On March 12, 2006, a Kaman K-1200 (registration N133KA) was involved in an aviation accident near Dexter, OR. Investigators recorded the probable cause as: Failure of the splines on the shaft providing turbine speed input to the turbine governor, resulting in overspeed. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
A helicopter experienced a power turbine and rotor system overspeed due to spline failure on the turbine governor input shaft. The splines failed from reduced lubrication caused by a partially blocked oil filter outflow orifice, which resulted from misalignment of the oil filter screen that allowed contaminants to bypass.
Event Description
While approaching a collection point with a load of logs, the helicopter's power turbine and rotor system entered a steady-state overspeed. This overspeed occurred because the splines on the shaft that provides turbine speed input to the turbine governor failed. Without this input, the governor erroneously detected a turbine underspeed and consequently supplied fuel in excess of what was needed to maintain the correct speed.
Findings
The splines failed due to a significant reduction in the flow of lubricating oil. This reduction resulted from the partial blockage of an oil filter outflow orifice. The outflow orifice became partially blocked because the oil filter screen was aligned in its housing in a manner that allowed some contaminants to bypass the filter and reach the orifice.
Emergency Procedure
The Kaman K-1200 Flight Manual contains an emergency procedure that allows the pilot to take manual (non-governed) control of the fuel input to the engine in the event of a governor failure. However, the pilot did not perform this procedure because he was not able to correctly identify the cause of the sequence of events.
Probable cause
Failure of the splines on the shaft providing turbine speed input to the turbine governor, resulting in overspeed. The splines failed due to reduced lubrication caused by partial blockage of an oil filter outflow orifice, which occurred because the oil filter screen was misaligned, allowing contaminants to bypass.
