Accident Overview
On September 28, 2010, about 1320 mountain daylight time, a Cessna 310R, registration N455TB, experienced a collapse of the right main landing gear while turning off the active runway onto a taxiway at Great Falls International Airport, Great Falls, Montana. The commercial pilot and his passenger were not injured. The airplane, owned by a member of the pilot's family, sustained substantial damage to its wings and empennage. The 14 Code of Federal Regulations Part 91 personal cross-country flight departed Lethbridge County Airport, Lethbridge, Alberta, Canada, about one hour before the accident, operating in visual meteorological conditions under a visual flight rules (VFR) flight plan between Canada and the United States.
Pilot's Account
According to the pilot, when he lowered the landing gear during approach, he did not get a green gear-down light for the right main landing gear, though everything felt and sounded normal. He checked the light bulb, which was not burned out. Having experienced a malfunctioning gear indicator electrical switch in the past, he assumed the green light was off due to a switch malfunction. He considered cycling the landing gear but decided against it, concerned that a broken part could cause the right main gear to remain fully retracted if brought back up. He continued the approach, made an uneventful landing and rollout, and slowed to normal taxi speed. When he turned off the runway onto the taxiway, and the airplane was nearly clear and straightening on the taxiway centerline, the right main gear began to fold back into the wheel well. The wing fell toward the terrain, contacting the surface and suffering substantial damage.
Post-Accident Inspection
Inspection of the right main landing gear activation and down-lock system revealed that the Upper Side Link (over-center down-lock arm) was undamaged and fully functional. The mid section of the Gear Strut Bellcrank arm was bent about 15 degrees from normal alignment, and the upper part of the arm had fractured about one inch below the point where its head was bolted to the gear strut. Fracture surfaces were at about 45 degrees to the axis, consistently smooth dull granular gray, with no evidence of previous oxidation or discoloration.
Cessna's Analysis
After reviewing the Cessna 310 Service Manual, the Investigator-In-Charge (IIC) contacted Cessna Aircraft Company. According to Cessna, the most likely reason the green light did not come on was that the gear extension mechanism had not moved the Upper Side Link to its over-center (locked) position. Thus, the Down Indicator Switch was not activated, the green light did not illuminate, and the "In Transition" light stayed on as reported. Both main gear were likely fully down, but the right main gear Upper Side Link was not locked. Consequently, any significant side-loads transferred to the tubular steel Outboard Push-Pull Tube. During the straight-ahead landing roll, side-loads were minimal, but when the pilot turned onto the taxiway, side-loads generated by the turn transferred to the Outboard Push-Pull Tube, which connects to a larger tubular steel landing gear activation Torque Tube on one end and to a cast aluminum alloy landing gear strut-mounted bellcrank on the other. Since the aluminum alloy bellcrank was not designed to withstand these side-loads, it failed in overload. Cessna further stated that the Upper Side Link would normally fail to reach the over-center/locked position only if the main gear activation system linkage was out of adjustment, due either to incorrect maintenance adjustment or usage-induced deterioration of linkage subcomponents. Once the bellcrank failed, it was not possible to determine which situation led to the misadjustment.
Maintenance Records
A review of the airplane's log books revealed that during an Annual Inspection at Salem Air Center approximately three months prior, an entry stated: "Checked landing gear rigging & down lock tensions." A Federal Aviation Administration (FAA) Airworthiness Inspector from the Portland Flight Standards District Office spoke with the Director of Maintenance (DOM) at Salem Air Center, who said the gear inspection was limited to a "gear swing," during which all gear activation and locking components functioned properly and moved to correct positions.