What happened
Following takeoff, the main landing gear of Flight 4317 failed to retract into the aircraft. The flight crew responded by performing the emergency checklist, which included extending the landing gear. The aircraft subsequently completed an uneventful landing.
The investigation
An examination of the aircraft revealed that the right landing gear retraction fitting had failed, resulting in a severed hydraulic line. Investigators also found that the quantity limiting valves were corroded, which restricted the movement of the respective pistons. As a result of these failures, both hydraulic systems were drained of fluid.
Further inspection of the fitting attachment bolts showed that the top bolt had lost torque and suffered a fatigue failure, while the bottom bolts failed due to overload. Additionally, the fitting spigot failed because of hydrogen embrittlement.
Following the incident, the operator conducted a fleet campaign and discovered 13 loose bolts in other 100 series aircraft. This led to the issuance of several airworthiness directives to address the loss of torque on attachment bolts, specifically requiring torque checks during the 300-hour landing gear inspection.