On December 19, 2011, at 1100, eastern standard time, a Piper PA-44-180, N790GK, registered to and operated by Airline Transport Professionals (ATP), incurred substantial damage when the right main gear collapsed during the landing roll at the Gwinnet County Airport (LZU), Lawrenceville, Georgia. The flight instructor and pilot rated student were not injured. Visual meteorological conditions prevailed and no flight plan was filed for the 14 Code of Federal Regulations Part 91, instructional flight. The flight originated, earlier that day, from the DeKalb-Peachtree Airport (PDK), Atlanta, Georgia, about 1000. While configuring the airplane for a power off stall, the pilots noted an unsafe gear indication after selecting the landing gear down position. There were two green lights, but the right main landing gear indicator light was not on. The flight instructor recycled the landing gear, but right main landing gear indicator light did not illuminate. The flight instructor changed the landing gear indicator light bulbs in an unsuccessful attempt to get the right main landing gear indicator light to ignite. He recycled the landing gear five more times, but the light still did not illuminate. The flight instructor followed the emergency landing gear extension procedures in the pilot’s operating handbook (POH), but the right main landing gear indicator light would not illuminate. The pilots reported the condition to an air traffic controller and he maneuvered the airplane for a low approach and the controller advised them that the gear appeared down. The flight instructor then entered the traffic pattern and landed on runway 07. During the landing, all three main landing gear were extended, but as the airplane slowed, the right main landing gear collapsed, the airplane veered off the runway, and struck a runway light. The airplane incurred damage to the right engine’s propeller, right wing outboard leading edge, right flap, and the right aileron incurred structural damage. The airplane was manufactured in 1979 and was equipped with two Lycoming O-360 series, 180-horsepower engines. The airplane's most recent 100-hour inspection was completed on November 16, 2011. At the time of the inspection, the reported aircraft time was 9781.3 total hours and the recorded hobbs time was 7073.6 hours. The accident occurred about 38 hours after the most recent 100-hour inspection. A postaccident examination of the right main landing gear revealed that the truss assembly hook assembly did not travel over the truss pin. Federal Aviation Administration inspectors tested the landing gear and the right main landing gear did not lock on any attempt. In addition, the right main landing gear truss bracket and stud had excessive friction and prevented the side brace from fully rotating, subsequently preventing the truss hook from traveling over the lock pin. The right main landing gear truss bracket pin had little or no grease present; however, the left main landing gear truss bracket pin had grease indications around the pin and zerk fitting. A review of the airframe maintenance records indicated that a new bushing was installed in the right main landing gear side brace mount assembly on November 16, 2011. The mechanic who performed the maintenance stated that he could not find any guidance in the maintenance manual on how to remove and install the new bushing; therefore, he replaced the bushing and verified that there was movement between the two parts, and then reinstalled the side brace assembly on the airplane. In the same maintenance entry the mechanic noted that he greased the landing gear and performed a landing gear retraction, extension, and free-fall test without defects. The Piper Aircraft PA-44-180/180T Airplane Maintenance Manual, Page 32-15, Installation of the Main Landing Gear, indicated that the procedures for installing the gear side brace link assembly were: A. Position link support bracket with swivel stud installed at its attachment point on the web of the spar and secure with bolts and washers. B. Ascertain that the upper and lower links are assembled with downlock hook, retraction fitting, c.. attached, and through travel of the links and downlock hook clearance checked. C. Attach the upper link to the swivel stud of the support fitting and secure with bolt, bushing, washer, nut and cotter pin. D. The actuating cylinder rod end bearing and lower side brace link may be attached respectively to the retraction fitting and strut housing during the adjustment of the landing gear. In addition, the airplane maintenance manual stated "when assembling components of the landing gear, lubricate bearings, bushings, and friction surfaces with proper lubricant as described in Chapter 12."