SkyWest Airlines Bombardier Aerospace, Inc. CL-600-2B19 accident

The airplane was damaged during a landing with the nose landing gear partially extended. As the airplane descended toward the destination airport, the crew moved the landing gear handle to the extend position. The pilots noted a "gear disagree" warning message displayed on the Engine Indicating and Crew Alerting System (EICAS) that showed the main landing gear were down and locked, and the nose gear was not fully extended. The pilots opted to land at an alternate airport and began to troubleshoot the gear indication with use of the airplane's Quick Reference Handbook (QRH). Despite their efforts, which included using the manual release handle, the nose gear failed to extend. The airplane touched down on the main landing gear and the nose lowered to the ground as the speed dissipated. Review of the CVR found that the flight crew followed each step of the Quick Reference Handbook checklist. The flight crew did not cycle the landing gear lever, nor was that action part of the QRH procedure at the time. The digital flight data recorder (DFDR) revealed that the main landing gear transitioned into the down and locked position but the nose gear remained partially extended between the uplocked and downlocked positions. A further review of the data disclosed the hydraulic pressure remained constant throughout the extension and did not show the pressure changes that should have occurred with normal system operation. The only pressure changes noted in the duration of the flight were consistent with the manual release system being activated. No increase of brake pressure was recorded during the gear extension attempt, indicating that there was no hydraulic backpressure on the retract side of the NLG actuator. The components of the nose landing gear (NLG) system were tested, revealing no anomalies or malfunctions that could have precluded normal operation. Several simulations were conducted exploring various potential problem scenarios; however, none of the simulations produced the hydraulic system and brake pressure readings on the accident flight's DFDR data. A review of several maintenance discrepancy databases for events where the NLG failed to extend into the downlock position found that the operator did not have a higher event rate of gear disagree messages than the rest of the worldwide fleet. Simulations and static analysis on the friction force of the uplock was performed, specifically detailing the interaction between the pin and the latch. As a result of the testing, areas of potential problems with the NLG extension systems (both normal and manual) under adverse conditions were found to include: friction within the system and the sequencing of the hydraulic pressure application. Bombardier subsequently modified the QRH procedure, adding another step to specifically recycle the gear selector handle.

Verbatim NTSB analysis and probable cause from the NTSB dataset