Cessna T337 N255 accident near Lewiston, Idaho

Probable cause

The liberation of an internal snap ring from a landing gear door actuator, which resulted in a complete loss of hydraulic fluid and a gear-up landing. Contributing to the accident was the failure of maintenance personnel to replace the snap ring during the last rebuild of the actuator.

NTSB analysis

The pilot reported that he was returning to the airport after an uneventful aerial survey flight. When the pilot extended the landing gear, the system began operating but the gear did not extend. Multiple efforts to extend the gear, including assistance by the passenger and coordination with his company's maintenance personnel, were unsuccessful. The pilot then burned off excess fuel and conducted a gear-up landing at the airport. The airplane sustained substantial damage to the lower fuselage.

Postaccident examination of the airplane revealed that two main landing gear door actuators had hyperextended and allowed all hydraulic fluid to be lost. Detailed examination indicated that the right actuator had hyperextended in flight, while the left one hyperextended due to landing gear door runway contact during the gear-up landing. The actuator rod in each actuator was normally travel-limited and contained by a single internal snap ring, which acted as the mechanical stop and retention device for the rod inside the actuator cylinder. For undetermined reasons, the snap ring was liberated from its groove in the right actuator, which allowed the actuator rod to travel beyond its normal limit, which resulted in the loss of hydraulic fluid and operational failure of the landing gear. Postexamination reassembly and testing of the landing gear system indicated that the hyperextended actuators were the only mechanical deficiencies in the system.

The airplane manufacturer's maintenance manual specified the replacement of the snap rings (retainers) upon rebuild of the actuator. According to the airplane maintenance records, all three main landing gear door cylinders (actuators) were rebuilt with new "O-rings" about 4 years before the accident. 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.

Review of industry documents revealed other occurrences of snap ring liberation and consequent landing gear system problems in this model airplane, and at least two different hardware options to prevent recurrence were available. One incorporated a new-design actuator that eliminated the accident failure mode, and one involved elimination of the subject actuators from the airplane. The operator reported that this modification was installed on the accident airplane when it was repaired after the accident. Either option would ensure against a recurrence of this accident scenario.

Text from the NTSB. Open the NTSB docket for WPR18LA215 for the full record.