Landing Gear Overhaul: Vital buffer between airframe and runway

Special skills in the "Landing Gear" business unit

Touchdown – the Airbus A340 has just landed. During this everyday procedure, the landig gear has to absorb high dynamic loads when up to 285 tons of aluminum and payload arrive on the airport runway piano keys. Here, the landing gear becomes the sole direct link between the airframe and tarmac and is vital to the safe conclusion of the flight. Like all other components which make up an aircraft the landing gear – often simply known as the 'legs' by aircrew and ground crew – is checked and maintained daily, repaired as required and fully overhauled at regular intervals. A full landing gear overhaul is a task for specialists that requires the use of special tools, equipment and technology. For this reason, for some long time many airlines have sub-contracted this work to organizations with commensurate equipment and know-how, such as Lufthansa Technik.

Based in Hamburg, Germany, for 40 years Lufthansa Technik has maintained one of the world's largest specialist aircraft landing gear maintenance and repair facilities. The 10,000 square meters base has a staff of more than 150, and has overhauled some 3,200 landing gears. 

Landing gear maintenance for Asia and USA

In the year 2000 Lufthansa Technik teamed with the American-owned landing gear overhaul organization Hawker Pacific Aerospace and further extended its activities in this sphere. Today, Hawker Pacific Aerospace is a wholly-owned company of the Lufthansa Technik Group. Lufthansa Technik's landing gear services division operates out of three locations: Hamburg, London and Sun Valley, California. While around 140 employees in Hamburg are specialized in the complete MRO portfolio for Airbus landing gears and the Boeing 737 NG, Bombardier CRJ and AVRO RJ, the 530 colleagues at Hawker Pacific in the UK and United States concentrate on the Boeing portfolio (incl. DC10, MD-11,-80,90) and certain Airbus types. In 2005 around 800 shipsets have been overhauled at the three locations.

China's largest airplane maintenance and repair organization is Ameco Beijing. The company has a broad range of landing gear overhaul, including Boeing 747 and 737, the Airbus A320 and A300-600 landing gear as well as the long-ranger Airbus types A330/AA340. Their customer list enumerates many Chinese and foreign airlines. 

D-Check Maintenance and Overhaul

Before each and every flight the landing gear oleos, tires and brakes are inspected by mechanics and flight crew. After no more than 120 months of flight operations a Boeing 747 landing gear is fully overhauled. Generally, the so-called D-check full overhaul will become due before expiry of this period. Depending upon the airplane type, the D-check is due every six to eight years, at which time it is completely disassembled, checked, repaired as necessary and reassembled. This likewise holds good for the landing gear. During the course of this operation the landing gear is examined to locate the smallest signs of damage such as tiny fractures or any indication of corrosion. The aircraft is first jacked up for the removal of tires and brakes; the landing gear legs, which weigh many tons, are then removed to a specialist workshop. Next, the landing gear legs are thoroughly cleaned and completely disassembled into their numerous components, which are then individually cleaned.

Any paint is then removed from components using an appropriate mechanical or chemical process. As in all its maintenance and repair operations, Lufthansa Technik places great emphasis on environmental protection. The company also has large airframe paint shops in which old paint is removed using a variety of environmentally-friendly processes. In particular, any spent process water is trapped and reprocessed for further use. 

Faults and Repairs

Next on the list is a search for any damage and corrosion using a variety of non-destructive test processes, plus careful optical checks of all components. In addition to checks for corrosion and fractures, specialists check the dimensions and chromed surfaces of all components. However small it may be, each and every component is subjected to the careful eye of an experienced technician and licensed inspector before it is released for repeat service.

When an inspection locates corrosion, the affected area is abraded just enough to remove all the damaged zone, but as little as possible of the underlying material. Only a certain defined allowable maximum of material may be removed; if more is necessary, then the component must be removed and replaced by a serviceable component. Moving components are linked by bushes made of a special bronze alloy, with main bolts holding moving parts together. Bushes are machined to the required dimensions and fitted; work is undertaken to the highest standards of mechanical precision and exactly within close tolerances.

The company has a chrome-plating shop for landing gear leg exposed parts, moving interfaces and bearings; the same workshop uses appropriate anti-corrosion processes to protect other components at risk. For example, in operation a wheel axle temperature can exceed 250ºC and is therefore coated with a special heat-resistant layer. The components are then subjected to a multi-lacquer process that protects them from the aggressive media to which they are exposed in everyday use.

After cleaning and repair, the roughly 10,000 components making up a single landing gear are again assembled into a complete working system. However, before it is again installed on the aircraft it is given a full functional test on a rig. For the technicians, the really exciting moment is when, following installation on the aircraft, the landing gear is given a full check. The big jet is positioned in a dock, with all its weight resting on hydraulic jacks and the landing gear mechanism retracted within a compact bay. Then the landing gear doors open, followed an instant later by the unfolding landing gear mechanism.

It takes Lufthansa Technik about five to six weeks to conduct a full landing gear overhaul for a widebody jet, faster than any other international maintenance and repair organization. Not too long ago, a job of this type could take up to 120 days, but this has since been drastically reduced by a continual process of optimizing work sequences, the processes employed and modern computer-based planning. 

A hundred new sets of tires per airplane

Wheels and tires are replaced much more regularly than are the landing gear legs; likewise, Lufthansa Technik has a special wheel and tire bay. During the course of its service life, an A340 will complete roughly 6,000 take-offs and landings, calling for about 25 sets of replacement tires, each 1.30 m high and weighing about 200 kg. The tires can be remolded several times before they are finally scrapped.

After it has completed about 250 landings, the wheel is removed to the wheel bay where it is thoroughly cleaned and checked. The wheel rim is subjected to eddy current and dye penetration checks that detect the onset of any tiny cracks. Once it has been passed as perfect, the wheel can be returned to service.

Each A340 has four landing gears with a total of twelve wheels. The airplane has a nose landing gear, a center landing gear and two main landing gears. When taking off at the gross weight of 271 tons, most of this is supported by the main landing gears.

The brakes are every bit as important as the tires. Airplanes have three types of brake: airbrakes (spoilers) on the wings, reverse thrust clamshells on the jet pipe, and conventional wheel brakes. The main landing gear wheels on the A340 have eight disc brakes. The brake forming part of each wheel assembly is made up of a package with a number of discs. Steel brakes have been progressively replaced by carbon composites that combine longer life with light weight. The nose landing gear and center landing gear have no brakes. 

40 years experience

For more than 40 years Lufthansa Technik has overhauled landing gears and their components for customer airlines. The company's experience with big jets extends back to the earliest Boeing 707, 727 and 737 types, all of which were serviced in the Hamburg workshops. Today, most of the overhaul work is for Airbus designs: A300, A310, A320, A321, A319 and A330/A340 as well as the Boeing 747. So far, Lufthansa Technik has overhauled landing gear for more than 95 airlines.

It takes Lufthansa Technik about five to six weeks to conduct a full landing gear overhaul for a widebody jet, and the company is now the international leader in this market segment. Not too long ago, a job of this type could take up to 120 days, but this has since been drastically reduced by a continual process of optimizing work sequences, the processes employed and modern computer-based planning.

It is good business sense to take great care of a Jumbo jet. The landing gear weighs about six tons and costs about 6 million Euros – without the brakes and tires. For the airline, repair is much cheaper than replacement. 

Ready for take-off

Within no more than 35 days - less for smaller airplanes - the landing gear has been installed, the wheels and brakes checked and fitted, new or remolded tires fitted. The repaired and overhauled landing gear is now one hundred per cent fit ready to resume its essential heavy duty, between airplane and runway.
Ready for take off – and the next landing.