N255

HISTORY OF FLIGHTOn August 8, 2018, about 1828 Pacific daylight time, a Cessna T337G airplane, N255, was substantially damaged to the lower fuselage during landing 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 under contract to and being operated by the United States Forest Service as a Title 14 Code of Federal Regulations Part 91 aerial survey flight. Visual meteorological conditions existed at the time of the accident and no flight plan was filed. The flight had 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 reported that the airplane was at a speed about 20 mph below the maximum landing gear operating speed when he selected the gear down. 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 damaging them by 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. PERSONNEL INFORMATIONFederal Aviation Administration (FAA) records indicated that the pilot held a commercial pilot certificate with a airplane single engine, multiengine, and instrument ratings. According to information provided by the pilot, he had approximately 4,460 total hours of flight experience, including approximately 632 hours in the accident airplane make and model. His most recent flight review was completed in May 2018, and his most recent FAA second-class medical certificate was issued in May 2018. AIRCRAFT INFORMATIONThe airplane was manufactured in 1975, as serial number P3370213. Operator information indicated that the airplane was equipped with two Continental Motors TSIO-360 series engines. The airplane was a centerline thrust configuration, and was equipped with retractable, tricycle-configuration landing gear. The airframe and both engines had each accumulated a total time (TT) in service of about 2,848 hrs. The engines had each accumulated about 880 hours TT since the most recent overhaul. The most recent annual inspection was completed on July 9, 2018.

The airplane manufacturer's published normal operating speed range for the airplane was 80 to 190 mph, and the caution speed range was from 190 to 230 mph. The maximum landing gear extension speed was 160 mph, and the maximum landing gear extended operating speed was 230 mph.

The landing gear was arranged in a tricycle configuration. All 3 landing gear retracted into the fuselage, and were aerodynamically faired by multiple doors for each wheel well. The aftmost and largest door for each of the two main landing gear wells extended from the fuselage bottom and partially up its respective fuselage side, resulting in an "L"- or bowl-like shape for the door. Due to their shape, these main landing gear doors are sometimes referred to as the "parachute" or "clamshell" doors.

Each main door was opened and closed by a dedicated hydraulic actuator attached to the door and the fuselage. Each actuator was equipped with an internal snap ring (sometimes referred to as a "circlip") that nested in a groove inside the actuator cylinder, functioned as a travel-limiting and retaining stop for the actuator rod in the actuator cylinder.

Each main door was hinged longitudinally along its upper (outboard) edge. The forward and aft door edges were oriented parallel to the airplane lateral axis. The inboard door edges were closest to the airplane centerline. During the landing gear extension cycle, each main door would swing down and then outboard, with a maximum travel of about 90°. When the doors were in transit or fully open, their shape and operating configuration resulted in significant airloads on the doors, and resultant aerodynamic drag on the airplane. These doors return to the closed position when the landing gear extension cycle is complete.

The airplane manufacturer's maintenance manual (MM) specified that after disassembly, the landing gear door actuator should be inspected for "cracks, chips, scoring, wear or surface irregularities which might affect…the overall function of the actuator." In the MM section for actuator re-assembly, the guidance specified to "Install all new packings and retainers." The internal snap rings qualified as retainers, and therefore were to be replaced during re-assembly of the actuators.

According to the airplane maintenance records "all three MG [main landing gear] door cylinders [actuators]" were "rebuilt" with "new O-rings" in July 2014, when the airplane had a TT of about 1,968 hrs. That entry was the most recent entry concerning the landing gear. The entry did not cite any additional details about the rebuilds, including whether new internal snap rings were installed. Discussions with the operator's Director of Maintenance indicated that the snap rings likely were not replaced during the actuator rebuild. The airplane accumulated more than 750 hours of operation between the actuator rebuild and the failure. METEOROLOGICAL INFORMATIONThe 1842 automated weather observation at LWS included winds from 090° at 5 knots, visibility 10 miles, clear skies, temperature 39°C, dew point 5°C, and an altimeter setting of 29.85 inches of mercury. AIRPORT INFORMATIONThe airplane was manufactured in 1975, as serial number P3370213. Operator information indicated that the airplane was equipped with two Continental Motors TSIO-360 series engines. The airplane was a centerline thrust configuration, and was equipped with retractable, tricycle-configuration landing gear. The airframe and both engines had each accumulated a total time (TT) in service of about 2,848 hrs. The engines had each accumulated about 880 hours TT since the most recent overhaul. The most recent annual inspection was completed on July 9, 2018.

The airplane manufacturer's published normal operating speed range for the airplane was 80 to 190 mph, and the caution speed range was from 190 to 230 mph. The maximum landing gear extension speed was 160 mph, and the maximum landing gear extended operating speed was 230 mph.

The landing gear was arranged in a tricycle configuration. All 3 landing gear retracted into the fuselage, and were aerodynamically faired by multiple doors for each wheel well. The aftmost and largest door for each of the two main landing gear wells extended from the fuselage bottom and partially up its respective fuselage side, resulting in an "L"- or bowl-like shape for the door. Due to their shape, these main landing gear doors are sometimes referred to as the "parachute" or "clamshell" doors.

Each main door was opened and closed by a dedicated hydraulic actuator attached to the door and the fuselage. Each actuator was equipped with an internal snap ring (sometimes referred to as a "circlip") that nested in a groove inside the actuator cylinder, functioned as a travel-limiting and retaining stop for the actuator rod in the actuator cylinder.

Each main door was hinged longitudinally along its upper (outboard) edge. The forward and aft door edges were oriented parallel to the airplane lateral axis. The inboard door edges were closest to the airplane centerline. During the landing gear extension cycle, each main door would swing down and then outboard, with a maximum travel of about 90°. When the doors were in transit or fully open, their shape and operating configuration resulted in significant airloads on the doors, and resultant aerodynamic drag on the airplane. These doors return to the closed position when the landing gear extension cycle is complete.

The airplane manufacturer's maintenance manual (MM) specified that after disassembly, the landing gear door actuator should be inspected for "cracks, chips, scoring, wear or surface irregularities which might affect…the overall function of the actuator." In the MM section for actuator re-assembly, the guidance specified to "Install all new packings and retainers." The internal snap rings qualified as retainers, and therefore were to be replaced during re-assembly of the actuators.

According to the airplane maintenance records "all three MG [main landing gear] door cylinders [actuators]" were "rebuilt" with "new O-rings" in July 2014, when the airplane had a TT of about 1,968 hrs. That entry was the most recent entry concerning the landing gear. The entry did not cite any additional details about the rebuilds, including whether new internal snap rings were installed. Discussions with the operator's Director of Maintenance indicated that the snap rings likely were not repla...