What happened
Upon lowering the landing gear, the pilot observed that the right main gear failed to reach the down and locked position. After notifying Air Traffic Control (ATC), the pilot was provided with vectors to the west of the airport to troubleshoot the malfunction.
The crew attempted to resolve the issue by recycling the gear handle multiple times. Following these unsuccessful attempts, the pilot initiated emergency gear extension procedures; however, the landing gear continued to indicate an intermediate position.
In a further attempt to verify the gear status, the pilot performed a low pass by the control tower. Because of the darkness at the time, tower personnel were unable to confirm if the gear was down. After several unsuccessful attempts to rectify the situation, the pilot elected to perform a gear up landing on the runway.
The investigation
Post-landing examination of the landing gear assembly revealed that the chain connecting the main landing gear torque shaft to the nose gear box had slipped off the sprocket at the nose gear box, leaving several teeth exposed.
This mechanical failure caused the landing gear sequence between the main gear and the nose gear to become desynchronized. Consequently, the nose gear retract nut bottomed out against the nose gear box, which prevented the full extension of the landing gear through both normal and emergency extension methods.