SkyWest Airlines Bombardier Inc CL-600-2B19 incident

During final approach, the transport-category airplane's right main landing gear (MLG) did not extend when the landing gear selector was placed in the "down" position. Shortly after, the flight crew noted a "gear disagree" warning message displayed on the engine indication and crew alerting system showing that the nose gear and the left MLG were down and locked but that the right MLG was in transit. The flight crew followed the quick reference handbook directions to troubleshoot the landing gear issue without success. The flight crewmembers then tried to manually extend the landing gear by pulling the landing gear alternate release handle multiple times; however, the right MLG failed to extend, and they subsequently landed the airplane with the right MLG retracted. During postincident activities, the airplane was lifted and an examination revealed that the right MLG remained in its full-up position within the wheel well.

The airplane's right MLG uplock pin exhibited signs of slight wear, consistent with in-service usage, and no abnormalities were found with the right MLG uplock assembly. Normal wear patterns were observed on the uplock latch with no discernable depth at the pin contact locations. The uplock pin's slight wear indicates that it might not have always rotated freely when it was in contact with the upper and lower surfaces of the uplock assembly latch as it should have per the system's design. Further, the worn pin could have increased the friction forces between the pin and the latch.

Functional testing of the incident airplane's 3A and 3B hydraulic system pumps showed that their output pressures were lower than specified. Analysis of hydraulic fluid samples taken from the No. 3 hydraulic system, including the right sidestay actuator and selected landing gear components, revealed that some of the fluid samples contained particles and fibers that exceeded the in-service size limits specified in the airplane's maintenance manual, and these contaminants likely restricted the hydraulic flow within the hydraulic assembly.

It is unlikely that any of these factors (the slightly worn uplock pin, the operation of the hydraulic pumps below their specified operating pressures, or the contamination within the landing gear hydraulic system) would have individually prevented the right MLG from extending; however, it is likely that the combination of these factors prevented the right MLG from extending. In addition, the overall landing gear system design, including modifications made in accordance with an airworthiness directive, did not preclude the consequence that the intermittent combination of these factors prevented the right MLG from extending normally.

Postincident examination of the cockpit revealed that the landing gear alternate release handle remained extended about 7 inches. Pulling up on the handle resulted in it moving about 3 additional inches to its fully extended position. The additional handle displacement resulted in the right MLG extending out of its wheel well. When the handle was released from its fully extended position, the handle automatically began to slowly retract, which caused the right MLG to stop extending. The handle had to be pulled and manually held in its fully extended position for the right MLG to extend to its down-and-locked position.

During postincident functional testing of the landing gear alternate release handle assembly, the handle's locking mechanism failed to maintain the handle in its fully extended position when a specified retract load was applied to the assembly. Disassembly of the handle assembly did not reveal any mechanical discrepancies. However, the assembly's inner housing and outer slider were found coated with an oily material consistent with lubricant; the origin of the oily material could not be determined. The alternate release handle assembly design specifications do not call for the application of lubricant on the inner housing or the outer slider; therefore, it is likely that the lubricant prevented the handle from remaining in its fully extended position.

Verbatim NTSB analysis and probable cause from the NTSB dataset