What happened
On September 27, 2013, a Bombardier DHC-8-402, registration 9A-CQC, operated by Croatia Airlines, was performing a scheduled flight (CTN 464) when the crew encountered a landing gear malfunction. During the approach to Zurich Airport, the crew discovered that the nose landing gear failed to extend. Despite attempts to resolve the issue, the aircraft was forced to perform a landing on runway 14 with the main landing gear down but the nose gear in the retracted position.
The investigation
Investigators examined the nose landing gear mechanism to determine why the extension failed. The inquiry focused on the WOW (Weight on Wheels) cover plate and its associated retaining devices. Technical analysis included testing the extension mechanism and inspecting the physical condition of the components involved in the gear's movement.
Findings
The investigation established that the crew was unable to extend the nose gear because the mechanism was immobilized. The primary cause was the failure of the two lower retaining devices of the WOW cover plate, which had broken and become wrapped around the two upper anchoring devices. This debris physically blocked the nose gear from extending.
Several contributing factors led to this breakage:
- The specific installation method of the WOW cover plate assembly generated lateral forces during the nose gear's movement, creating mechanical tension on the retaining devices.
- Imperfections in the weld beads were present in the area of the lower retaining device, which weakened the component.
Safety action
Following the accident, the aircraft manufacturer, Bombardier, issued an All Operator Message (AOM No. 581) on October 2, 2013. This message reminded operators to ensure the proximity cover was secure before flight and suggested that ground crews exercise diligence when attaching or removing tow bars. It also recommended that a tactile inspection of the nose gear WOW cover be considered during pre-flight checks. Additionally, the manufacturer initiated a design study to implement a new mounting bracket that attaches the cover plate to the inner cylinder rather than the thrust collar, specifically to eliminate the lateral forces that caused the initial failure.