Printed headline: Shifting Gears
Commercial aircraft landing gear are designed to absorb energy from aircraft weighing up to 375 tons touching down at speeds of around 175 mph. Not surprisingly, they are among the most robust systems on an aircraft.
Like many parts of modern jets, the outward appearance of landing gear has not changed significantly over the past 50 years. The basic tricycle layout of two main gear plus a nose system which retracts into the fuselage or wing root is favored by most aircraft designs, with the only significant difference—at least at a cursory glance—being the number of wheels on the main gear bogies.
To cope with higher weights, widebody aircraft have more wheels than narrowbodies do. Stretched aircraft also alter weight and the center of gravity, affecting landing gear design. One example is the Airbus A350-1000, which features a six-wheel main gear rather than the four on smaller A350 types.
Another factor is the thinner, composite wings of new-generation aircraft, which “require different landing load-sharing and might impact the available space where the landing gear is stored during flight,” says Gilles Bouctot, chief operating officer of Safran Landing Systems.
Outward appearances aside, the systems themselves have evolved enormously thanks to new materials and avionics, as well as the changing demands of airlines, aircraft manufacturers and regulators.
UTC Aerospace Systems (UTAS) supplies landing gear for the Airbus A380, A350-1000, Boeing 737 MAX, 747-8 and 767. Paul Lavigne, its director of landing gear engineering, says that lowering cost and weight are two of the aircraft OEM’s main priorities.
“Cost is managed through a combination of technology application, process management and continuous improvement,” he says, adding that evolving technologies such as additive manufacturing will offer further benefits in the future.
Meanwhile, new materials such as advanced composites and alloys can help lower weight, although a prerequisite for this is appropriate experience and capability in system design.
Advances in computer-assisted design have streamlined and improved the development of most aircraft components. Landing gear are no exception, and design engineers can now test variables such as loads, stiffness, stresses and stability using mathematical models that allow accurate simulations without the need for building physical prototypes.
“Finite-element analysis and multibody simulation have advanced to a big extent, with the result that landing gear integration in terms of attachment interface, space envelope and structural analyses can be managed without hardware mockups,” says Francis Carla, managing director and chief technical officer at Liebherr Aerospace & Transportation, which produces and services the A350 nose landing gear, as well as landing gear systems for the Embraer E-Jet E1 line, Bombardier C Series and Comac C919. He adds that virtual reality is another digital aid when it comes to teaching engineers about the assembly, installation and maintenance of landing gear.
Safran Landing Systems provides landing gear for the A320neo, A350-800/900, A380 and 787, as well as several regional aircraft. Safran’s Bouctot agrees that lower weight is one of the key improvements sought by aircraft OEMs. “It drives us into reassessing the architecture, the thickness of parts as well as using lighter material,” he explains.
Bouctot says Safran uses digital mockups, 3D tolerancing, model-based simulation engineering and global load models to pursue marginal gains in all those areas, while at the same time increasing system complexity, performance and safety. Examples include enhanced anti-skid features and braking optimization.
Another application of simulation is about mitigating aircraft noise, although landing gear’s contribution to that is increasingly focused on the development of quieter engines and sleeker airframes. Old-fashioned wind-tunnel testing is still a great help in this respect, but advances in computational fluid dynamics mean that more noise modeling can now happen on-screen.
Computer models also can simulate the effect of bird strikes or tire tread separation, “which has enabled UTAS to optimize the structure and mechanisms associated with these events and reduce the weight of our products,” says Lavigne.
As airlines drive for reduced maintenance costs and improved operational efficiency, reliability and performance expectations for gear manufacturers increase.
“Airlines are pushing for reduced turnaround times and extended maintenance practices and increased overhaul intervals, in conjunction with increased product maturity at entry into service,” adds Lavigne.
At Liebherr, Carla points out that an “increased number of landings affects the landing gear design in terms of engineering strength and endurance wear.” He adds: “Subsequently, life cycle cost is impacted, to an extent that depends on the change in the operational profile.”
Landing gear manufacturers also note that airlines increasingly expect product maturity at entry into service, which necessitates a rigorous design and testing process.
“This is driving increased maturity-testing to not just meet requirements but also better understand margins and design capabilities to expand the landing gear performance envelope as well,” says Lavigne.
Materials and Systems
Carbon fiber may hog the headlines when it comes to fuselage and wing development, but higher-strength metal alloys are proving equally transformative for other parts of aircraft. New steel and titanium alloys contribute to stronger, lighter landing gear structures, while composites also are used where appropriate.
“Application of composite and titanium landing gear structures and actuation are in development to further reduce weight, corrosion potential and cost,” Lavigne explains.
For production items, a big driver of new material use is the EU’s REACH regulation, which aims to restrict the use of certain harmful chemicals. This has led to widespread replacement of cadmium with zinc-nickel plating and the development of chromium-free paints for metals.
On the systems side, higher hydraulic system pressures are the gateway for smaller, lighter actuation systems. The A380, A350 and 787 all feature system pressures of 5,000 psi, a big leap from the standard 3,000 psi of most other aircraft.
Future platforms may leave hydraulics behind altogether and move to electrically actuated landing gear systems. The 787 already features electric brakes, while electrically powered nose wheel systems are in use with some airlines. Safran’s Bouctot adds that a fully electric nose gear is in development.
“Electric power generation and alternate power distribution architectures will enable increased use of electrical and electro-mechanical actuation in landing gear systems equipment such as actuators and uplock assemblies,” says Lavigne.
Landing gear also will benefit from increased Wi-Fi connectivity, allowing equipment to transmit health monitoring data through the aircraft maintenance reporting system, and reducing the number of electrical harnesses.
Landing gear certification has changed over the years from a test-based approach supported by analysis to an analysis-based approach backed up by testing.
“The landing gear certification basis is unique, considering the single-point failures and associated failure criticality for structural components within the landing load path,” says Lavigne.
Critical structural components are governed by FAA and European Aviation Safety Agency regulations regarding principal structural elements. According to Carla, this includes stringent requirements such as obtaining material data from 10 different samples from 10 material batches. He also flags the need for “extravagant generation of lower-bound fatigue curves for fatigue analysis and fully traceable production processes with 100% inspection and documentation.”
Subsystems, meanwhile, are subject to validation and verification processes, also known as requirements-based engineering, which aims to coordinate safety objectives within the development process.
“Safety requirements are driving more verification activities, in particular systems and software used for braking or steering,” says Bouctot.
Alongside cost and weight improvements, aircraft OEMs also expect gains in equipment durability and maintainability. Mean time between overhauls for landing gear has roughly doubled in the past 20 years, with the average for today’s equipment standing at about 15,000 cycles.
This means that the latest aircraft will probably undergo only one landing gear overhaul during their lives, as opposed to the standard two for the previous generation. Landing gear manufacturers therefore are increasingly reliant on new sales rather than aftermarket services income—a trend that runs in the opposite direction to engine OEMs.
“As our products are more robust today, we expect the aftermarket revenue to be proportionally smaller in the future,” says Bouctot.
Nonetheless, Carla says that competition in the landing gear aftermarket is tightening, while airline expectations grow.
“Engineering know-how, business intelligence, data management, predictive maintenance, specifically designed processes and a . . . worldwide logistics organization are key for us to make the difference in this market,” he says.
Achieving better reliability and durability is a combination of design, technology and maintenance practice. UTAS, for example, has emphasized product maturity during development, using some of the design tools mentioned above, while also simplifying servicing techniques. On the technology front, it has improved shock absorber sealing and is researching increased use of health monitoring for predictive and preventive maintenance.
Eventually, landing gear durability may improve to the extent that the systems are no longer classified as life-limited parts.
“Increased use of composites, titanium and stainless steels, wear performance improvements such as nonmetallic bearings, improvements in corrosion protection through augmented use of nickel plating to prevent fretting and corrosion, along with health monitoring systems will allow further increases in overhaul intervals and the potential to achieve an on-condition product for widebody platforms,” Lavigne says.