Cessna T337 N255

Summary

On 9 August 2018 at about 01:28 local time, a Cessna T337 registered N255, operated by Northern Air Inc, was involved in an accident near Lewiston, Idaho, United States. 2 people were on board and nobody was injured. The aircraft was substantially damaged. The NTSB has published a probable cause for this accident; it is quoted in full below.

Date

9 August 2018 at 01:28

Classification

Accident

Location

Lewiston, Idaho, United States

Nearest airport

Lewiston Nez Perce County (LWS)

Coordinates

46.3750, -117.0100

Aircraft

Cessna T337

Registration

N255

Category

Airplane

Year built

1975

Engines

2

Operator

Northern Air Inc

Operating rule

Part 91: General Aviation

Purpose of flight

Aerial Observation

Phase of flight

Not recorded

Route

Lewiston → Lewiston

Aircraft damage

Substantial

Weather

VMC

Light

Day

NTSB number

WPR18LA215

Nobody was injured.

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.

Quoted verbatim from the NTSB record. This site does not paraphrase or interpret it.

Read the full NTSB narrative

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.

Quoted verbatim from the NTSB record.