HISTORY OF FLIGHTOn January 17, 2013, about 1740 Pacific standard time, a Cessna 172RG, N6290V, landed gear-up with the left main landing gear partially extended, at Roberts Field, Redmond, Oregon. Professional Air was operating the airplane under the provisions of 14 Code of Federal Regulations Part 91. The certified flight instructor (CFI) and pilot undergoing instruction were not injured. The airplane sustained substantial damage. The instructional flight departed Bend Municipal Airport, Bend, Oregon, about 1515. Night visual meteorological conditions prevailed, and no flight plan had been filed. During the landing approach into Bend, the pilot undergoing instruction was unable to fully extend or retract the left main landing gear. After multiple unsuccessful troubleshooting attempts, the CFI elected to divert to Redmond, retract the nose and right main landing gear, and perform a gear-up landing. During the landing roll, the airplane sustained substantial damage to the underside of the fuselage. TESTS AND RESEARCHComponent Examination Postaccident examination revealed that the pivot assembly for the left landing gear actuator had broken at the fillet radius transition between the splined and non-splined portions of the pivot. The actuator assembly was sent to the Cessna Materials and Process Laboratory in Wichita, Kansas, for examination under the oversight of an engineer from the Federal Aviation Administration (FAA). Examination of the fracture surfaces revealed heavy smearing, typical of torsional fractures. The un-smeared pockets of the fracture surface displayed the presence of closely spaced fatigue striations, with the splined portion of the pivot displaying longitudinal cracking at the root of 36 of the 49 splines. The longitudinal spline cracking extended to the center of the pivot shaft, in a star pattern to depths of approximately 0.32 inches, effectively reducing the cross section of the shaft. A couple of these longitudinal cracks were broken open, revealing a fracture surface containing closely spaced fatigue striations. Material and chemical analysis as well as tensile testing was performed on the splined pivot, with results indicating that it met the engineering drawing requirements for its material type, 7175 aluminum. Landing Gear Service Bulletins On May 14, 2001, the FAA issued Airworthiness Directive AD 2001-06-06, which required the inspection of main landing gear pivot assemblies of certain Cessna 172 airplanes for cracks, in accordance with Cessna Service Bulletin SEB90-1 Revision 3. The service bulletin required replacement of any cracked main landing gear pivot assemblies on the affected airplanes, and the installation of new shaft bushings that were designed to enhance the service life of the pivot assembly. Cessna issued this service bulletin in March 1999. It stated that once complete, compliance was no longer necessary unless the airplane had experienced a landing gear overload condition, or if the brakes exhibited "spongy" operation that could not be attributed to brake component wear or improper servicing. The service bulletin stated that compliance with the inspection portion required about 10 hours of time per pivot assembly to complete. Maintenance Records Examination of the airplane's maintenance records revealed that SEB90-1 Revision 3 was complied with in June 2001, at a tachometer time of 5,544 hours. In May 2010, at a tachometer time of 7,379.6 hours, the airplane sustained a hard landing, requiring the removal of the propeller and engine for inspection, repair to the right wingtip, and a "hard landing inspection." No mention was made indicating that SEB90-1 had been complied with since that time. Additionally, the work order revealed that the total time spent performing the hard landing inspection was 4.02 hours. According to representatives from Cessna, the maintenance manual makes no mention of a hard landing inspection, and that items in such an exam are left to the discretion of the mechanic. The airplane's tachometer indicated 8,274 hours at the time of the most recent annual inspection, 2 days prior to the accident. Maintenance records indicated that the actuator was the original component, and had not been serviced or overhauled since the airplane's manufacture in 1980. A similar failure to that observed in the accident airplane occurred with a Cessna 172RG in Kissimmee, Florida, on November 8, 2011. See NTSB report number ERA12LA270 for further details.