NTSB Factual Report

On August 8, 2018, about 1828 Pacific daylight time, a Cessna T337G airplane, N255, sustained substantial damage when it landed with its landing gear retracted on runway 30 at Lewiston Perce Nez Airport (LWS) Lewiston, Idaho. The commercial pilot and his passenger were not injured. The airplane was registered to Northern Air Inc, and was operating under contract for the United States Forest Service as an "air attack" firefighting support mission. Visual meteorological conditions existed at LWS for the landing. The flight originated from LWS about 5 hours before the accident.

According to the pilot, the airplane operated normally for the mission, and for most of the landing approach. However, when the pilot selected the landing gear to the extended position, the gear doors opened, the electric motor that pressurized the hydraulic extension system continued to run, but the landing gear failed to extend. The pilot pulled the circuit breaker for the pump motor and abandoned the approach. He then flew a short distance away from LWS so that he could attempt to troubleshoot and rectify the problem. The pilot cycled the landing gear control five or six times, but all efforts to operate the landing gear were unsuccessful. He then employed his passenger to assist him with running the checklists and other troubleshooting activities. The pilot and passenger determined that the landing gear system hydraulic fluid reservoir that was accessible to them in the cabin was empty. The pilot contacted his maintenance personnel by radio, and they assisted in additional attempts to correct the situation; these attempts included replenishing the reservoir with oil and water. Despite those efforts, the landing gear could not be successfully extended. The pilot decided to burn off extra fuel before returning to conduct a gear-up landing.

During the return approach, the pilot secured the front engine, and "bumped" the propeller with the starter to position the blades horizontally, in order to prevent damage from runway contact. The occupants unlatched a cabin door in order to enable assured opening after landing. The pilot landed the airplane gear up on runway 30, and the airplane slid to a stop within a few feet of the runway centerline. The pilot shut down the aft engine, and secured the airplane. Both occupants exited, and no fire or other problems occurred.

Post accident examination of the airplane revealed the runway slide had ground through several of the lower skin panels and structural members, substantially damaging the airplane. Examination of the landing gear system revealed that the circlips that retained the actuator rods in both the left and right actuators for the main landing gear "parachute" doors had been liberated from their retention grooves. Loss of this circlip enables hyper-extension of the actuator rod, and permits hydraulic fluid to exit the actuator.