Accident Details
On November 8, 2011, at approximately 1850 eastern standard time, a Cessna 172RG, registration N5095V, operated by Sun State Aviation Inc., was substantially damaged during a gear-up landing at Kissimmee Gateway Airport (ISM), Kissimmee, Florida. The aircraft was conducting an instructional flight under Title 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed, and no flight plan was filed.
The certified flight instructor reported that while practicing maneuvers with a private pilot, they noticed the absence of a positive gear-down-and-locked indication. The right main landing gear was extended only about halfway. The crew recycled the landing gear and attempted manual extension without success. After further troubleshooting proved ineffective, the instructor returned to ISM and performed an emergency gear-up landing on runway 33. During the landing, the underside of the fuselage sustained substantial damage. Neither the instructor nor the private pilot was injured.
Examination by a Federal Aviation Administration (FAA) inspector revealed that the splined stub shaft of the right main landing gear pivot assembly was fractured, with a portion of the shaft remaining in the pivot assembly housing. The fractured components were sent to the NTSB Materials Laboratory in Washington, DC.
Metallurgical Findings
Metallurgical examination of the separated portion of the splined stub shaft fracture surface showed characteristics consistent with a torsional fatigue fracture with multiple origins. Cracks propagated in transverse and longitudinal directions, with longitudinal cracks aligned with the corners of the spline roots. The final fracture of the stub shaft was due to overload of the resulting reduced cross section.
Maintenance and Inspection History
At the time of the accident, the airplane had accumulated 7,755 total hours and had operated for about 20 hours since its most recent 100-hour inspection. Prior to the accident flight, a company mechanic observed hydraulic fluid on the landing gear leg and assumed a leak from the pivot gear O-rings, which were replaced.
On May 14, 2001, the FAA issued Airworthiness Directive AD 2001-06-06. This directive required inspection of main landing gear pivot assemblies on certain Cessna 172 airplanes for cracks, replacement of any cracked assemblies, and installation of new shaft bushings per Cessna Service Bulletin SEB90-1. According to a Cessna representative, the inspection requirements of SEB90-1 were required to be repeated only if the airplane experienced a landing gear overload condition or if the brakes exhibited spongy operation not attributable to brake component wear or improper servicing.
The Cessna Model 172RG Service Manual contained Supplemental Inspection Number 32-10-01, which called for 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 and repeated every 500 hours thereafter. Maintenance records showed compliance with AD 2001-06-06 on June 20, 2001, at a total airframe time of about 5,961 hours. However, no records indicated that Supplemental Inspection Number 32-10-01 had ever been performed. This inspection was not required for aircraft operated under Part 91.
Similar Occurrence
A similar failure occurred to a Cessna 172RG in Spain on August 5, 2008. According to the investigating authority's report, that aircraft had 7,618 total hours and 360 hours since its respective pivot assemblies were inspected for cracks.