More than just "rubber on steel"
Aircraft tyres are subjected to enormous factors and stress during use. The maximum take-off weight of a Boeing B747-400 at Lufthansa is 394,600 kilograms. The tremendous weight placed on the wheels, coupled with high speeds during take-off and landing, causes the tyres to heat up from friction and flexing. The main wheels of commercial aircraft are mounted on fixed axles which cannot be steered, and they must be able to handle both radial and axial forces when the aircraft makes a turn. The tyres are strongly deformed during such a turn, and the result may be the formation of cracks in the tread.
But tyres must also stand up to the stress from external factors such as uncleared runways (damage from foreign objects), quality of the runway surfaces and variations in temperature (ground/cruising altitude). When one considers the general conditions they are used under, it comes as no surprise that wheels (tyres and rims) require extensive servicing, even though their design appears simple enough. This work is done by Lufthansa Technik AG's wheel workshop in Frankfurt. All of the work required for aircraft wheels, from changing tyres to repair work to the complete overhaul of a wheel, can be done here.
General data of main aircraft wheels
Aircraft model Weight Rim (kg) Weight Tyre (kg) Diameter B737-500 (mm) B737-500 39,6 72 1016 B747-400 74,4 110 1244
20,000 wheels per year
An average of 20,000 completely mounted wheels leave the wheel workshop every year. The volume of work at the workshop fluctuates depending on the season of the year. This is partly a result of the greater number of flights during peak travel seasons, but also comes from the higher temperatures during the summer months which cause greater wear and tear on the tyre treads. Working on the tyres involves a lot of steps because the manufacturer's instructions, which are anchored in the CMM (component maintenance manual), must be strictly observed, the only exceptions being those which have been approved by Lufthansa Technik's engineering department.
Every wheel that comes into the wheel workshop passes through numerous stations until, after a thorough inspection in the form of a dual release in conformity with JAA and FAA regulations, it is approved in the final check. All of the incoming wheels are subjected to a receipt inspection and all of the relevant data is entered in a computer system. The staff at this station look at the wheel data from its lifecycle card and its external condition (paint damage, damage to rims and tyres, completeness of the components, etc.), then decide what path the wheel will follow through the workshop. The wheels are first dissembled as far as necessary to do the required work, and the components are handed on for cleaning.
Several checks and procedures
After being cleaned, the rims are then, depending on the required extent of the check, subjected to a destruction-free material inspection by semi-automatic processing equipment. The most frequent applications here are paint penetration procedures or turbulence checks. If there is any doubt, or wherever there are places which are hard to get to, the personnel perform an additional manual check using ultrasonic measurements. If the inspection required the removal of the paint, the rims must be repainted and can then be transferred straight to assembly for further processing.
Rims are checked according to service intervals which vary according to manufacturer and aircraft type; the intervals are entered and saved in a special computer system for every wheel part number and every individual serial number. This system can be used to track the rim during the entire time it is in the workshop. The data is kept on digital storage media and can be traced back for years. For the tyres, the technical service in the wheel workshop ends after they have been removed from the rim. The tyre manufacturers pick up the tyres and, if possible, completely redo the tread. Theoretically, this can be done as frequently as ten times and is subject to the manufacturers' strict quality demands.