At the appropriate time during the approach sequence, the co-pilot of the Boeing 737, who was the handling pilot for the sector, called for the landing gear to be extended. When the pilot in command placed the landing gear lever to the “down” position, a loud thump was heard and the “gear safe” green light for the right main gear illuminated immediately. This was followed by the illumination of the left main and nose landing gear lights, consistent with a normal extension sequence. The aircraft rolled approximately 4 degrees to the right while the gear was extending. This was counteracted by a left roll control input. As the crew had received indications that the landing gear was safely locked down, they continued the approach and completed a normal landing. Ramp maintenance staff briefly inspected the aircraft but did not find any immediate cause for the reported thump. The aircraft was then placed on jacks for a retraction test. When the landing gear lever was selected to the “up” position, the right main landing gear moved inboard approximately 15 cm before a grinding noise was heard. The test was immediately suspended and the landing gear was extended. When access panels were removed, it was found that the actuator beam arm inboard lugs and beam hanger had fractured. The rear wing spar, landing gear beam, aileron bus cable, pulley bracket, aileron and spoiler cables and hydraulic lines had been damaged extensively following the fracture of the lugs and hanger. Inspection of the actuator beam arm revealed that the fracture of both lugs was due to stress corrosion cracking.