the equipment to measure it. We have purchased an extension for our Bore gage to measure this bearing and determine if the cylinder is still serviceable, and offer a repair to bring it back to overhauled dimensions."
Another tricky process that many shops cannot offer is changing the axle on the main gear. This requires a bit of practice and in fact there are a couple of overhaul shops that send us all of their struts to replace the axles because they don't feel comfortable changing axles. We have seen some that other shops have tried where they ruined the socket that the axle inserts into. In the field, an axle is not available as a replacement part number — the entire assembly must be replaced if you have a damaged axle. Interestingly, it's relatively cheap to replace the axle alone, so it's worth it to send it out for an axle replacement. He adds that there are only around a dozen FAA-approved chrome plating facilities in the US and this is why the turnaround time on chroming is so long.
He also warns not to send landing gear to just "any type" of chrome shop. "If the shop is not FAA-approved, we won't even deal with them. The reason is that the plating must be done according to manufacturer's specifications, which is often proprietary information. On the 1900s, it's an industrial hard chrome that is machined down to specs, and it is hydrogen embrittled for hardening and once that is done, it doesn't flake off."
Landing Gear Overhaul A behind the scenes look at some of the steps involved in this inspection By Thomas Davis Photos by Joe Escobar November 2001 There is an old adage in...
DC9/MD80/737 Airstairs The forgotten ones By Greg Napert September 2000 There was a time when the image of a passenger aircraft pulling up to a small terminal and deploying a...