Landing gear has it tough. It must withstand temperature and environment extremes, take a considerable pounding from runways and remain completely safe and functional during even the roughest of aircraft landings. For maintenance, repair and overhaul purposes, that means flight hours and cycles are continuously recorded, ensuring the gear remains safe and flightworthy.
British engineering consultancy Atkins has teams of engineers experienced in the design, manufacture, analysis and in-service support of landing gear systems, and the company works with aircraft manufacturers to resolve issues. This engineering work may be used to justify modifications to landing gear or implementation of service bulletins. “Most gear is overhauled before reaching the end of its life,” says Andrew Munday, advanced engineering practice director at Atkins Aerospace. “At that point, any damage will need to be recorded, assessed by the OEM and repaired by the MRO. They may then update the record for remaining service life for that particular set of gear.”
Of course, takeoff and landing are the major culprits when it comes to landing gear damage. The stresses induced by heavy landings are large; the biggest loads any gear will experience are in a vertical direction. When an aircraft touches down, the wheels spin up to aircraft speed, causing an initial drag on landing gear comparable to the drag caused by braking. During cross-wind landings, the sideways motion places loads on the landing gear side-stay, which also has to resist the horizontal loads that arise when the aircraft turns on the ground. On multi-axle “bogies,” the main strut experiences very high torsion during low-speed and high steering-angle turns, which is why there is the addition of aft axle-steering on some gears.
Landing gear has to be made of tough stuff to survive all this buffeting. “Because they are subject to a large load at least once per flight—on landing—landing gear must be able to withstand a high number of load cycles, both in terms of frequency of event and the level of load,” Munday says. These punishing load cycles mean that metal fatigue is a concern—that is, preventing landing gear components from cracking under repeated loads. Despite all these pressures, landing gear rarely fails under large, one-off, extreme landings. Most landing gear sets are made with super-strength steel that can withstand a lot of punishment. This material is, however, sensitive to corrosion, which creates notches on the gear that must subsequently be “blended out” by the MRO provider, says Munday.
In addition to being strong and safe, landing gear must also be as light as possible and meet operational requirements for retraction, extension and turning. If a landing gear system fails to extend or retract, “this might result in dramatic landings on foam-covered runways, but rarely in anything more serious than a damaged aircraft and a foamed-up runway,” Munday says.
Tragically, the crash of Air France Flight 4590 in July 2000, the only fatal accident during the Concorde’s 27 years of operation, demonstrated the devastating impact of another type of failure—a tire blowout. “Managing the impact of tire debris after a blowout is thus an important activity for engineers,” says Munday.
Lufthansa Technik repairs landing gear for aircraft from across the Airbus and Boeing fleets and has added new landing gear repair capabilities for the Airbus A380 and Boeing 747-8. It is also preparing to repair landing gear for the Boeing 787. The MRO has three landing gear service locations to support its global customers—in Hamburg, Germany; London and Sun Valley, California. The facility in London is purpose-built and handles around 300 landing gear legs a year. It is decked out with paint-removal equipment, hydraulics and electric shops, nondestructive testing (NDT) equipment, a machine shop and a full plating shop.
Repairs carried out there include chemical plating, such as with cadmium for corrosion prevention, electroplating with nickel and chrome plating for surfaces exposed to high heat and wear. Globally, Lufthansa Technik also provides a spare asset pool for exchange of landing gear—an inventory that requires substantial investment, the company says.
“For example, a brand-new Boeing 777-300ER [landing gear] shipset might cost $20 million,” says Sandra Eckstein, Lufthansa Technik vice president for aircraft systems and landing gear. Documentation is “back to birth” for traceability, containing the total landing gear flight cycles for each set. Finding the correct documentation can be a challenge for older aircraft types such as the Boeing 757 or Boeing 767, especially when the landing gear has undergone its second or third overhaul or has had several operators, Lufthansa Technik says.
One MRO specializing in older aircraft is Aerotek Aviation Engineering of Poole, England. Managing Director Richard May says the company’s expertise in hydraulic and electrical systems makes it well-suited to repairing landing gear, and it specializes in regional aircraft up to the size of Boeing 737s and Airbus A320s, as well as Sepecat Jaguars and BAE Systems Hawks for the military, and all wheeled landing gear for helicopters. Aerotek Aviation Engineering sometimes deals with landing gear that is 40 years old. On-site testing facilities include NDT using dye penetrant, in which dye is sprayed on the assembly before an ultraviolet light inspection to reveal any cracks or deficiencies within the metal. Aerotek engineers also carry out inspection via eddy current and magnetic-particle testing.
During magnetic-particle testing, ferrous particles in a wet suspension are applied to a part. These are attracted to an area in which the magnetic field fluxes, indicating where a repair may be required. Aerotek also has special bays for stripping paint from landing gear that can accommodate changes to the strip medium in the coordinate measuring machine. Other capabilities include shot peening facilities—used to harden and strengthen metal structures—and a machine shop used to fabricate bushings and other components.
May points out that with some older aircraft, such as the 737, it can be cheaper to buy a new set of legs than to overhaul them. As the fleet of older 737s is broken up, sets of legs can be picked up at relatively low prices, he adds. “Landing gear is subject to formidable stresses, which is why we do a lot of testing with NDT,” May says. “Corrosion is always also an issue.” He tells the story of a landing gear in which part of the gear was hollowed out, meaning it collects water and requires inspection with a camera on an endoscope. “But you get these fantastic old aircraft—and people want to keep flying them,” he notes.
If passenger aircraft are converted for other uses—in forestry, for example, or flying in extreme weather conditions—the landing gear will often remain the same and must be maintained for the aircraft’s new role. Parts availability: “Because we can fabricate as well as repair—we are in the process of gaining a license to manufacture on-site in the UK, too—we are better placed than most for a quick turnaround, thanks to our workshop.” A typical turnaround might be 8-10 weeks for a landing gear set at Aerotek.
Lufthansa Technik has a typical turnaround time for landing gear repair of 30-55 days. “Our competitive turnaround times are a direct result of lean principles,” Eckstein says. The Lufthansa Technik Landing Gear Services facility has Japan Civil Aviation Bureau (JCAB) approval, one of the highest quality-standards achievements in the industry. Part of the lean methodology has seen color coding introduced in operations for managing disassembled parts. Black-painted trolleys are used only for parts that are freshly disassembled and awaiting cleaning. Trolleys for parts awaiting inspection are purple, while yellow trolleys are used for repair and overhaul parts. Each trolley has a photograph of the components required on each tray, and shadow boards highlight any missing items.
In what direction might future landing gear design evolve? Atkins Aerospace believes landing gear will incorporate additive-manufactured parts where the design is complex and requires custom—and time-consuming—manufacturing processes. Landing gear is also likely to commonly incorporate electric-taxi drives to reduce fuel consumption. “Most of these concepts feature a motor in the nose wheel,” says Munday. “This means the engines do not need to be used to make the aircraft taxi from the stand to the runway and can also save time by not needing to hook up a tug or tractor to push the aircraft back from the gate.”
The greater use of composites in landing-gear structures may improve their resistance to corrosion, reducing maintenance requirements, Atkins suggests. The use of big data from aircraft sensors is potentially highly valuable for improving the operation and life of landing gear. Munday says in the future, landing gear will have multiple sensors recording strains on major components. “By combining the stress data with the recorded loads on the airframe, an accurate prediction of fatigue life for all components can be made,” he says.
“This means that servicing can be done when needed, rather than at a set, conservative interval, reducing the maintenance cost, but also increasing the availability of the aircraft. Artificial intelligence will be used to spot trends and then identify causes of serious damage to allow the designs to be refined,” Munday notes.
Landing gear In Numbers
60,000 The number of landings commercial aircraft must be able to carry out safely before changing landing gear
10,000 The number of landings military aircraft must be able to carry out safely before changing landing gear
3% Fixed landing gear ‘s proportion of aircraft weight
6% Retractable landing gear’s proportion of aircraft weight
85-90% Efficiency of landing gear energy absorption
-112F Operating temperature for retracted landing gear
6,447 lb. The weight of each Boeing 747-400BCF body landing gear