ERA12LA070

The pilot was not able to obtain a positive gear down-and-locked indication and noticed that the right main landing gear (MLG) only extended about halfway down. After unsuccessful troubleshooting efforts, he performed an emergency gear-up landing, which resulted in substantial damage to the underside of the fuselage. Examination of the landing gear system revealed that the stub shaft of the right MLG pivot assembly was fractured and exhibited characteristics consistent with a torsional fatigue fracture containing multiple origins. Before the accident flight, a company mechanic observed some hydraulic fluid on the MLG leg and assumed that there was a leak from the pivot gear O-rings, which he replaced. The Federal Aviation Administration previously issued an airworthiness directive (AD), which included an inspection of MLG pivot assemblies for cracks, in accordance with a manufacturer's service bulletin. The AD did not call for repetitive inspections. At the time of the accident, the airplane had been operated for about 7,755 total hours and about 1,800 hours since the time of AD compliance. The airplane service manual contained a supplemental inspection that included initial visual and eddy current inspections of the MLG pivot assembly for cracks in the spline area at 3,000 hours in service with repeated inspections every 500 hours thereafter. The visual and eddy current inspections had not been nor were they required to be performed on the accident airplane.

On November 8, 2011, about 1850 eastern standard time, a Cessna 172RG, N5095V, operated by Sun State Aviation Inc., experienced a landing gear malfunction and was substantially damaged during a gear-up landing at the Kissimmee Gateway Airport (ISM), Kissimmee, Florida. The certified flight instructor and a private pilot were not injured. Visual meteorological conditions prevailed and no flight plan had been filed for the instructional flight that was conducted under the provisions of Title 14 Code of Federal Regulations Part 91. The flight instructor reported that he was practicing maneuvers with the private pilot when he noticed that they were not getting a positive gear down and locked indication. In addition, the right main landing gear was only extended about halfway. They recycled the landing gear and attempted to manually extend the gear without success. After additional troubleshooting efforts were also unsuccessful, the flight instructor returned to ISM and performed an emergency gear-up landing to runway 33. During the landing, the underside of the fuselage sustained substantial damage. Examination of the landing gear system by a Federal Aviation Administration (FAA) inspector revealed that the splined stub shaft of the right main landing gear pivot assembly was fractured, and a portion of the shaft remained in the pivot assembly housing. The fractured components were forwarded to the NTSB Materials Laboratory, Washington, DC. Metallurgical examination of the separated portion of the splined stub shaft fracture surface revealed characteristics consistent with a torsional fatigue fracture containing multiple origins. The cracks appeared to have propagated in the transverse and longitudinal directions, with the longitudinal cracks aligned with the corners of the roots of the shaft’s splines. The final fracture of the stub shaft was due to overload of the resulting reduced cross section. At the time of the accident, the airplane had been operated for about 20 hours since its most recent 100-hour inspection, and had 7,755 total hours. Prior to the accident flight, a company mechanic observed some hydraulic fluid on the landing gear leg and assumed that there was a leak from the pivot gear O-rings, which he replaced. 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, the 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, in accordance with Cessna Service Bulletin SEB90-1. According to a Cessna representative, the inspection requirements of Cessna Service Bulletin SEB90-1 were required to be repeated only any time an airplane had experienced a landing gear overload condition or if the brakes exhibited "spongy" operation that could not be attributed to brakecomponent wear or improper servicing. The Cessna Aircraft Company Model 172RG Service Manual contained a Supplemental Inspection Number 32-10-01 that included a visual and eddy current inspection of the main landing gear pivot assembly for cracks in the spline area initially at 3,000 hours in service, with repeated inspections every 500 hours thereafter. According to maintenance records, AD 2001-06-06, which included Service Bulletin SEB90-1 was complied with on June 20, 2001, at a total airframe time of about 5,961 hours. There were no records found that indicated Supplemental Inspection Number 32-10-01 had ever been performed on the airplane. Nor was the inspection required for airplanes operated under Part 91. A similar failure to that observed on the accident airplane occurred to a Cessna 172RG, in Spain, on August 5, 2008. According to the investigating authority’s report, that airplane had been operated for 7,618 total hours, and 360 hours since its respective pivot assemblies were inspected for cracks.