Anderson, IN, USA
N92640
WSK PZL MIELEC M-18A
The pilot was marking a field in preparation for an aerial application when he experienced what he thought was an engine failure. He stated that the engine was still running, but it was not producing power. The pilot pushed the power lever forward; the engine power did not increase, but the engine temperature increased. He stated that the engine was idling fast and that he forced the airplane into a bean field about 150 to 200 yards ahead of a tree line. The pilot ground looped the airplane before contacting the trees. A postaccident examination of the engine did not reveal any anomalies. A bench test of the propeller governor revealed that the maximum speed was above the nominal limit and the minimum speed was below the nominal limit. The control lever position was set higher than the specified position, which indicated that the governor had been adjusted in an attempt to regulate the propeller speed. X-rays and a teardown revealed that the propeller governor speeder spring was fractured. A metallurgical examination revealed a preexisting fatigue crack that, over time, resulted in the failure of the spring. The failed speeder spring would have resulted in a loss of oil pressure to the propeller piston, which allowed the propeller blades to rotate to a high blade angle, which then decreased the engine speed and overall power output of the engine. Maintenance records showed the propeller governor was overhauled on May 19, 2009. The speeder springs did not have serial numbers, and, therefore, the maintenance history of the speeder spring could not be determined.
On July 25, 2011, at 1417 eastern daylight time, a WSK PZL MIELEC M18A airplane, N92640, collided with the terrain near Anderson, Indiana, during an aerial application flight following a loss of engine power. The airline transport rated pilot was not injured. The airplane was substantially damaged. The flight was being operated by Forest Air Services under 14 Code of Federal Regulations Part 137 without a flight plan. The local flight originated from Marion, Indiana, about 1335. The pilot stated he was beginning to mark his second field when the accident occurred. As he started the second marking pass, he experienced what he thought was an engine failure. He stated the engine was still running, but it was not producing power. He turned away from a nearby town and corn field, and located a bean field in which to land. The pilot pushed the power lever forward and the engine power did not increase, but there was an increase in engine temperature. He stated the engine was idling fast and he forced the airplane onto the ground about 150 to 200 yards prior to a tree line. The pilot ground looped the airplane prior to contacting the trees. The airplane was equipped with a Hartzell 5-bladed, hydraulically operated, constant speed propeller, model number HC-B5MP-5BL, hub serial number EVA3036. The blades contained varying degrees of impact damage. Impact marks between the link arms and the bottom of the blade counterweights indicated that the blades were not in either the feather or reverse position at impact. A postaccident inspection of the propeller did not reveal any failure or malfunction that would have precluded normal operation. The engine, a Honeywell model TPE331-11U-612G, serial number P44089C, was last installed on the accident airplane on October 28, 2009. The engine was shipped to Honeywell for a teardown inspection under the supervision of the Federal Aviation Administration. The teardown inspection did not reveal any failure or malfunction that would have precluded normal operation of the engine. The fuel control, fuel pump, propeller governor, fuel nozzles, and fuel manifold were removed for further testing. All of the components tested normally with the exception of the propeller governor which showed the minimum stop adjustment was out of limits. The governor was found to be delivering sufficient oil volume and pressure. The propeller governor, Woodward part number 897410-16, serial number 1758842 was subsequently examined at Woodward. A bench test of the governor revealed the maximum speed was 44 rpm above the nominal limit at the maximum control lever position of 112 degrees, and the minimum speed was 410 rpm below the nominal limit. The maximum control lever position, which should have been 84 +/- 5 degrees, was set at 112 degrees. The maximum speed to minimum speed control lever position was set at 30 degrees and it should have been set to 10.2 degrees. A non-Woodward lockwire was present on the control lever indicating that adjustments had been made to the control lever position. The propeller governor was x-rayed. The x-ray appeared to show a fracture of the speeder spring, part number 1522-666. Two of the spring coils were in contact with each other near the spring seat. Disassembly of the governor revealed the speeder spring was fractured at a point 1-1/2 to 1-3/4 coils from the end of the spring. A metallurgical examination of the speeder spring revealed the fracture was a result of a fatigue crack which propagated through the fracture surface. Contact marks were visible between the coils. The depth and wear of marks on the contacting surfaces of the coil adjacent to the fracture indicate the spring continued to function after the fracture was initiated. The surface finish of the spring adjacent to the fracture origins contained pitted areas. Maintenance records show the propeller governor was overhauled on May 19, 2009. Speeder springs do not contain serial numbers and therefore the history of the fractured spring could not be determined. Further testing on an exemplar spring was performed by Woodward to determine the change in load rating of the spring at a condition where wear was occurring before and after the fracture occurred. This was accomplished by measuring the spring both prior to and after an electrical discharge machine cut through 90 percent of the coil. Prior to the cut with an applied load of 8 pounds, the spring length was measured at 1.1842 inches; with a 16 pound load, the spring length was 1.0707 inches. Following the cut with the same loads applied, the spring length was 1.1657 inches and 1.0642 inches respectively. In addition, it was calculated that, during the time the crack was present, but prior to complete failure, the resulting governor speed output would have been 244 rpm (6.5 percent) lower than with a non-cracked spring. This value would would be an additional 2.7 percent lower in speed.
The fatigue failure of the propeller governor speeder spring, which resulted in the pilot’s inability to control engine output power.
Source: NTSB Aviation Accident Database
Aviation Accidents App
In-Depth Access to Aviation Accident Reports
