History of Flight
On April 18, 2014, about 1340 Pacific daylight time, a McDonnell Douglas Helicopter (MDHI) 500N, registration N522GS, experienced a right rear landing gear strut failure during landing at Santa Monica Airport (SMO), Santa Monica, California. The owner/pilot was operating the helicopter under 14 Code of Federal Regulations Part 91. The private pilot and three passengers were not injured; the helicopter sustained substantial damage to the front landing gear structure. The local personal flight departed Camarillo, California, about 1315, with a planned destination of Santa Monica. Visual meteorological conditions prevailed, and no flight plan had been filed.
The pilot reported that during landing he slowly set the helicopter down on a landing pad and felt the helicopter "drastically" slip to the right. He then lifted the helicopter off the pad, at which time a passenger on board verified the right strut was broken. A passenger reported hearing a "loud pop" on touchdown.
Aircraft and Airworthiness Directive
The 1993 McDonnell Douglas 500N helicopter was subject to Airworthiness Directive (AD) 2007-12-23, which required modifications to the strut and fairings and continuing inspections. The AD referenced MD Helicopter Service Bulletin SB500N-022, which describes the modifications. The increased size of the rivet hole and the polished surface around the hole suggested that the modifications had been performed. The AD required an initial dye penetrant inspection 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 Federal Aviation Administration (FAA), AD 2007-12-23 was complied with at an airframe total time of 2,248.3 hours; at the time of the accident, a total airframe time of 2,274 hours was reported. The FAA noted that there were no dates on the record for the AD compliance, and it was assumed that the AD was complied with during the aircraft's 100-hour/annual inspection, which was performed in December 2013, according to maintenance records.
Tests and Research
Magnified visual examinations of the fracture surfaces by a National Transportation Safety Board materials specialist uncovered faceted features and arrest marks consistent with fatigue cracking on both sides of the inboard holes. The remaining fracture area was matte gray and highly textured, consistent with overstress separation. The fatigue appeared to initiate at multiple origins on opposite sides of the hole bore. The fatigue cracks propagated circumferentially short distances into the strut wall, with slightly greater penetrations adjacent to the outer diameter of the strut. Visually, the fatigue measured about 0.043 inch (aft) and 0.054 inch (forward) at the outer surface of the strut.
About 1.5 inches above the fracture, the outer diameter of the strut measured approximately 2.368 to 2.374 inches, with an inner diameter measuring 1.998 to 1.999 inches. The inner and outer diameters were not concentric, consistent with the engineering drawing. The wall thickness measured about 0.167 inches thick at the inboard location above the fracture.
Engineering representatives of the manufacturer stated that the material was specified as aluminum alloy AA1 7075 in the T73 temper condition per AMS-A-227712. X-ray fluorescent spectroscopy confirmed the composition as consistent with aluminum alloy 7075. For further information, see the NTSB Material Lab Factual Report in the docket for this accident.