Accident Overview
On August 19, 2014, about 0740 mountain daylight time, an Air Tractor AT-401, registration N4542W, operated by Tri-County Ag of Wray, Colorado, was involved in a forced landing in a field near Wray, Colorado. The pilot, the sole occupant, sustained a minor injury. The airplane was substantially damaged. The flight was conducted under 14 Code of Federal Regulations Part 137 as an aerial application operation. Visual meteorological conditions prevailed, and no flight plan was filed. The local flight had originated from Wray Municipal Airport (2V5) at approximately 0700.
Pilot Report
According to the pilot's accident report, he had just completed spraying a field and was climbing out to return to home base when he felt a subtle vibration from the engine and noticed manifold pressure dropping rapidly. Upon attempting to increase manifold pressure by opening the throttle, the engine began to vibrate violently and lost all power. The pilot executed a downwind forced landing. To avoid a drainage ditch and power lines ahead, he applied heavy braking, causing the airplane to nose over. The vertical stabilizer and rudder, cockpit, and left wing sustained crushing damage.
Engine Examination
On October 14 and 15, the engine was partially disassembled and examined. Engine oil drained from the rocker arm sump contained carbon, aluminum, and steel filings. Compression checks of all cylinders were performed with the following results: Cylinder 1: 0/80, Cylinder 2: 50/80, Cylinder 3: 0/80, Cylinder 4: 20/80, Cylinder 5: 25/80, Cylinder 6: 75/80, Cylinder 7: 65/80, Cylinder 8: 55/80, Cylinder 9: 55/80.
Cylinders 1, 3, and 4 were removed for further examination. The intake valve in the number 3 cylinder had a ridge worn on its face and the valve seat was excessively worn. The exhaust valve in the number 1 cylinder was stuck open and binding in the valve guide; it was removed with extreme difficulty. Exhaust valve clearances were measured and found to be less than the Pratt and Whitney overhaul limits of .003" to .0055" (maximum wear limit .010"). Measured clearances and dimensions: Cylinder 1 clearance .002" (stem .610", guide .613"), Cylinder 3 clearance .0022" (stem .611", guide .6132"), Cylinder 4 clearance .0022" (stem .611", guide .6132").
Probable Cause
None stated in the source.