NTSB Factual Report

On September 28, 2010, about 1320 mountain daylight time, a Cessna 310R, 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, but the airplane, which was 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, which departed Lethbridge County Airport, Lethbridge, Alberta, Canada, about one hour before the accident, was being operated in visual meteorological conditions. The pilot had been on a visual flight rules (VFR) flight plan between Canada and the United States.

According to the pilot, when he lowered the landing gear during his approach to the destination airport, although everything felt and sounded normal, he did not get a green Gear-Down light for the right main landing gear. He therefore checked the light bulb to make sure it was not burned out, which it was not. Because he had experienced a problem with a malfunctioning gear indicator electrical switch in the past, he assumed that the green light was not illuminated because of a switch malfunction. The pilot considered cycling the landing gear, but decided against it as he was concerned there might be a broken part that could possibly result in the right main gear staying fully retracted once it had been brought back up. The pilot therefore continued his approach, made an uneventful landing and rollout, and slowed the airplane to normal taxi speed. Once at normal taxi speed, he turned off the runway onto the taxiway. When the airplane was almost completely clear of the runway and straightening out on the taxiway centerline, the right main gear began to fold back up into the wheel well. As the landing gear folded, the wing fell toward the terrain, ultimately contacting the surface and suffering substantial damage.

A post-accident 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 able to function as designed once the activation system had moved it into the over-center/locked position. That same inspection determined that the mid section of the Gear Strut Bellcrank arm had been bent about 15 degrees from its normal alignment, and that the upper part of the arm had fractured about one inch below the point where its head was bolted to the gear strut itself. It was further noted that the fracture surfaces of the arm were all at about 45 degrees to the axis of the fracture, were a consistently smooth dull granular gray, and showed no evidence of previous oxidation of discoloration.

After reviewing the main landing gear activation and locking system portion of the Cessna 310 Service Manual, the Investigator-In-charge (IIC) contacted Cessna Aircraft Company to discuss the circumstances and findings of this accident sequence. According to Cessna, the most likely reason for the green light not coming on during this accident sequence was that the gear extension mechanism had not moved the Upper Side Link (over-center arm) to its over-center (locked) position. When this occurred the Down Indicator Switch was not activated, and therefore the green "Down" light in the cockpit did not come on, and the "In Transition" light in the cockpit stayed on as reported by the pilot. At that point, both main gear were most likely fully down, but the right main gear Upper Side Link was not in the locked position. Therefore, any significant side-loads were transferred to the tubular steel Outboard Push-Pull Tube, which is part of the gear activation system. With the right main landing gear in this alignment/condition, it remained in the down position during the straight ahead landing roll with its minimal side-loads, but when the pilot began turning the airplane off of the runway to enter the taxiway, the side-loads generated by the turn were transferred to the Outboard Push-Pull Tube. The Outboard Push-Pull tube is connected 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 gear strut-mounted bellcrank was not designed to withstand the side-loads generated by the turn, it then failed in overload.

The Cessna representative further stated that the main landing gear Upper Side Link would normally only fail to go to the over-center/locked position if the main gear activation system linkage was out of adjustment, and that there were only two situations that would lead to the linkage being out of adjustment. The first would be if maintenance personnel incorrectly adjusted the linkage, and the second would be from usage-induced deterioration of the many linkage subcomponents. According to Cessna, once the bellcrank failed, it was no longer possible to determine which of the two aforementioned situations led to the linkage being out of adjustment.

A review of the airplane's log books by the IIC revealed that during an Annual Inspection that took place at Salem Air Center approximately three months prior to the accident, an entry was made in the log book that stated, "Checked landing gear rigging & down lock tensions." As a follow-up to this information, a Federal Aviation Administration (FAA) Airworthiness Inspector from the Portland Flight Standards District Office, talked with the Director of Maintenance (DOM) at Salem Air Center. That DOM said that the gear inspection was limited only to a "gear swing," during which it was determined that all of the gear activation and locking components functioned properly and moved to their correct positions.

NTSB Probable Cause

The uncommanded retraction of the right main landing gear during taxi due to the failure of the landing gear extension system to move the right main gear upper side link into the locked position for undetermined reasons.