FTW97LA202

While conducting an aerial application swath run the engine lost all power and the pilot conducted a forced landing in a field with high vegetation which was unsuitable for a forced landing. Examination of the engine provided evidence of failure of both the left magneto drive gear and crankshaft idler gear. Examination of the aircraft records provided information that over 247 hours had been accumulated on the aircraft since the last 100 hour inspection and that an Airworthiness Directive directed at the magnetos had been complied with.

On May 27, 1997, at 1030 mountain daylight time, a Piper PA-25-235, N4829Y, conducted a forced landing in a field north of Lamar, Colorado, following a complete loss of power during an aerial application flight. The commercial pilot was not injured and the aircraft sustained substantial damage. Visual meteorological conditions existed for this aerial application flight conducted under Title 14 CFR Part 137 and no flight plan was filed. The flight departed Lamar at 1005.

According to the pilot, he was on his last swath run when the engine lost all power. He said he had refueled the aircraft approximately 15 to 20 minutes prior to the accident and had about 25 to 30 gallons of fuel on board when the engine lost power. Following the accident, the pilot said he checked the fuel in the aircraft, in his fuel tank, and the fuel in his other aircraft and found no water or other contaminates.

An engine examination was conducted by the investigator-in-charge when the aircraft was removed to a repair facility. An attempt was made to run the engine following an external examination. The engine would fire but would not start. The engine had accumulated 437 hours since overhaul.

The left magneto, Bendix part number 74830 was removed and exhibited a crack in one impulse coupling flyweight, three broken teeth, and the pivot pin for the flywheel had excessive play.

The idler gear was missing enough teeth that it would not drive the magneto gear, and due to the idler gear missing teeth, engine timing with the cam shaft rendered the engine inoperative. (See the attached diagram.) No other discrepancies were found in the accessory section of the engine and the right magneto operated in a normal fashion.

According to the pilot, the aircraft was out of service for an extended period of time and was flown for 105 hours between August of 1990 and May of 1996. According to the aircraft log, an annual inspection was conducted on July 6, 1990, and an annual and 100 hour inspection of the engine was conducted on May 2, 1996. Since that inspection, recorded time on the engine/aircraft was 247.41 hours. The log also provided information that an Airworthiness Directive on the magnetos had been complied with.

According to the mechanic who assisted in the examination of the engine following the accident, an on-time 100 hour inspection may have shown timing being out of tolerance and led to finding damage to the magneto drive prior to failure. A timing check is required during this inspection.