CHI07LA108

The pilot reported that upon reaching approximately 75 knots during the takeoff roll, he felt a surge from the left engine. He noted that the "surging continued and positive directional control could not be regained," at which point he closed the throttles and elected to abort the takeoff. The pilot attempted to maintain directional control by using differential braking and rudder inputs. The airplane subsequently departed the left side of the runway and impacted an airport sign, causing the nose landing gear to collapse, before coming to rest. Post accident testing of the left propeller governor unit revealed that the component pressure gain was approximately one-half of the manufacturer's specification, resulting in the unstable engine operation.

On April 11, 2007, at 1115 central daylight time, a Swearingen SA-226T, N125WG, piloted by an airline transport pilot, was substantially damaged during an aborted takeoff on runway 16 (5,000 feet by 150 feet, asphalt) at Chicago Executive Airport (PWK), Wheeling, Illinois. Instrument meteorological conditions prevailed at the time of the accident. The business flight was being conducted under 14 CFR Part 91 on an instrument flight rules (IFR) flight plan. The pilot and four passengers on-board were not injured. The flight was originating at the time of the accident and had an intended destination of Shreveport Regional Airport (SHV), Shreveport, Louisiana.

The pilot reported that upon reaching approximately 75 knots during the takeoff roll, he felt the left engine surge. He noted that the "surging continued and positive directional control could not be regained," at which point he closed the throttles and elected to abort the takeoff. The pilot attempted to maintain directional control by using differential braking and rudder inputs. The airplane subsequently departed the left side of the runway and impacted an airport sign, causing the nose landing gear to collapse, before coming to rest.

Post accident testing of the left engine propeller governor indicated that it did not meet the manufacturer's requirements. Specifically, the pressure gain across the unit was approximately one-half that required by manufacturer's specifications. Bench testing revealed that a shaft input speed of approximately 80 RPM was required to provide a pressure increase of 150 pounds-per-square inch (psi). The specification required an input speed of between 20 and 40 RPM in order to obtain the 150 psi pressure increase.

Subsequent testing and teardown inspection of the governor unit revealed that the flyweight toe extensions were worn. This altered the position of the flyweights during operation and reduced the component pressure gain.

Aircraft maintenance records indicated that an overhaul of the governor unit was completed on November 22, 2000. It had accumulated approximately 913 hours time in service at the time of the accident. Detailed records of the overhaul were not available. According to the component manufacturer, the flyweights are not normally replaced during overhaul. However, the flyweights are replaced when wear is observed. The manufacturer's recommended overhaul interval was 3,500 hours.

A representative of the propeller manufacturer stated that a governor unit exhibiting a low gain condition, such as the unit on the accident airplane, may cause unstable engine operation such as the surging reported by the accident pilot.