History of Flight
On August 8, 2018, about 1828 Pacific daylight time, a Cessna T337G airplane, registration N255, was substantially damaged to the lower fuselage during a gear-up landing at Lewiston–Nez Perce County Airport (LWS), Lewiston, Idaho. The commercial pilot and passenger were not injured. The airplane was operated by Northern Air Inc under contract to the United States Forest Service as a Title 14 Code of Federal Regulations Part 91 aerial survey flight. Visual meteorological conditions prevailed, and no flight plan was filed. The flight originated from LWS about 5 hours before the accident.
According to the pilot, the airplane performed normally until the landing gear was selected down. The gear doors opened, the electric motor pressurizing the hydraulic extension system continued running, but the landing gear did not extend. The pilot reported the airplane's speed was about 20 mph below the maximum landing gear operating speed when he selected gear down. He pulled the circuit breaker for the pump motor and abandoned the approach. The pilot flew a short distance away to troubleshoot.
Cycling the landing gear control five or six times did not resolve the issue. The pilot and passenger discovered the accessible hydraulic fluid reservoir in the cabin was empty. They contacted maintenance personnel by radio, who assisted in replenishing the reservoir with oil and water, but the gear still could not be extended. After burning off extra fuel, the pilot prepared for 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. The occupants unlatched a cabin door. The pilot landed gear up on runway 30, and the airplane slid to a stop near the centerline.
Personnel Information
Federal Aviation Administration records showed the pilot held a commercial pilot certificate with single-engine, multiengine, and instrument ratings. He had about 4,460 total flight hours, with 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 Information
The airplane was manufactured in 1975, serial number P3370213. It was equipped with two Continental Motors TSIO-360 series engines in a centerline thrust configuration and had retractable, tricycle-configuration landing gear. The airframe and both engines had accumulated about 2,848 hours total time (TT) since new. The engines had about 880 hours TT since most recent overhaul. The most recent annual inspection was completed on July 9, 2018.
The normal operating speed range was 80 to 190 mph, caution range from 190 to 230 mph. Maximum landing gear extension speed was 160 mph, maximum extended operating speed was 230 mph.
Landing Gear System and Maintenance
The landing gear retracted into the fuselage, covered by multiple doors. The aftmost doors for the main gear, called "parachute" or "clamshell" doors, extended from the fuselage bottom and sides. Each main door was opened and closed by a hydraulic actuator with an internal snap ring (circlip) that functioned as a travel-limiting and retaining stop. During extension, doors swung down and outboard up to about 90°, creating significant drag.
The maintenance manual specified that after disassembly, actuators should be inspected for cracks, chips, scoring, or wear, and that all new packings and retainers should be installed during re-assembly. The internal snap rings qualified as retainers.
According to maintenance records, all three main landing gear door actuators were rebuilt with new O-rings in July 2014, when the airplane had about 1,968 hours TT. The entry did not specify whether new snap rings were installed. Discussions with the operator's Director of Maintenance indicated the snap rings likely were not replaced. The airplane operated over 750 hours between the rebuild and the failure.
Wreckage and Examination
The airplane came to rest near the runway 30 centerline, about 100 ft before taxiway F. All landing gear were retracted, with doors in various transit positions. Damage was confined to belly skins, structure, and landing gear doors. Rear propeller tips had minor grinding damage consistent with runway contact while the engine operated.
Examination found the same failure in both left and right main door actuators: the internal snap ring had liberated from its groove in the cylinder but remained captive on the actuator rod. This allowed hyper-extension of the rod, permitting hydraulic fluid to exit the system. The landing gear system had no hydraulic fuses, and the reservoir was empty. Loss of fluid rendered the system inoperative in both normal and emergency modes. No damage to the snap rings or cylinders was observed.
Both main doors were open about 90°, with inboard edges contacting the runway. The right door edge was slightly damaged; the left door had about 3–4 inches of material missing along its inboard edge. Evidence suggested the right actuator failed first, causing fluid loss and disabling the system, while the left actuator's snap ring liberated during runway contact.
After the snap rings and rods were reinserted and the hydraulic system replenished, the landing gear system operated normally during jack testing, with no anomalies.
Additional Information
The operator reported a Canadian operator with 18 Cessna 337 airplanes had 3 occurrences of snap ring liberation since 1980. FAA Service Difficulty Reports also documented similar events. The airplane manufacturer offered an alternate actuator design using a threaded cap instead of a snap ring. An aftermarket company, RT Aerospace, developed an FAA supplemental type certificate replacing the moving doors with fixed panels having gear cutouts, reducing drag and eliminating two actuators and four hydraulic lines. This modification was not installed on the accident airplane but was added during repair.