What happened
While en route to its destination airport, the helicopter's hydraulic system caution light illuminated and an associated aural warning sounded. The pilot decided to continue to the intended destination; however, as the aircraft neared the runway, it lost hydraulic boost. This resulted in the pilot performing a run-on landing on the runway.
Following this initial incident, a company mechanic removed the hydraulic pump and pulley, discovering that the hydraulic pump drive shaft splines and the coupling sleeve exhibited excessive wear. A new hydraulic pump was installed, and after a test flight, the aircraft was returned to service. Approximately three weeks later, while flying the same helicopter, the pilot experienced a similar series of events, which again resulted in a run-on landing. There was no damage to the aircraft during this second event.
The investigation
Following the second incident, the hydraulic system test switch was identified as faulty and was replaced. Subsequent inspections and functional testing of the removed hydraulic switch revealed no anomalies. Further examination of the hydraulic pump drive shaft and coupling sleeve showed that while the mating splines of both components were severely worn, enough material remained to allow the coupling sleeve to drive the hydraulic pump.