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Helicopter Power Turbine Overspeed Due to Spline Failure from Oil Filter Misalignment (N133KA)

Dexter, OR, US

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.

Sourcesthe U.S. National Transportation Safety Board (NTSB) historical archivePrimary reportUpdated 2026-07-04Data APIEditorial standards
Aircraft registered N133KA
Aircraft registered N133KA. Photo: Tequask / CC BY-SA 4.0, via Wikimedia Commons

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.

Frequently asked questions

What happened in the 12 Mar 2006 Kaman K-1200 accident near Dexter, OR?

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…

What aircraft was involved and where did it happen?

The accident on 12 Mar 2006 involved a Kaman K-1200, registration N133KA, at Dexter, OR.

What was the probable cause of the accident?

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.

More accident analyses

Investigation report by the U.S. National Transportation Safety Board (NTSB) historical archive. Original record: https://carol.ntsb.gov/event/20060327X00352. This page is a structured re-presentation; facts and quotes are in the National Transportation Safety Board (NTSB), United States.