On September 26, 2002, at 1135 mountain daylight time, a Cessna 172RG, N6525R, registered to Lynch Flying Service doing business as Edwards Jet Center as a 14 CFR part 91 practical test flight, landed with one main landing gear not locked in the extended position at Billings Logan International airport, Billings, Montana. Visual meteorological conditions prevailed at the time and no flight plan was filed. The aircraft was substantially damaged and the private pilot and check airman were not injured. In a written statement, the pilot reported that after completing commercial maneuvers for the commercial check ride, the flight returned to the airport to accomplish touch-and-go landings. While in the pattern, the landing gear was extended, however, the right main landing gear would not fully extend and lock. The pilots attempted to extend the landing gear with no success. Eventually the aircraft was landed with the left main and nose gear extended. When the aircraft settled on the right side, the aircraft skidded off the runway. The right wing contacted the surface resulting in minor damage, however, the right side horizontal stabilizer was substantially damaged. During an inspection of the aircraft by a Federal Aviation Administration Principal Maintenance Inspector from the Helena, Montana, Flight Standards District Office, the right main landing gear actuator, part number 9882015, was found broken. Maintenance records indicated that on August 28, 2001, at airframe total time of 4389.5 hours, Cessna Service Kit SK172-151, Main Landing Gear Pivot and Actuator modification was accomplished. The kit provided parts and instructions to replace the inboard bushing on the main landing gear pivot and the bushing in the actuator cap with a single bushing of softer material. Approximately 329 hours had been accumulated on the aircraft since the modification. The main landing gear actuator was sent to the National Transportation Safety Board, Materials Laboratory, Washington D.C., for examination. The Mechanical Engineer reported that the gear housing was cracked in two locations. The first crack was oriented radially, traversing the most forward mounting hole. The second crack initiated at the inside diameter of the gear housing portion and had propagated radially, stopping at the hole for the roller bearing. The fracture faces for the first crack revealed radially oriented lines which indicated multiple origins on the exterior surface of the gear housing portion of the actuator and from the intersection with a bolt hole. There were two fractures located at the second crack position. One fracture on the left side of the rack and one fracture on the right side of the rack. Examination of the left and right fracture faces revealed radially oriented lines. Some of these lines indicated their origin on the inside diameter of the gear housing portion of the actuator. The Mechanical Engineer reported that the features at both crack locations in the actuator housing were consistent with an overload event. Examination also revealed two areas of disturbed material located on the upper surface of the hole, forward of the roller, and normally occupied by the rack as it was extended and retracted. The remaining portion of the actuator body revealed a disturbed surface located on the lower surface of the hole normally occupied by the rack. Score marks were noted longitudinally oriented towards the cylinder portion of the actuator body. The score marks at the cylinder portion of the actuator body were deeper and the process had deformed material into the o-ring groove. The scoring process had continued past the o-ring, on to the rear edge of the groove and had deformed material at the inside chamfered edge. The piston/rack assembly was extracted from the cylinder portion of the actuator for further examination. The three visible rack teeth displayed longitudinally oriented smear marks on their flanks and crowns. The gear teeth that are meshed with the sector gear when the landing gear is in the "up" position were examined. Smearing marks were noted to the crowns and rack teeth. All of the smearing on the rack teeth was in a longitudinal direction. The components were then sent to Cessna Aircraft Company, Product Engineering for further evaluation. The Material and Process Engineer reported concurrence with the findings of the NTSB report. The Engineer further stated, "Severe wear of the piston rack teeth was present, suggesting possible problems with the meshing of the sector gear with the rack gear teeth. Wear was also observed on the rack opposite to the gear teeth, due to rubbing contact with the roller, suggesting significant contact force between the rack and the roller." The fracture surfaces were observed utilizing a scanning electron microscope. The fracture surfaces indicated the presence of closely spaced fatigue striations on the fracture surfaces that were located at the housing roller. The fracture surfaces that were located at the forward attaching bolt were indicative of overload.