Mcdonnell Douglas Helicopter 500N N522GS accident near Santa Monica, California

Probable cause

The fracture of the right aft landing gear strut due to undetected fatigue cracking in an inboard rivet hole.

NTSB analysis

The private pilot/owner of the helicopter was conducting a local personal flight. The pilot reported that, upon landing, he slowly sat the helicopter down on a landing pad and then felt it "drastically" slip to the right. He then lifted the helicopter off the pad, at which time a passenger verified that the right aft landing gear strut was broken. A passenger reported that, upon touchdown, he heard a "loud pop."

The Federal Aviation Administration (FAA) had previously issued an airworthiness directive (AD) applicable to the accident helicopter make and model to detect cracks that could result in the failure of the strut and subsequent loss of helicopter control during landing. The AD required modifications to the landing gear strut rivet holes and fairings and subsequent initial dye penetrant inspections followed by continuing 10x magnified visual inspections at intervals not to exceed every 100 hours time in service or during each annual inspection. According to the FAA, the AD was complied with at an airframe total time of 2,248.3 hours. At the time of the accident, the total airframe time was 2,274 hours.

Examination of the right aft landing gear strut revealed that it had fractured due to undetected fatigue cracking in an inboard rivet hole. The fatigue cracks were very small, and given their size, they likely would not have been detectible by the existing required visual inspections. The cracks initiated after the rivet hole diameter was increased; therefore, they initiated after the initial penetrant inspection.

Although it is possible that the fatigue cracks were the result of a degradation in material properties or, more likely, due to greater loads from a previous hard landing or normal landing on primarily the right skid, the investigation was not able to determine the origin of the fatigue cracks based on the available information.

Text from the NTSB. Open the NTSB docket for WPR14LA253 for the full record.