Landing Gear OEMS Adopt Lighter Weight, More Sustainable Materials

Landing gears are getting greener, with greater use of lighter-weight materials and more environmentally benign anti-corrosion technologies.

“Landing gears are manufactured using a combination of high- and low-strength steel, stainless steel, aluminum and titanium,” explains Raul Cruz-Alvarez, accountable manager at Landing Gear Technologies, a Miami-based commercial airliner landing gear MRO specialist. “With the use of more titanium parts on the current and upcoming designs and engineering threads, today’s landing gears are more durable and require less maintenance, therefore extending the useful life of the parts.”

Corrosion and hydrogen embrittlement, Cruz-Alvarez points out, are landing gears’ biggest enemies. High- and low-strength steel and aluminum components are especially vulnerable to damage due to corrosion and induction of hydro-gen—-on the steel parts—during the overhaul process.

“The use of high-velocity oxygen fuel [HVOF] used on the Boeing 787-series landing gears will greatly reduce the induction of hydrogen during the overhaul process. In fact, some first-tier airlines and Boeing are currently introducing this process to the other aircraft landing gears that were not originally manufactured with HVOF,” he says. He also notes that the use of HVOF, instead of hexavalent chrome and sulfamate nickel, has less environmental impact.

“What we are seeing beyond the horizon for landing gear MRO is the complete elimination of hexavalent chrome and sulfamate nickel,” he says. “However, HVOF as a replacement for hexavalent chrome and sulfamate nickel may be years away for the MRO side, but we must all be ready today.”

Cruz-Alvarez also says developments in certain lubricants and corrosion-inhibiting compounds (CIC) used in the last 25 years have greatly improved the corrosion-inhibiting process for the landing gears between overhauls. “Today’s landing gears come in after 10 years of dealing with differential temperatures and pressures, water, snow, salt and the infamous deicing fluids, not to mention between 5,000 and 20,000 flight cycles since the last overhaul, and once you remove the locking mechanism, the nuts come off with very little effort and much less corrosion,” he says.

In contrast, Cruz-Alvarez notes that when he started to overhaul commercial landing gear in the mid-1980s, removing the nuts involved a process that usually caused collateral damage to the mating parts and threads due to the amount of corrosion.

Meanwhile, landing gear OEMs continue to push the technology envelope with material selection.

“When it comes to raw material, we see great improvements in high-strength, corrosion-resistant steels, which now provide almost the same strength characteristics as the noncor-rosion-resistant, high-strength steels used today for landing gear structures,” says Tim Lammering, director of landing gears at Liebherr-Aerospace in Germany. “Being much less prone to corrosion, overhauls will become much easier and, potentially, flight cycles between overhauls can be extended.” He adds that without the need for corrosion protection, these materials have shown greater resistance to foreign object damage.

Lammering points out that Liebherr-Aerospace continues to apply new manufacturing processes to replace those that will be restricted by the European REACH (Registration, Evaluation, Authorization and Restriction of Chemicals) regulation, as well as the upcoming Green Deal regulations. Green Deal describes a series of legislative proposals and regulatory reforms to reduce Europe’s carbon emissions to zero by 2050. He cites the use of high-strength steels as one example.

“For sustainability and environmental protection, each step not required in the manufacturing process is a positive one,” Lammering stresses. “With high-strength corrosion-resistant steels, no corrosion protection, in terms of galvanic surface treatment, is required. Thus, today’s critical galvanic processes can be removed from the manufacturing of structural parts made from these materials.”

Asked about any research being done on thermoplastic and other composites for landing gear applications, Lammering says Liebherr-Aerospace is “continuously observing composite technology” for potential use in landing gear structural parts fabrication. “Currently, they show no benefit over modern metallic parts,” he says. “From our point of view, if composites are applied, they would most likely be to struts and braces, since they are simple parts without major functional surfaces. They are also better shielded by the landing gear bay and doors against foreign object damage when compared to other landing gear parts.”

Lammering adds that for Liebherr’s customers, a major concern is the time to market for incremental changes in the landing gear structure, such as those specifically being applied to freighter or higher maximum-take--off-weight variants during an aircraft’s life cycle.

“The landing gear is impacted almost every time due to changes in loads and fatigue spectrums. Hence, fast methods are required for showing compliance of the existing landing gear to those changes that do not need time and cost-efficient testing,” he says. “With acceptance of the airworthiness authorities, we have been successfully applying our strategy of ‘verification by simulation’ for landing gear structures over the past several years, providing the fastest time to market of incremental changes for our customers. To do that, we have applied high-fidelity models and simulations without the need for time and cost-intensive testing.”

In fact, the need for the landing gear to manage increased loads has been driving the push for much more durable and lightweight structures, reports Satish Narayanan, executive director of engineering for landing systems at Collins Aerospace. He confirms that the OEM has been incorporating composites and high-strength titanium material into nonstructural landing gear components, and it is developing composite materials for structural components. “Both types of material can provide a substantial reduction in weight compared to steel, and both technologies enable lower production energy use, which in turn reduces the environmental impact of our production operations,” he says.

Narayanan also says Collins has been investing in chopped-fiber composites and thermoplastic in situ composites to better understand the structural integrity and material properties of the parts produced. “We are collaborating with our customers in developing the certification basis for their use and anticipate deploying them into aircraft platforms in the near future,” he says. “As we validate use of—and implement—composites on nonstructural parts of the gear such as brackets, our focus has shifted to the most weight-sensitive parts of the gear associated with principal structural elements like actuators, struts, pistons and brake rods.”

Narayanan says thermoplastic composites enable the use of lower-weight, more durable landing gear on a wide spectrum of platforms—from regional and business jets to single-aisle and large twin-aisle aircraft. However, he stresses that while Collins is exploring applications for all these aircraft types, the best immediate opportunities seem to be on widebody aircraft, where the landing gear structure is a larger proportion of the overall aircraft weight.

He also notes that Collins expects that composite parts will be more damage-resistant. “We are in the midst of substantial testing and development to understand and learn more about the structural longevity and damage tolerance of composite parts,” he says.

For lighter-weight structural components, however, Narayanan says high-strength titanium alloy offers great promise in the short term. “We have proven our additive manufacturing for nonstructural components, enabling more organic and producible designs with attention to acoustic noise and aerodynamics while enabling reduction in part count and procurement lead time,” he says.

Narayanan adds that there is a sustainability benefit with composites and high-strength titanium. “The process used to manufacture thermoplastic composites consumes less energy, and the material is highly recyclable, reusable and durable, which means longer use and less waste,” he points out. “The use of high-strength titanium and composites significantly lowers the use of energy to manufacture them. Finally, the elimination of the need for corrosion protection will limit material of concern usage and disposal.”

At GKN Fokker in the Netherlands, the main focus has been on the development of carbon-fiber-reinforced plastic (CFRP) landing gear components to reduce weight, according to Peet Vergouwen, technical authority for materials and processes.

“Fiber-reinforced materials can provide the most benefit if load paths are clearly defined or can be influenced by stiffness distribution,” he explains. “Therefore, struts and braces, which are axially loaded, are the obvious starting point, since they have the highest weight-reduction potential compared to baseline metal versions.”

Vergouwen adds that for more complex loaded parts, such as trailing arms or main fittings, weight reduction will not be as high but will be appealing from an environmental and life-cost perspective. Combined with this are the environmental benefits of composite materials due to their inherent corrosion resistance and the elimination of hazardous surface-protection coatings commonly used on metallic parts.

He also explains that CFRP composite parts, when combined with corrosion-resistant fittings and interfaces, require inspection but little or no scheduled maintenance.

“We expect that the typical fixed-life limitation that is commonly used in metal landing gear parts can ultimately be eliminated for composite structures,” he says. “In the event that replacement is required, an additional benefit for composite landing gear parts is the much shorter manufacturing lead time, which is only a few weeks instead of many months for metal forged parts, minimizing the number of spares.”