HISTORY OF FLIGHT On January 10, 2009, about 1100 Pacific standard time, a Cessna 172RG, N6272R, landed with the right main landing gear partially extended at Carson Airport, Carson City, Nevada. National Pilot Academy was operating the airplane under the provisions of 14 Code of Federal Regulations (CFR) Part 91. The certificated commercial pilot and flight examiner were not injured; the airplane sustained substantial damage. The local personal flight departed Carson Airport, about 0930. Visual meteorological conditions prevailed, and no flight plan had been filed. The pilot reported that after an uneventful preflight check, takeoff, and flight in the practice area he began the return trip towards Carson City. About 5 minutes later he heard a “loud clunk” from underneath the airplane, and assumed it was the hydraulic system energizing. A few minutes later he became alarmed when he heard the sound again. As he approached the crosswind leg for the landing runway, he attempted to extend the landing gear but observed an unsafe indication of the landing gear indicator lights. He visually examined the right main landing gear, and confirmed that it had not fully extended. He recycled the landing gear extension system, operated the manual pump, and performed, “g-loading” maneuvers but the landing gear remained partially extended. After conferring with maintenance crew, the pilot landed the airplane with the nose and left landing gear retracted. During the landing sequence the airplane sustained substantial damage to the belly structure. National Pilot Academy was a flight training facility, and the accident airplane was primarily used for flight training. TESTS AND RESEARCH Examination of the right main landing gear actuator revealed that it had sustained fractures around its mounting holes, and appeared to have failed. A mechanic from the operator reported that he observed a similar failure on another Cessna 172RG in their fleet (N5242V), and had retained the failed part. The accident actuator and associated piston and the actuator, piston, and sector gear from N5242V were sent to the Safety Board Materials Laboratory for examination. N6272R Actuator Examination Examination of the actuator body revealed a part number 9882015-2 and serial number 4580; the assembly date was noted to be 4Q79 indicating the fourth quarter of 1979. An examination of the actuator body revealed cracks at three locations adjacent to the sector gear opening. One crack was located at the forward mounting hole, and had propagated through the entire body, leaving a gap between the fracture surfaces. A second crack was located between the inner surface of the body and the rear mounting hole but had not opened up or propagated through the body. A third crack was located between the inner surface of the body and the hole containing the piston roller bearing; the crack had not opened up or propagated through the body. Microscopic examination of the fracture at the forward mounting hole revealed that the edges of both faces were jagged, and their surfaces displayed a uniformly colored rough grainy surface. The surfaces were further examined utilizing a scanning electron microscope (SEM); a survey of the fracture surface revealed ductile dimples, typical of an overload event. Examination of the piston revealed mechanical damage to the forward edge of the piston rod adjacent to the first partial gear of the piston rack. Further examination of the damaged area revealed that material had been deformed rearwards, towards the piston, and downwards, towards the adjacent partial gear tooth on the rack. To the rear of the deformed material was an arc-shaped indentation. Scoring was noted on the edge (at the intersection of the crest and the flank) of the adjacent partial gear tooth. Similar but more heavily defined scoring was observed on the forward edge of the next gear tooth. All gear teeth on the rack exhibited smearing on their forward and aft flanks, consistent with the sliding action of gear contact. N5242V Actuator Examination Due to a machining operation on the actuator body, no identification plate was observed and no casting marks were noted; as such the history of the part could not be determined. Examination of the actuator body revealed similar cracks in the forward and aft mounting holes to the accident part; however, no crack was observed in the roller bearing area. Survey of the forward mounting hole failure surfaces utilizing a SEM revealed ductile dimple features surrounding a woody (fibrous) core, also typical of an overload event. Mechanical damage similar to that observed on the accident actuator was noted on the exemplar actuator piston and its associated rack gears. Examination of the sector gear revealed arc-shaped concave indentations on the crest and flank of the last sector gear tooth normally fully engaged with the rack when the piston is fully extended (landing gear retracted) and an arc-shaped indentation on the flank of the adjacent tooth. Installation Examination The operation, rigging, and installation of the actuator were examined on an exemplar airframe in the presence of representatives from the NTSB and Cessna. During the examination it was noted that no provisions for rigging adjustment were included within the airframe. Additionally, extension and retraction were restricted through the use of end-stops and micro-switches that controlled the engagement of the hydraulic system. In order to facilitate examination of the sector gear and rack interaction during operation, the cap from the actuator was removed. The landing gear was then cycled and the internal gear interaction was documented. Inspection of the sector gear and rack in the gear-up and gear-down position revealed that the sector gear teeth did not impinge on the piston surface adjacent to the gear rack. A complete metallurgical exam report is contained within the public docket for this accident. Airplane Maintenance Records An FAA Inspector examined the airplane's maintenance logbooks. He reported that they contained no entry for Cessna Service Bulletin SEB01-2 compliance. This service bulletin addressed repeated inspections for specific fatigue induced cracks in an area of the actuator body adjacent to the piston roller bearing. The operator reported the last 100-hour inspection occurred on January 8, 2009, at a total airframe time of 5,031 flight hours. Examination of the operators 100-hour inspection control log did not indicate an entry for Cessna Service Bulletin SEB01-2. The NTSB laboratory examination did not observe the polishing required by compliance with the Service Bulletin.