Incident
An uneventful flight from Salzburg concluded with a landing incident at Vienna-Schwechat Airport. After being cleared to descend, the crew prepared for an approach to runway 34. During the landing gear extension, they observed that the left main landing gear remained retracted in its wheel well and that the corresponding green indicator light on the cockpit panel did not illuminate. The crew elected to continue the approach.
Upon touchdown on runway 34, the aircraft began to deviate to the left, subsequently veering off the runway. It came to rest on a grassy area near taxiway D and taxiways B5 and B6. All five occupants evacuated the aircraft safely. The aircraft sustained damage beyond repair.
Investigation
Examination of the aircraft's landing gear system revealed the cause of the malfunction. A metallic foreign body was found lodged between the valve seat and the ball of a spring-loaded ball check valve within the undercarriage servo valve of the left main landing gear. This obstruction prevented the check valve from closing as designed, allowing hydraulic fluid to flow directly to the landing gear cylinder without moving the piston rod.
Because the piston rod did not move as intended, the mechanical unlocking hook for the left undercarriage remained inactive. Consequently, the landing gear was not deployed.
Probable Cause
The accident was caused by a metallic foreign body between the valve seat and ball of the spring-loaded ball check valve in the undercarriage servo valve of the left main landing gear, which prevented the check valve from closing as intended and directed hydraulic fluid to the landing gear cylinder without moving the piston rod. Consequently, the mechanical unlocking hook of the left undercarriage was not controlled, and the landing gear did not deploy.
Contributing Factors
Two contributing factors were identified. First, the crew did not utilize the option to abort the approach, execute a go-around, and enter a holding pattern to perform troubleshooting, as outlined in the operator's operations manual and the aircraft manufacturer's emergency and abnormal procedures manual. Second, the emergency landing gear extension system was not used.