Gearing up

Landing gear manufacturers are coming under increasing pressure to reduce weight and deliver cost savings. Ed Hill speaks to Liebherr Aerospace's chief technical officer, Heiko Luetjens, about its approach to making these systems.
Whatever the latest technology employed to make aircraft as efficient, reliable and comfortable as possible in flight, it means nothing if a plane cannot take off and land safely.

The undercarriage or landing gear is one of the most safety critical parts of any aircraft and also one of the hardest worked. Manufacturers of such systems need to ensure they are reliable and robust but also push for the weight and fuel saving advantages that can be achieved with new materials and technologies and demanded by airline operators. Landing gear systems are one of the heaviest components of an aircraft; usually around 5% of the take-off weight so any reduction in heaviness is highly desirable.

Liebherr Aerospace and Transport is one of the largest suppliers of landing gear systems to the OEMs. Part of the giant Liebherr engineering group, it is currently involved in eight landing gear programmes for aircraft including the latest passenger jets, military trainers, business jets, combat aircraft and helicopters. The aerospace division employs more than 4,400 staff and as well as landing gear systems it also produces flight control, actuation and air management systems and carries out maintenance contracts.

Heiko Luetjens is managing director and CTO for flight control/actuation systems, landing gear systems and hydraulics at Liebherr-Aerospace and Transport.

“Liebherr-Aerospace is developing landing gear systems for the COMAC C919 and ARJ21, the AleniaAermacchi M-346 military trainer and the AgustaWestland AW189,” he begins. “Among the series programmes are the Embraer 170, 175, 190 and 195, the Eurofighter, and the AgustaWestland AW139. For the Bombardier CSeries we need to develop and qualify two different landing gear systems; one for each version of the aircraft, the CS100 and the CS300. The same goes for the Airbus A350 XWB: we are basically responsible for two nose landing gear, the A350-900 nose landing gear and the A350-1000 nose landing gear. For development activities we have built up a specific development organisation and for legacy activities we have a separate and independent programme management arrangement.”

Tough technology

Landing gear is subjected to some of the most extreme forces in operation, especially at touchdown. It also has to be safeguarded against factors such as foreign object damage and corrosion.

“We take the possibility of operational hazards into account in our design work,” Luetjens continues: “All needs regarding corrosion and heavy forces are considered at very early stages and demonstrated by analysis and test. In particular, we carry out thorough test campaigns for all system components as well as for the complete systems. The tests repeat several times the life duration of a system during its operational life covering such key aspects as endurance, fatigue and stress.

“We have made investments worth several millions of Euros in our test facility where our systems and components are thoroughly tested within their wide operating spectrum and according to the contracts of our customers. With the developed test results we generate the front-loading regarding ageing aircraft so that we can provide our customers' aircraft with robust products.

“In order to guarantee the time to market, especially for new projects, we have to deal with and manage items with long lead-times such as forgings.”

The big stuff

Landing gear components, especially on some of the civil aircraft programmes that Liebherr supplies to, can be very large presenting particular engineering challenges. Machining operations can involve deep hole drilling, turning and milling of parts made from exotic alloys with high surface finish requirements.

“We have invested considerably in dedicated machining centres, which feature the latest milling and deep-hole drilling technologies and enable us to increase productivity in the production of landing gear components and other parts,” Luetjens comments. “For example, we recently set up a new milling centre for parts of the A350 XWB nose landing gear and for the Bombardier CSeries landing gear housings. For the A350-1000 nose landing gear, we use high-strength steel (300M-type), as well as cadmium-free coatings. We have installed new facilities for surface treatments such as High Velocity Oxygen Fuel Spraying (HVOF) coating so all our core competences and processes are in-house.”

Most of Liebherr Aerospace work on landing gear systems in Europe is carried out at its sites in Lindenberg, Germany and Toulouse, France. For example, on the A350-800/900 the company is responsible for the nose landing gear which includes the retraction and extension actuators, the steering actuation mechanism and hydraulic valve controls.

The company also operates manufacturing sites in Guarantingueta, Brazil and in Nizhny Novgorod, Russia and additional service centres in many sites around the globe including the USA, Dubai, Singapore and Shanghai, China.

Once the systems have been assembled at its plants they are shipped to the OEM's assembly facilities such as Toulouse for Airbus aircraft or Montreal, Canada for Bombardier's CSeries.

“The gear installations are done by the aircraft manufacturers,” states Luetjens. “In some cases, Liebherr-Aerospace is responsible for “dressing up” the landing gear systems, which means assembling hydraulic tubes, wirings, lights, wheels and brakes, before delivery to the customer. Our landing gear ship sets are completed and tested before we deliver them. The strength of our organisation lies within our satellites that we have built closely to all the final assembly lines in order to support our customers. The key challenge is the time to get a fix.”

Cleaner and greener

Although Liebherr Aerospace is involved in many current landing gear programmes the company continues to look to the future to see how it can improve its systems.

The company recently signed up to the European Commission's Clean Sky 2 initiative which aims to reduce CO₂ emissions per passenger kilometre by as much as 75% by 2050.

Liebherr is not only looking into more lightweight materials from which to build its landing gear systems but also the control mechanisms. Electric control systems are looking increasingly more attractive compared to their hydraulic counterparts for many moving parts of aircraft, including the landing gear.

“In the design and manufacturing of landing gear systems, we face several challenges and demands,” Luetjens explains. “For example, we need to reduce the weight by using titanium and carbon fibre material, and apply new surface treatments to fulfil environmental and OEM needs. The steps towards a more electric aircraft are a major challenge as suppliers have to demonstrate the advantages of electrical systems compared to hydraulic systems. Moreover, we need to reduce operating and overhaul costs, and to supply systems and components with longer life duration.”

The company is also working on E-taxiing systems in which aircraft's rolling movements on the ground are powered by an electric drive system attached to the landing gear (potentially powered by fuel cells) so emissions, noise and fuel consumption levels can be reduced between landing and take-off.

“Landing gear is playing a major role in the next generation of aircraft,” Luetjens concludes. “Of course, we are carrying out studies on integrated composite material, in particular for nose landing gear. We are also pursuing R&D activities with a view to increasing the amount of titanium parts in our systems. Our aim is to offer our customers minimum 10% weight reduction for the next generation of aircraft. We are working on these areas in order to reach Technology Readiness Level (TRL 6) for all topics. In addition to weight reduction and the simplification of system architectures, we are also working on E-taxiing in order to reduce drastically NOx, CO₂ and noise emissions.”

www.liebherr.com