Landing Some Information: A fresh perspective on why landing gear need an overhaul

Oct. 1, 1999
Landing some information

A fresh perspective on why landing gear need an overhaul

By Greg Napert

October 1999

Thousands of pounds of pressure bearing down thousands of times — hard landings; missed approaches; dirty, dusty runways; 105 degrees F to -60 in 30 minutes; student pilots; overheated brakes. Welcome to the life of a landing gear. You know - those brilliantly engineered devices that, if the rest of the aircraft were made as well, would put aircraft maintenance personnel in the same room as the Maytag repairman.

It's true, landing gear are not very problematic relative to the rest of the aircraft, yet all it takes is one landing gear failure to realize why it is they were engineered the way they are. And contrary to what one might think, a quick breakdown of landing gear at the end of its cycle can give pause to the technician over the need for regularly scheduled care and inspection.

So it is, that AMT magazine travelled to Professional Aircraft Accessories (PAA) in Titusville, FL, to take a close look at a landing gear overhaul facility and the process of reconditioning landing gear — landing gear that have reached the end of its design life.

PAA specializes in the overhaul of Beechcraft 1900 and King Air landing gear. Joseph Gramzinski, Chief Inspector of the landing gear overhaul department for PAA, says the typical overhaul period for Beech 1900 and King Air series of aircraft landing gear is 10,000 cycles or five years, although some airlines that have been allowed to extend that to 12,000 cycles. In reality, however, airlines never make it to five years. They reach the 10,000 or 12,000 cycle limit way before the five-year period — typically within two to three years.

Aging aircraft
With regularly scheduled overhauls, landing gear can actually be in service for years. In fact, many landing gear in operation are 30-plus years old — a concern for those in the overhaul business.

According to Gramzinski, "One of the things that we are seeing in the shop more frequently as time goes on is severe corrosion — particularly with older magnesium components used in landing gear. Older King Air gear were manufactured with magnesium, which is a lightweight and very strong material." Unfortunately, it's come to light that magnesium has inherent problems that make it unsuitable for landing gear use. The biggest problem being corrosion as a result of dissimilar metals and exposure to elements. A steel bushing inside a magnesium housing or grease fittings, for instance, will all show signs of corrosion around the area where the magnesium and steel are touching — particularly where there is an opportunity for moisture to enter between the two metals."

This nut exhibits dissimilar metal corrosion on the threads and will need to be replaced.

"I have also seen gears cracked in the corroded area. Beech has since grown wise to the problems related to magnesium and has replaced all magnesium parts with aluminum. Aluminum is adequate in terms of strength, with the advantage of being much more resistant to corrosion."

"It's not very often that we even get a magnesium gear that is re-useable today. Many of them have been in service for many years and it's time to replace them. I would say that four out of 10 magnesium gears are shot and need to be replaced," says Gramzinski. All of the 1900 landing gear are aluminum so they don't present the same kinds of problems. However, 1900 landing gear carry their own set of problems, the major one today being with the 1900D landing gear.

"The problem with the 1900D is that when Beech built the 1900D, a much bigger and heavier aircraft, they used the same gear that was designed originally for the 1900," says Gramzinski. "The result is that 1900D gear exhibit much higher rates of cracks and wear than gear that are used on the 1900."

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.

Older King Air magnesium cylinders are prone to corrosion, heavy pitting, and dissimilar metal corrosion at bushings and grease fittings.

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."

This older 1900 nose gear on the left requires additional inspection of the welds for cracking. The new design on the right eliminates this inspection requirement.

"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."

Extreme care must be taken to make sure the strut is depressurized prior to disassembling or removing any hardware on the drag link.

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."

"On the chrome finishes, the main causes for rejection are corrosion, scoring, and wear. What we actually do is check the cylinder for wear to make sure it's within acceptable dimensions. Scoring that cannot be felt with your fingernail and can be buffed out on the lathe is reusable. Otherwise, any scoring that is beyond buffing out is cause for rejection and requires re-plating. Discoloring is typically not a problem and can easily be polished to look like new."

The inside surface of the bearing on the left is damaged and requires replacement of the phenolic surface. This is a repair that can be accomplished under a process specification.

He continues, "The Beech manual is quite vague. It says to look for grooves and gouges and doesn't really give any limits. We reject anything that cannot be polished out or re-plated. The upper housing is actually quite forgiving in terms of gouges and dents. We can usually remove most minor damage and blend-out most nicks that are within reasonable limits."

Gramzinski says that it is also prudent at overhaul to replace all hardware and seals — this includes all nuts, bolts, clips, seals, etc. "It's not worth having the entire assembly fail for a nut that costs a few dollars."

Landing light mounts are prone to corrosion due to dissimilar metal contact. These should be checked and cleaned occasionally.

A typical overhaul on the landing gear can be accomplished within five days, unless the components need re-plating. Re-plating of the chrome will add another two weeks to the overhaul process. But, because PAA typically have many replacement components in stock for the 1900, it can usually incorporate a reconditioned part from its inventory into the overhaul and complete it within the five-day period.

Repair techniques save $
"We have worked hard over the years developing repairs aimed at saving our customers money. We have developed several FAA-approved process specs for re-plating bores and re-anodizing shafts and bushings to restore the original dimensions to many components that would otherwise be thrown away. This requires an investment in engineering and FAA red tape, but it's worth it to reduce overhaul costs," he says.

For instance, PAA has developed a repair for the bore of the torque link attach points, for which it was having to throw away the cylinder or strut.

The upper bearing of the main gear must be inspected for wear during overhaul.

"This was a $5000 part and we decided to machine out the bore and install a bushing. We had the repair engineered and approved by the FAA and use it regularly."

Some repair schemes are not quite as simple as installing a bushing. Some are in critical areas where material cannot be removed to accomplish the insertion of a bushing. "This is why you've got to make an investment in engineering and determine what can and cannot be done," he says. "A recent study we did for one of our airline customers determined that through incorporation of all of our repair schemes, they saved just under $300,000 in a period of two years."

Repairs offered by PAA include bushing repairs to the upper main gear and nose gear housing, and the lower nose strut, repair of internal bearings in the nose cylinder, replating and repair of the axle assemblies, and re-chroming of the tube assemblies. A recently added repair is the replacement of the phenolic coating on the main gear lower bearing. "This bearing costs around $300 to replace. We have developed a procedure to reline this bearing with new phenolic — cost is around $110."

Protection from corrosion
Gramzinski says there are several steps that can be taken to keep the landing gear from corroding severely during the next in-service period.

One of those is to use a poly-amid epoxy primer instead of the traditional zinc chromate. This epoxy hardens to create a very durable corrosive-preventive primer and is a great improvement to the zinc chromate used by many shops.

"We also recommend taking the extra step of applying primer to the inside surfaces of the cylinder on the non-bearing surfaces," says Gramzinski.

He adds that PAA coats all exposed surfaces, to include the axle with either permanent or temporary protection from the elements so the gear can be stored for significant periods of time. For instance, the axles are coated with a petroleum-based wax and all the landing gear shipped by PAA are filled with hydraulic fluid and all air is removed from the system.