NTSB Factual Report

On July 16, 2009, at 0640 central daylight time, a Cessna A188B, N9747G, sustained substantial damage during a forced landing to a field near Suring, Wisconsin, after a loss of engine power. The pilot was not injured. The 14 CFR Part 137 aerial application flight had departed from a local field and was maneuvering when the loss of power occurred. Visual meteorological conditions prevailed at the time of the accident. No flight plan was filed.

The pilot reported that the airplane was operating normally during the previous flights prior to the accident flight. He reported that on the accident flight, the engine began to run very rough during a turn at low altitude. The engine suddenly seized so he made an emergency landing to a wheat field. During the landing, the left main gear collapsed and the left wing and nose of the aircraft dug into the soft soil, sustaining substantial damage.

The inspection of the engine confirmed that it had seized. The 300 horsepower Continental IO-520-D engine, serial number 572952, was shipped to the engine manufacturer for a teardown inspection.

The teardown inspection revealed that the exterior crankcase exhibited minimal exterior damage. The number one, two, three, four, and five main bearing supports exhibited fretting. The number three main bearing support also exhibited main bearing shift. The crankcase backbone exhibited fretting between the number two and three main bearing supports. A red sealant had been applied to the crankcase backbone. The number one, two, and three camshaft journal supports exhibited fretting.

The number two crankshaft main bearings had extruded from their crankshaft positions in the left and right case halves. The bearings exhibited mechanical damage that was consistent with bearing shift, rotation, and extrusion. Fragments of the bearing were found in the oil sump. The number three crankshaft main bearings exhibited mechanical damage that was consistent with bearing shift. The babbit was smeared, exposing the copper layer.

The number three connecting rod exhibited mechanical damage and was fractured at the base of the I-beam. Fragments of the connecting rod cap exhibited mechanical damage. Fragments of connecting rod bolts and nuts were fractured through and exhibited mechanical damage consistent with overload signatures.

The crankshaft was fractured through at the number four short cheek at the number two main journal. A metallurgical examination of the fracture surface revealed crack arrest marks, which were indicative of a fatigue fracture which emanated from the surface.

An examination of the engine logbooks revealed that engine’s total time was about 1,778 hours and it had about 500 hours since the last major overhaul. The engine’s last overhaul was conducted on September 30, 1986. The last annual inspection was conducted on October 23, 2008.

The Teledyne Continental Aircraft Engine, Service Information Letter, SIL98-9A, issued on November 17, 1998, recommended that an IO-520-D engine be overhauled every 1,700 hours or at least every twelve (12) years. It recommended that aircraft used in aerial spraying should have the engine overhauled after the accumulation of 1,200 hours or twelve (12) years, whichever occurs first.

NTSB Probable Cause

A loss of engine power resulting from the fatigue failure of the crankshaft due to the loss of engine case through-bolt torque. Contributing to the engine failure was the operator's failure to overhaul the engine in accordance with the manufacturer's recommended guidelines.