The emerging use of composites, more durable metals and alternative corrosion-resistant coating processes is being incorporated in landing-gear design and maintenance. For landing-gear service specialists, this could be a game changer.
“The increasing use of composites in landing gear will accelerate a trend toward more on-condition maintenance and away from heavy overhauls,” says Tjaard Sijpkes, head of technologies for Fokker Landing Gear in Helmond, Netherlands. “That is the future of landing gear.”
Fokker Landing Gear, a business unit of GKN Aerospace, is a landing-gear OEM and MRO, supplying sets for the in-production Bombardier Q400 twin turboprop airliner. As a repair station, the company services landing gear on the Boeing 737 and Fokker’s legacy airliners, including the Fokker 50 turboprop and Fokker 70 and 100 jets. It is also a landing-gear component supplier and repair facility for some military aircraft, which is where Fokker’s carbon-fiber and polymer-resin composites are being used, says Sijpkes.
“Composites have been shown to be more durable, provide weight savings, have greater corrosion resistance and are less expensive than conventional metal systems,” says Sijpkes. “Also, composites have shorter lead times, with delivery in as little as 2-3 months. For many metal components, it can be as much as two years.”
Sijpkes adds that Fokker is studying the use of composites for its airliner landing-gear components but is unable to disclose any details. At the same time, Fokker is adopting new coating processes to eliminate the use of chrome and cadmium in favor of more environmentally friendly high-velocity oxygen-fueled (HVOF) thermal spray methods using a cobalt powder. Cobalt, he explains, creates a hard, more wear-resistant surface, especially in sliding surfaces involving bearings and seals.
Lutz Wierschin, general manager of HAECO Landing Gear Services in Xiamen, China, predicts that landing gear will continue to use high-strength steel and titanium alloys. “That is not likely to change in the foreseeable future. What has changed is the surface treatment of the steel components,” he says.
Wierschin points out that in today’s MRO environment, there are two commonly used surface treatment methods to make the steel more durable and corrosion-resistant: chemical plating, which uses cadmium and phosphate, and electroplating, which incorporates chrome or nickel. Nickel is primarily used for salvage repairs on major components, while chrome is applied to surfaces subject to wear and high heat conditions, he notes.
“In the landing gear (MRO) business, chrome will be replaced using HVOF thermal spray coatings in which a titanium-based powder is applied and liquefies when heated,” says Wierschin. “The final product is harder than chrome, more wear-resistant and should last longer. And because it has a higher surface density, it will ultimately reduce friction.”
Wierschin reports that extended times between overhauls (TBO) by landing gear OEMs indicate that HVOF thermal spray coatings will perform better for heat and wear resistance—and require less rework. “This has already been confirmed by military applications, so I believe we will see the same in commercial systems,” he says. “While it will never totally replace electroplating, we do see increasing demand for HVOF during the next 3-5 years.”
Thermal spray coatings have been OEM-approved for the entire landing gear on the newest jets, such as the Airbus A320neo and A350 and the Boeing 787 and 737 MAX, as well as for some specific landing-gear parts for legacy aircraft programs, Wierschin says. Still, he cautions that as the new fleets proliferate, landing-gear MROs will have to decide if there is a business case for the $3-5 million investment in HVOF equipment.
“As new landing gear hit the overhaul market, that’s when there will be a business case,” says Wierschin. “Especially since it is expected that the landing-gear OEMs will not want to give operators the option of using chrome plating.”
In that regard, the European Union’s REACH (Registration, Evaluation, Authorization and Restriction of Chemicals) will mandate that MRO providers in the EU countries change some plating technology methods, mainly those involving cadmium and chrome, says Martin Vabr, component and landing gear maintenance manager for Czech Airlines Technics in Prague. The MRO provides full landing-gear services for the 737 family as well as individual component maintenance on Airbus A320, Embraer ERJ 135/145 and ATR 42/72 landing gear.
“HVOF and zinc-nickel plating are the next developments in coating technology,” says Vabr. “Zinc-nickel plating is approved as a less toxic alternative to cadmium plating. However, in comparison, it does not yet provide as great a resistance to corrosion as cadmium. As a replacement for cadmium—it is still an open issue.”
Vabr also notes that while there is currently “no replacement option for chrome plating,” there are restrictions for chromium trioxide, under REACH Annex XIV, which requires MROs to submit an application for a permit to use it.
As for HVOF, Vabr points out that it already has been used on new types of landing gear, such as those on the Airbus A350, in conjunction with new advanced materials, such as high-strength titanium. “It is a high-level surface treatment,” he says.
Interestingly, the new and in-development technologies in landing-gear maintenance are not always easily available, as intellectual property and patent issues factor into the mix. Vabr points to tiodizing, an anodizing process applied to titanium components, as an example.
“For tiodizing—and other special coatings—there are already problems due to the limited capacity of providers, and patent issues,” he says. “Based on our experience, the turnaround time (TAT) of some component repairs by our subcontractors must often be prolonged to five months due to the lack of providers of this service on the market. To work with this prolonged TAT, we have put in place detailed planning and prearrangement of spare parts.”
According to Andreas Tielmann, vice president of aircraft systems at Lufthansa Technik in Hamburg, the technology shift in landing gear “is relatively modest.” He cites two trends. “One is toward more HVOF coating instead of chrome coating, and the second is [the use of] more titanium parts,” he says. “They are much more corrosion-resistant and are now replacing some steel parts, such as torsion links.”
Along this line, Tielmann says that newer landing-gear systems are being manufactured with fewer but more geometrically advanced parts with more complex shapes. That achieves the same “function integration” as older systems that use more parts with simpler geometry, he says.
TBOs, which are now generally running more than 10 years—with limited line maintenance between overhauls—are not likely to change dramatically, says Tielmann, noting that for some landing gear, TBOs could reach 12 years.
“Although 10 years is the most common landing-gear TBO today, we see a trend toward higher TBOs on most newly designed landing gear,” he states, naming some members of Embraer’s E-Jet family that are now at 12 years. Embraer, in fact, confirmed a 12-year TBO on the E170 and E175 landing gear, as well as a 10-year TBO on the E190 and E195.
Asked if the changes in landing-gear materials and maintenance will require MROs to invest in new supporting technologies, Tielmann says that will depend on several factors.
“HVOF coating will require new technology during landing-gear overhaul, whereas titanium parts require relatively little overhaul work,” he points out. “We see few composite applications in landing gears. However, if composite designs [were to] spread, this would require very different maintenance technologies from today’s steel and aluminum designs.”
The logical question, then, is whether the landing-gear OEMs will want to assume a greater role in the aftermarket. It also raises questions about the impact of the new landing gear on independent MROs.
“To service the airlines, an MRO will need the capital to invest in rotables, and to make this investment pay, it would have to capture a very large fleet size,” says Wierschin at HAECO Landing Gear Services. “This is especially true for widebody aircraft, where a single landing-gear ship set for the Boeing 777-300 ER costs about $20 million and more than $30 million for a Boeing 747-8. That gives the OEMs an advantage in the aftermarket.”
The other factor affecting the independent MRO is the 12-year TBO, which he says will likely result in some of the smaller landing-gear service centers exiting the business.
“An airline needs to work very closely with the MRO in order to achieve lower ownership costs,” counters Lufthansa Technik’s Tielmann. “In general, we see significant increase of material pricing for the spare parts and complete landing gears, but at the same time there is a general trend of increasing time between overhauls, which reduces cost. This means there is very little need for an airline to keep spare landing gear in between the overhauls. That’s in line with the overall market requirement—that the MRO provides the exchange landing gear.”
Landing-gear MROs should experience a stable market over the next three years, according to Chris Doan, vice president of consultancy Cavok. “The global fleet will generate just over 1,800 gear sets that will come due in 2016, with about a 5% increase in each of the next two years,” Doan says. “North America, Western Europe and Asia-Pacific [will lead] the numbers of gear sets forecast for overhaul in the next few years.”
Doan notes that the exceptions to the 2016-18 forecast will be Africa and India, with increases of 25% and 50%, respectively, in 2017. “While that will be relatively small in absolute numbers, it will represent a challenge for the operators in those regions,” he says. “Along with the issue of out-of-service time, finding a convenient and reliable vendor to handle the additional gear overhauls will be the next challenge. While there may be global capacity, it is often difficult for a small carrier to identify [a provider]. And a distant overhaul shop can add time and expense to the process.”
Not surprisingly, narrowbody gear repairs are projected to rise modestly over the next three years, while those for widebodies will rise more slowly. “The [Airbus] A319/320 and the [Boeing] 737-700/800 lead the pack in narrowbody gear overhauls, with steady but relatively low year-over-year growth anticipated,” says Doan. “In the widebody category, the [Boeing] 777 will see a sizable 50% jump in overhauls between 2016 and 2017, so that is one for the gear shops around the world to anticipate for capacity planning.”