On heavy aircraft, the need to spread the weight over a large area has resulted in the use of multiple wheel undercarriages. The undercarriage unit normally consists of shock absorber strut, at the lower end of which a bogie beam is pivoted, and the axels are attached to each end of the beam. On some aircraft the rear pair of wheels swivels on the bogie beam, and castors when the nose wheel is turned through a large angle, on others the upper torque link member is replaced by a pair of hydraulic jacks which, when nose wheel steering is applied, rotates the whole bogie. Castor or steering prevents excessive torque on the undercarriage leg and minimizes tire scrubbing during turns. For normal operation the swiveling pair of wheels is locked in line with the fixed pair. Break torque at each wheel is transmitted through compensating rods to the shock absorber strut, thus preventing excessive loads on the boogie beam. On retractable landing gear a levelling strut or ‘hop damper’ provides a means of positioning the bogie beam at suitable angles for retraction and landing, this strut is usually connected into the hydraulic system to prevent retraction if the bogie is not at a suitable angle, and combines the functions of hydraulic ram and damper unit.

Maintenance.

In addition to the lubrication, testing and maintenance of landing gear described in previous paragraphs, particular care and higher standards of workmanship are necessary with bogie undercarriages. Since this type of undercarriage is fitted to heavy aircraft, the materials used are of very high strength and great care is taken in the manufacture, heat treatment and finishing of the components. However, these materials are usually more susceptible to failure from scratches, indentations or corrosion, then materials of lower strength. All servicing functions should therefore be carried out with special care, particularly with regard to lubrication, the lack of which could result in corrosion or hydrogen embrittlement. If any surface damage is found during inspection, it should be repaired strictly in accordance with the instructions and limitations specified in the manufacturer’s manuals. Or if no adequate guidance is given, in accordance with an approved repair scheme.