ERA11IA172

The pilot reported hearing a “pop” sound from the nose gear area when the gear was extended for the landing. Toward the end of the landing roll, the nose gear collapsed. A postaccident examination of the nose gear revealed that the bolt that holds the downlock drag brace joint link to the airframe structure had fractured. A metallurgical examination revealed that the bolt failed in reverse bending fatigue, which is associated with the bolt having insufficient clamping pressure (looseness). The airplane’s last required inspection was about 70 flight hours and 40 gear cycles before the nose gear collapsing, and the nose gear downlock system rigging should have been inspected at that time.

On January 16, 2011, at 1500 eastern standard time, a Mitsubishi, MU-2B-20, N52CD, owned by Hayes Landing, LLC and operated by an individual, had the nose landing gear collapse during landing at the Lowcountry Regional Airport (RBW), Walterboro, South Carolina. The pilot and passenger were not injured and the airplane incurred minor damage. Visual meteorological conditions prevailed and an instrument flight plan was filed for the personal flight, operated under the provisions of Title 14 Code of Federal Regulations Part 91. The flight departed from the Northeast Regional Airport (EDE), Edenton, North Carolina, earlier that day, about 1330. The pilot stated that the flight was uneventful. During the gear extension, on approach to the airport, he heard a “pop” as the nose gear lowered. During the landing roll the nose gear collapsed, resulting in minor damage to the nose gear doors and the skin behind the nose gear area. Maintenance personnel discovered the attaching bolt for the drag strut for the down lock mechanism assembly fractured. The bolt had an estimated in service time of 500 hours with an estimated 290 gear cycles. The fractured bolt was retained for further examination. The airplane’s last one year/100 hr/200 hour inspection was performed on September 14, 2010, at airframe total time of 8,053 hours. At the time of the accident the airframe had accumulated a total time of 8,124 hours. The National Transportation Safety Board Materials Laboratory conducted a metallurgical examination on the fractured bolt. The examination revealed the bolt failed in reverse bending fatigue. The bolt holds the down lock drag brace joint link to the airframe structure. The airplane’s manufacturer maintenance manual calls for and indicates when to check for proper rigging of the nose gear down lock system. Manufacturer Service Bulletin (SB) No. 188, dated July 13, 1981, which was developed due to several incidents were the improper adjustment of the down lock mechanism led to the failure of the nose landing gear. The SB requires inspection of the mechanism for proper adjustment (rigging). As a result of this incident the manufacturer published a Service News letter, dated September 5, 2011, outlining the importance of proper rigging of the down lock mechanism, making reference to the SB No. 188, and informing maintenance personnel and operator were to obtain the correct rigging tools.