NTSB Factual Report

On May 2, 2008, about 1840 Pacific daylight time, a Rockwell International 690C, N980AK, sustained substantial damage following a collapse of both main landing gear while taxiing for takeoff at the Minden-Tahoe Airport (MEV), Minden, Nevada. The commercial pilot and his sole passenger were not injured. Visual meteorological conditions prevailed for the 14 Code of Federal Regulations (CFR) Part 91 flight, and an instrument flight rules (IFR) flight plan was filed. The proposed cross-country flight was destined for the Monterey Peninsula Airport (MRY), Monterey, California.

In a telephone interview with the NTSB investigator-in-charge (IIC), as well as in a statement provided to the IIC by a Federal Aviation Administration (FAA) airworthiness aviation safety inspector, the pilot reported that while taxiing to runway 16 and just prior to making the final right turn from the taxiway onto the runway, "I [he] heard a 'Pop' to my [his] left and almost simultaneously felt the airplane 'Bump' or 'Hop'. Then the plane began to sag and swerve to the right. Within a very short distance (+/- 25 feet) the right propeller struck the ground, and the aft fuselage touched the ground, sliding approximately 10 feet before coming to a stop." When asked by the IIC if the landing gear selector lever had been inadvertently moved to the UP position, the pilot said that it had not, and that it had remained in the DOWN position. The pilot stated that the airplane had just recently come out of an annual inspection, where some work was performed on the landing gear system. The pilot added that the airplane had about 14 hours on it since the time of the inspection.

In a statement dated May 27, 2008, the FAA airworthiness inspector reported that he examined the airplane at the pilot/owner's hanger on May 5, 2008. The examination revealed that the nose gear remained extended, the right main landing gear was retracted further than the left main landing gear, and the lower surfaces of the fuselage, consisting of skins, formers, stringers, and bulkheads were substantially damaged. The inspector reported that further examination revealed that the left main landing gear's inboard clevis (P/N 12758), which attached the main landing gear's inboard hydraulic actuator to the left main landing gear trunnion boss, was fractured. The inspector further noted that "...the clevis was incorrectly installed. A bolt, washer, and barrel nut fastens the clevis into the landing gear housing boss. When we removed the broken off end of the clevis that fits into the landing gear boss, we found that the bolt was not screwed into the barrel nut. The bolt had been forced in and was impinging into the edges of the hole in the landing gear housing, the holes in the hollow clevis end, and the end of the barrel nut." The inspector noted no anomalies with the left outboard actuator assembly and associated components, or the landing gear's bungee cords. Additionally, the inspector reported no anomalies with the right main landing gear.

In a letter to the FAA dated June 30, 2008, submitted by the Director of Maintenance (DOM) for Aero-Air, LLC, the firm which performed the most recent landing gear maintenance on the accident airplane, the DOM, in response to the FAA inspector's statement that "the clevis was incorrectly installed," responded with the following: "The barrel nut can be inserted, and if allowed to drop can be missed by the bolt. In doing so however, the bolt is still engaged in both the gear trunnion and the clevis pin. With the barrel nut being slid down to the bottom of the clevis, it allows the bolt to narrowly go over the top of the nut, very slightly changing the angle of the bolt. This causes the bolt to tightly thread into the opposite opening in the clevis body. The bolt would still be tight and thusly retaining the clevis in the trunnion as it should. The breakage of the clevis pin would not be caused by this installation."

In a statement provided to the IIC by a local certificated airframe and powerplant mechanic who was asked to remove the clevis bolt from the landing gear, the mechanic stated, "I was requested to remove the bolt from the collapsed landing gear. The bolt rotated relatively easily for removal and the nut appeared to be incorrectly located. I cannot verify the bolt was threaded into the nut. There were a number of people in the hangar, including representatives from the FAA."

Both portions of the fractured clevis were sent to the NTSB Materials Laboratory in Washington, D.C., for further examination. A Senior NTSB Metallurgist reported that magnified optical examinations of the fracture region revealed large shear lips completely through the wall thickness, which was indicative of an overstress separation. No indications of pre-existing cracks or corrosion were noted on the fracture surface. Deformation accompanying the fracture indicated bending stresses were present at the time of the fracture. The deformation indicated that the bending direction was parallel to the plane of rotation of the clevis bolt. The reason for the overstress separation was undetermined. (Refer to the attached Materials Laboratory Factual Report.)

On April 25, 2000, 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.

A review of maintenance records revealed that the airplane's most recent annual inspection was completed on March 28, 2008, at an aircraft total time of 5,191.2 hours. The inspection included a 5 year/3,000 hour main and nose landing gear and actuator inspection, which included the replacement of the following parts; left main landing gear inner body with overhauled P/N ES12394, and left and right main landing gear glands with new P/N ED10311. During the inspection Twin Commander Service Letter No. 376 was complied with. The airplane had accrued 13 hours since the inspection at the time of the accident.

On May 20, 2008, at the request of the IIC, and under the supervision of the FAA airworthiness inspector, a new clevis was installed on the left main landing gear assembly where the clevis failure had occurred. Subsequently, four normal landing gear retractions and extensions, one emergency landing gear extension, and then four additional normal retractions and extensions were performed. The inspector reported that the landing gear system worked satisfactorily, including the sequencing valves, with no discrepancies or malfunctions noted.

According to the Gulfstream Commander Model 690C/690D Maintenance Manual, General - Landing Gear Description and Operation, hydraulically actuated clam shell doors enclose the main landing gear wheel wells and cycle to the open and closed position each time the main landing gear is extended or retracted. Photographs taken about 25 minutes after the accident indicate that the clam shell doors were in the closed position. The manual further states that when the landing gear control lever is placed in the DOWN position, hydraulic pressure on the retract side of the landing gear actuating cylinder is released. Hydraulic pressure is simultaneously applied to the inboard main landing gear actuating cylinder, and nitrogen pressure reacting on the downside of the outboard actuating cylinder piston helps the inboard hydraulic actuating cylinder to extend the main gear. Positive locking in the down position is assured by bungee cords attached to drag braces of each main landing gear. Additionally, unintentional retraction of the landing gear is prevented by a mechanical safety latch on the landing gear control.

NTSB Probable Cause

The collapse of both main landing gear for undetermined reasons.