On May 30, 2008, about 1100 Pacific daylight time, N115AB, an Aero Commander 690A, sustained substantial damage following a landing gear collapse during landing roll at the Tacoma Narrows Airport (TIW), Tacoma, Washington. Neither the certificated airline transport pilot rated flight instructor, the first pilot, or the airline transport pilot student, the second pilot, were injured. Visual meteorological conditions prevailed for the local instructional flight, which was conducted in accordance with 14 Code of Federal Regulations (CFR) Part 91, and a flight plan was not filed. The flight departed TIW about 1000.

In a statement dated June 19, 2008, the second pilot, who was the owner of the airplane and the flying pilot, reported that he had engaged the services of the first pilot to provide refresher training. The second pilot stated that prior to the accident landing he had completed about 8 to 10 touch-and-go landings, and on the last landing during rollout he experienced a retraction of the right main landing gear. Local maintenance personnel reported substantial damage to the right underside area of the fuselage.

A post accident examination of the airplane by a Federal Aviation Administration (FAA) airworthiness aviation safety inspector revealed that the right main landing gear inboard clevis, P/N ED12758, which serves as the connection between the hydraulic actuating cylinder and the right main landing gear body, had failed.

The NTSB investigator-in-charge (IIC) retained both halves of the broken clevis, which were forward to the National Transportation Safety Board's Materials Laboratory in Washington, D.C., for further examination. A Senior Metallurgist reported that the fracture region revealed large shear lips completely through the wall thickness, indicative of an overstress separation. No indications of preexisting cracks or corrosion were noted at the fracture. The deformation indicated that the bending direction was parallel to the plane of rotation of the clevis bolt, with the clevis end bending forward. The reason for the overstress separation was undetermined. (Refer to the attached NTSB Materials Lab Factual Report.)

On April 25, 2005, Twin Commander Aircraft Corporation issued Service Letter No. 376, "Inspection of Main Landing Gear Clevis and Drag Brace Installations," as a result of field reports of cracks found in clevis P/N ED 12758. Compliance instructions indicated that inspection was recommended prior to further flight if the landing gear had not undergone inspection within the last 500 hours, and to reinspect at 500 hour intervals.

Maintenance records indicate that on November 1, 2006, during a 150-hour inspection at an aircraft total time of 7,670.9 hours, the rod bearing on the right main landing gear inboard actuator was replaced with a new rod end, P/N HMX6G. Maintenance records also revealed that during this inspection, repairs were made to the cracked right inboard main landing gear trunnion. The airplane's most recent annual inspection was performed on October 16, 2007, at an airframe total time of 7,673.1 hours. At the time of the accident the airframe total time was 7,673.8 hours. Maintenance records also indicate that during a 150-hour inspection performed on September 6, 2005, at an aircraft total time of 7,661.6 hours, the main and nose landing gears were inspected and overhauled. Additionally, both main landing gear and nose gear actuators were overhauled, including emergency actuators, and all hydraulic and pneumatic landing gear hoses were replaced and lead checked. (Refer to the maintenance records attached to this report.)

On July 30, 2008, under the supervision of the IIC, the accident airplane's landing gear system was examined by parties to the investigation. A replacement right main landing gear inboard clevis was installed. The examination consisted of multiple landing gear extension and retraction cycles, during which all three landing gear were observed to extend and retract normally. No landing gear anomalies were observed.

According to the Gulfstream Commander 690A/690B Maintenance Manual, Section VI, Landing Gear, Wheels and Brakes, Landing Gear Operation, when the landing gear control lever is in the down and locked position, hydraulic pressure is being applied to the inboard main landing gear actuating cylinder, as well as assisted by the outboard main landing gear (hydraulic-pneumatic) actuating cylinder to extend and lock the main gear. Bungee cords attached to the main landing gear drag braces assure that the main landing gear is locked in the down position. Unintentional retraction of the landing gear is prevented by a safety latch located to the left of the landing gear control lever. A postaccident examination revealed no anomalies with either the right main landing gear actuating cylinder or the bungee cords.

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