What happened
A hydraulic fluid leak, caused by a deteriorated O-ring in the landing and gear selector solenoid valve, rendered the normal landing gear extension system inoperative. Upon realizing the issue, the crew attempted to use the alternate gear extension system, but they were unable to extend the right main landing gear. The first officer pulled the main landing gear alternate extension cable until it reached its limit, yet the right main landing gear uplock actuator failed to release.
Following advice from company maintenance, the crew performed a procedure to retract the nose gear and the left main gear using the normal system before attempting another normal extension. During this second attempt, only the left main landing gear extended. The aircraft's number 2 hydraulic system was found to be depleted, with only 1.5 quarts of fluid remaining. After an unsuccessful attempt to retract the left main landing gear, the captain performed a landing with only the left main landing gear extended.
The investigation
An examination of the alternate gear system revealed that the main gear extension cable would bind once the left main landing gear uplock actuator released. While subsequent testing on jack stands reduced the amount of binding, all tests showed that the left main landing gear uplock actuator would release before the right uplock actuator.
Further inspection of the main landing gear alternate extension cable showed that the plastic coating was missing in two locations near a pulley. Additionally, investigators found the right main landing gear wheel well to be excessively dirty, with areas of exfoliated white paint that exposed the underlying primer.