Keeping landing gear airworthy is an undertaking. Bernie Baldwin hears from specialists on the challenges they face in carrying out that task
Airlines would probably love it if their aircraft didn’t have to carry around huge weighty castings which only get used for a small fraction of the journey. However, such castings constitute the bulk of every landing gear on aircraft and, not only do they have to be carried, they have to be maintained and kept in full working order.
One of the major challenges when maintaining metal castings in landing gear is corrosion. Maintenance specialists are continually developing new repair techniques to enhance the preventative qualities they employ to limit corrosion, as Pastor Lopez, president MRO Group at GA Telesis, explains.
“There have been many improvements on aircraft systems such as the vast use of composite on the Boeing 787 for instance, but landing gears have seen little change over the past 20 years. To prevent corrosion, some of the OEMs have introduced titanium and/or composite as the material of choice for critical parts. Titanium was used in the design of the 747-100 landing gear. These material types are not susceptible to corrosion. In addition to that, Boeing increased the use of HVOF (high-velocity oxygen fuel) coatings in the 737NG family to protect against the elements,” he remarks.
“The correct application of finishes such as cadmium and chromium is critical to deter corrosion from forming on the surfaces. It is worth remembering that landing gears are not only exposed to the elements, but also stay on wing for long periods of time, usually ten years. This, together with proper maintenance (use of grease, other lubricants and so on) while in service, shall ensure an optimal on-wing performance and a decrease in overhaul cost during the next cycle,” Lopez states.
Innovation in anti-corrosion techniques and repairs is similarly an ongoing process at Revima. Pascal Hocquigny of the company’s Landing Gear Methods Innovation Centre outlines the processes his company takes in dealing with the problem.
“First, we have to reveal all evidence of corrosion during overhaul. For that, there are two steps of inspection: visual and non-destructive tests. Nondestructive inspection methods, such as magnetic particle inspection (MPI) and fluorescent penetrant inspection (FPI) are used,” Hocquigny begins. “These methods need large, sophisticated equipment to control the largest parts of landing gears and Revima is equipped with the most modern equipment for these inspections. Our personnel is obviously certified, but above all they have a long experience of controlling the various parts that make up a landing gear.
“Second, we have to remove corrosion and limit the stress concentrations which can initiate stress corrosion during service,” he continues. “One of the first steps during overhaul is a heat treatment to relieve stress on all surfaces at 190°C for a minimum of four hours. But it’s also very important to minimise the effects of stress concentrations in transition areas. For example, when a coating such as chrome or nickel is applied, the coating must exhibit proper runouts to minimise stress concentration and avoid any corrosion during service.
“Several in-service corrosion fractures are caused by improper plating techniques and runout conditions, and base metal damage caused by poor blending or machining control. Revima has therefore invested in new plating tools, grinding, milling and turning machines in its factories to obtain the best quality,” Hocquigny states.
“Third, the restoration of protective finishes is absolutely decisive for the resistance of parts against corrosion. Plating and conversion coatings are applied in very good conditions to avoid any microstructural damage, such as hydrogen embrittlement. Low hydrogen-embrittlement, cadmium plating is always the best protection of high-strength steel against corrosion,” he adds.
“The application of this surface treatment must be done on base metal free of corrosion pits, local stress concentrations and mostly protected by a compressive stress layer which will limit the progression of corrosion cracks realised by a shot peening machine, as with the very modern digital machine in the Revima ASIA factory. After cadmium plating and conversion coating, paint is the ultimate protective finish.”
Touchdown is probably the most crucial moment for landing gear, as hard landings can obviously affect their performance. Tests are therefore carried out after a hard landing is reported.
“It is the operator’s ultimate responsibility to inspect an aircraft after a hard landing,” Lopez reports. “Depending on the force experienced during landing, the airline will make the decision to send the landing gear to a shop. Typically, the OEM recommends that all finishes, including all plating, are removed from each individual component involved in the incident. Each part is then to be quarantined for six months.
“Each part will then be submitted to non-destructive testing to see if they developed a crack while in quarantine due to residual stresses. If cracks are noted, the OEMs and operators will err on the side of caution and remove the part from service. Parts that do not show any cracks can be finished following the CMM (component maintenance manual) and returned to service.”
Revima’s Hocquigny elucidates further. “Hard landings will produce an overheating of components and this can change the original steel temper and mechanical properties of the affected area,” he observes. “The inner cylinder is the part which is the most impacted. The degree to which the mechanical properties are changed depends on the temperature and duration of exposure.
“Overheating can result in overtempered martensite (OTM) or untempered martensite (UTM) formations in the base metal [martensite is a very hard crystal structure in steel formed by rapid quenching]. UTM formations may be accompanied by heat-induced cracking within these overheated areas that can propagate while in service. This heat damage can be detected in service by local NDT inspection such as magnetic particle inspection (MPI) and fluorescent penetrant inspection (FPI).
“Barkhausen inspection [a form of electromagnetic testing] is very interesting to detect base metal heat damage under chrome plating or other protective finishes. This technique can be used successfully to screen components with suspect damage,” Hocquigny notes.
“After performing a chrome strip and temper etch (using nital, a mixture of nitric acid and alcohol) inspection on all suspect axles, heat damage is generally removed by carefully machining the base metal. Afterwards, another temper etch inspection is done to ensure the machining did not create more heat damage.”
The Revima executive adds that damage is generally shallow and can be removed by machining. After overhaul operations are completed, the component is returned to service. “However, some heat damage is so severe the heat-treated condition of the material is altered in adjacent areas. This widespread reduction in metal hardness may indicate that the component cannot be salvaged,” he states, somewhat echoing Lopez. “Discoloration of the enamel, primer, or chrome or evidence of cadmium damage may require the heat-damaged component be removed from service.
“Overheating affects components to various degrees. In some instances, only finish durability is degraded. This may result in a shorter than planned time between component overhauls,” Hocquigny warns.
While hard landings are a source of damage that can be mitigated by the skill of the operator, foreign object damage is usually beyond their control, as Lopez notes. “When landing gears roll down a runway, small stones and rocks may be on a runway. Some of the landing gear components are hit with these items.
“The DC9 and old 737, for instance, used to have gravel deflectors installed at the base of the wheel,” he recalls. “However, these components added to the aircraft weight and were removed in subsequent models. The cost of the units (maintenance, weight on the aircraft and cancelled or delayed flights) exceeded the benefits. The centre landing gear on the DC10 had a special epoxy applied to the back of the cylinder to avoid these types of damages. As topcoat (paint and primer) evolved, there have been fewer requirements for special protective finishes to prevent rock impacts.”
On the operational front, as with any part of the industry at present, the Covid-19 pandemic has been having an effect. Lopez explains how GA Telesis has been coping with changes in demand and management of the supply chain, both in terms of inbound material and deliveries back to the operator.
“At GA Telesis, we are committed to supporting customers while providing a safe environment for our teammates. We have taken many steps to ensure the wellbeing of our team, which have proven very effective,” he emphasises. “This has led us to have uninterrupted operations throughout the crisis. We have also taken steps to ensure we continue to support incoming work such as large orders of material from our suppliers and OEMs to ensure we have more than adequate supply of parts; increasing our internal manufacturing capabilities (both facilities, components and composites, now have FAA approval for fabrication); and diversification of the subcontractors that perform critical processes.
“Time and time again, this industry has managed to excel during difficult times, and we are confident we will do that again,” Lopez summarises, his attitude most likely matching the thinking in many an MRO shop around the world.