An automatic landing on runway 27 was carried out with the first officer being the pilot flying. The right main landing gear failed immediately upon touchdown. The aircraft slid along the runway and came to a halt some 1,600 metres further on, resting on its right flaps, slats and wingtip. An emergency evacuation was carried out. Examination of the aircraft showed that the right main gear strut cylinder had fractured below the attachment trunnions, releasing the wheels/axle assembly back into the flaps and wing/fuselage fairing. The lower portion of the failed cylinder remained attached only by the sidestay. A closer inspection revealed the presence of an anomaly on the forward outer surface of the cylinder fracture, which subsequent examination identified as a fatigue crack measuring approximately 3.5 mm long and 1.1 mm deep. CAUSAL FACTORS: 1. The right main landing gear cylinder failed immediately upon touchdown due to the application of spin-up drag loads on a section of the cylinder containing a major fatigue crack 3.2 mm long and 1.0 mm deep and several other smaller cracks associated with it. 2. The origins of these fatigue cracks could not be identified but other embryonic cracks were found which were associated with surface irregularities arising from a grit-blasting process during manufacture. Abnormal loading, possibly due to an occurrence of a mode of fore-and-aft vibration known as ‘gear walking’ is thought to have been responsible, at some time in the aircraft’s history, for propagating the cracks to a depth at which continued growth was possible under normal loading. Alternatively, some abnormal loading may have relaxed the beneficial compressive surface stresses induced by shot-peening at the critical section and allowed propagation from the same surface defects. 3. Inspection and other mandatory preventive measures taken following two similar accidents did not prevent the occurrence of this third accident. This was probably due to the small size of cracks which are required to be detected before reaching a critical dimension.