What happened
A pilot in command was operating an aircraft with a safety pilot onboard. Following several approach attempts, the crew attempted to extend the landing gear, but only the nose gear successfully deployed. Because the main landing gear remained retracted, the pilot in command requested that the second pilot perform the landing due to the latter's greater level of experience.
Prior to touchdown, the propellers were feathered. The aircraft subsequently made a hard landing, which resulted in the crushing of the lower aft fuselage. There were no injuries reported following the incident.
The investigation
An examination of the aircraft's hydraulic system identified contamination within the system. A mechanic previously servicing the aircraft noted that this contamination prevented the main gear uplock cylinder from releasing, which in turn prevented the extension of the main gear. The aircraft was not equipped with an alternate landing gear extension system; consequently, the failure of the hydraulic uplock valve to release rendered the main gear unextendable.