What happened
As the aircraft descended toward its destination airport, the crew moved the landing gear handle to the extend position. The pilots observed a "gear disagree" warning on the Engine Indicating and Crew Alerting System (EICAS), indicating that while the main landing gear were down and locked, the nose gear was not fully extended.
The crew decided to divert to an alternate airport to troubleshoot the indication using the Quick Reference Handbook (QRH). Despite performing procedures including the use of the manual release handle, the nose gear failed to extend. The aircraft touched down on its main landing gear, and the nose subsequently lowered to the ground as the aircraft slowed. The incident resulted in aircraft damage.
The investigation
A review of the Cockpit Voice Recorder (CVR) confirmed that the flight crew followed every step of the QRH checklist. The investigation found the crew did not cycle the landing gear lever, though such an action was not required by the QRH at that time.
Data from the digital flight data recorder (DFDR) showed that while the main landing gear transitioned to the down and locked position, the nose gear remained stuck between the uplocked and downlocked positions. Analysis of hydraulic pressure revealed it remained constant during extension and lacked the pressure changes expected during normal operation; the only recorded changes were consistent with the activation of the manual release system. Additionally, no increase in brake pressure was recorded during the extension attempt, suggesting there was no hydraulic backpressure on the retract side of the nose landing gear actuator.
Testing of the nose landing gear components revealed no mechanical anomalies or malfunctions. Simulations of various failure scenarios failed to replicate the specific hydraulic and brake pressure readings recorded during the flight. Furthermore, a review of maintenance databases showed the operator did not have a higher rate of gear disagree messages compared to the global fleet.
Static analysis and simulations regarding the friction force of the uplock, specifically the interaction between the pin and the latch, identified that potential issues in the extension system—both normal and manual—could be caused by system friction and the sequencing of hydraulic pressure application.
Findings
- The nose gear remained partially extended between the uplocked and downlocked positions.
- Hydraulic pressure did not exhibit the fluctuations characteristic of normal system operation during the extension attempt.
- Testing identified that friction within the system and hydraulic pressure sequencing are potential factors in gear extension issues under adverse conditions.
- Bombardier subsequently updated the QRH to include a step for recycling the gear selector handle.