WPR09LA100 · Cessna 210A

During a post-maintenance test flight following the replacement of both the engine-driven hydraulic pump and the hydraulic power pack, both main landing gear failed to fully extend. The overhauled pump and the serviceable power pack were replaced as corrective action to address a repetitive problem with the airplane's hydraulically driven landing gear and flap actuating systems. Although the landing gear had been extended with the emergency extension hand pump during the preceding post-maintenance test flight (when the gear failed to fully extend using the normal system), it failed to do so during the accident flight. The pilot therefore landed with the main gear partially extended, resulting in the airplane departing the runway and impacting rough terrain. A series of post-accident inspections and tests did not reveal any evidence of anomalies, malfunctions, or leaks associated with the airplane's hydraulic plumbing, hydraulic fittings, or engine-driven hydraulic pump. The hydraulic power pack, which controls the function of the flap system, as well as both the normal and emergency gear extension and retraction systems, was also tested and disassembled for internal inspection. Although the power pack , which was described by the testing technician as an older unit needing overhaul, displayed a number of minor discrepancies during the testing and inspection process, it successfully passed a functional test sequence. The technician also stated that the power pack may have failed to operate the flaps, the normal landing gear extension, and emergency landing gear extension modes because of particulate contamination of the fluid restricting the movement or alignment of a poppet valve. He stated that any such contamination might be dislodged by the impact forces created during the accident sequence, and then flushed from the system by the flow of fluid during the functional bench test sequence.

Verbatim NTSB analysis and probable cause from the NTSB dataset