What happened
On September 3, 2015, at approximately 10:27 UTC, a Cessna 550 Bravo was on final approach to Runway 34 at Vienna International Airport (LOWW) following a flight from Salzburg. During the landing sequence, the crew noted that while the nose gear and right main gear indicators showed green, the left main gear indicator remained unlit.
After performing a landing gear test using the rotary switch, the pilots observed the green lights illuminate and a cockpit warning tone sound. Believing the initial lack of indication was a display error, the crew proceeded with the landing. Upon touchdown, the pilot flying noticed the left wing dipping toward the runway. The left main gear had failed to extend, leaving the aircraft in a retracted state. Despite attempts to use differential thrust and aileron control to maintain the aircraft on the runway, the left wingtip struck the pavement. The aircraft slid off the runway into the grass between taxiways, eventually rotating 180 degrees before coming to a halt. All five occupants—two crew members and three passengers—evacuated the aircraft without injury.
The investigation
The Austrian SUB examined the aircraft's landing gear control valve, the wiring, and the sensors. The investigation focused on why the gear failed to deploy despite the successful completion of the landing gear test. Technical analysis of the left main gear control valve revealed that a metallic foreign object was lodged between the valve seat and the ball of the spring-loaded check valve. Additionally, investigators found significant corrosion within the downlock sensor, though this was determined not to be the primary cause of the accident.
Findings
- A metallic foreign object in the left main gear control valve prevented the check valve from closing properly, allowing hydraulic fluid to bypass the piston rod without moving it. This failure prevented the mechanical unlocking hook from being actuated, leaving the gear retracted.
- The crew elected to continue the landing despite the unlit left gear indication, assuming a cockpit display error.
- The crew did not utilize the functional emergency gear extension system.
- The crew did not declare an emergency or initiate a go-around to troubleshoot the gear indication discrepancy.
Safety action
- The SUB issued a recommendation to the aircraft operator to ensure that all approved manuals and procedures are strictly followed to ensure safe flight operations.
- A recommendation was issued to the manufacturer regarding the potential need for defined service lives for landing gear wiring and sensors, as these components are subject to extreme weather conditions and were found to have significant internal corrosion.