Accident Details
On May 2, 2008, at about 1840 Pacific daylight time, a Rockwell International 690C, registration N980AK, experienced a collapse of both main landing gear while taxiing for takeoff at Minden-Tahoe Airport (MEV), Minden, Nevada. The commercial pilot and sole passenger were not injured. Visual meteorological conditions prevailed for the 14 CFR Part 91 flight, and an instrument flight rules flight plan was filed. The intended destination was Monterey Peninsula Airport (MRY), Monterey, California.
According to the pilot, while taxiing to runway 16 and just before making a final right turn onto the runway, a popping sound was heard from the left side, followed by a bump or hop. The aircraft then began to sag and swerve to the right. Within approximately 25 feet, the right propeller struck the ground, and the aft fuselage contacted the ground, sliding about 10 feet before stopping. The pilot stated that the landing gear selector lever remained in the DOWN position and was not inadvertently moved.
Examination
An FAA airworthiness inspector examined the aircraft on May 5, 2008. The nose gear remained extended; the right main landing gear was retracted further than the left. Substantial damage was noted to the lower fuselage skins, formers, stringers, and bulkheads. The left main landing gear's inboard clevis (P/N 12758), which attaches the inboard hydraulic actuator to the trunnion boss, was fractured. The inspector reported that the clevis was incorrectly installed: the bolt was not screwed into the barrel nut but forced in, impinging on the edges of the housing hole and the clevis end.
The maintenance firm, Aero-Air, LLC, responded that the barrel nut could be inserted and missed by the bolt, yet the bolt could still engage the trunnion and clevis pin, remaining tight. They asserted this installation would not cause the clevis to break.
A local mechanic who removed the clevis bolt noted it rotated easily and the nut appeared incorrectly located, though he could not verify threading.
Both fractured clevis pieces were sent to the NTSB Materials Laboratory. A metallurgist found large shear lips indicative of overstress separation, with no pre-existing cracks or corrosion. Bending stresses were present, parallel to the plane of rotation of the clevis bolt. The reason for the overstress separation was undetermined.
Maintenance History
On April 25, 2000, Twin Commander Aircraft Corporation issued Service Letter No. 376 regarding inspection of main landing gear clevis and drag brace installations due to field reports of cracks in clevis P/N ED 12758. The most recent annual inspection was completed on March 28, 2008, at 5,191.2 hours, including a 5-year/3,000-hour gear and actuator inspection. Parts replaced included the left main landing gear inner body and glands. Service Letter No. 376 was complied with. The aircraft had 13 hours since inspection at the time of the accident.
On May 20, 2008, under FAA supervision, a new clevis was installed on the left main landing gear. Subsequent testing including normal and emergency gear cycles showed no discrepancies.
According to the maintenance manual, hydraulically actuated clam shell doors enclose the main landing gear wheel wells. Photographs taken about 25 minutes after the accident showed the clam shell doors in the closed position.