NTSB Factual Report

On April 30, 2007, at 1643 central daylight time, a Beech S35, N8820M, sustained substantial damage during a forced landing to a cornfield after a loss of engine power near North Central Missouri Regional Airport (MO8), Brookfield, Missouri. The commercial pilot, the only occupant, was not injured. The 14 Code of Federal Regulations Part 91 flight departed John Bell Williams Airport (M16), Raymond, Mississippi at an unknown time and was en route to Fort Dodge Regional Airport (FOD), Fort Dodge, Iowa. Visual meteorological conditions prevailed at the time of the accident. No flight plan was filed.

The pilot reported that the engine was "operating normally" as he was flying at 6,500 feet mean sea level when the "engine blew up." He made a MAYDAY call and flew best glide speed as he navigated to the nearest airport located about 15 nautical miles away. The pilot elected to execute a gear up forced landing to a field about two miles from the airport, but the airplane impacted a ditch during the emergency landing. The pilot exited the airplane with no injuries.

A Federal Aviation Administration airworthiness inspector examined the airplane at the accident site. The inspection revealed that the engine had two holes in the crankcase. One of the holes was on the number 4 cylinder side of the crankcase. The number 4 cylinder had separated from the crankcase and the number 4 connecting rod was not attached to the number 4 crankcase pin. The number 4 rod cap, bolts, and nuts were found lying loose in the engine cowling. The number 4 cylinder top left stud and three cylinder deck stud nuts were also found lying loose in the engine cowling.

The engine was sent to Teledyne Continental Motors for an engine teardown inspection. The inspection revealed a hole in the crankcase approximately 3 inches by 3 inches above the number 4 cylinder, which included the 11 and 1 o'clock deck studs. The number 4 connecting rod and rod cap exhibited signatures that were consistent with an overload event that deformed and/or sheared the threads on the nuts and bolts. The number 4 cylinder exhibited skirt damage between the 5 and 6 o'clock position and fretting signatures on the base flange. Cylinder number 4 deck stud nuts 6, 7, and 8 were reported missing from the engine at the accident scene, and subsequently found inside the engine cowling of the aircraft. Cylinder number 4 deck studs 6, 7, and 8 were securely attached to the engine crankcase and the threads were clean and undamaged. The 2 o'clock cylinder deck stud was fractured and missing. The remaining deck studs and thru-bolts all exhibited ductile fractures indicative of tensile overload.

The remaining five cylinders were removed from the crankcase. The thru-bolt and deck stud break-away torque measurements were recorded for each thru-bolt and deck stud. The acceptable torque limits for the deck studs was 490 - 510 inch/pounds, and the limits for the thru-bolts was 690 - 710 inch/pounds. The inspection revealed that numerous thru-bolts and studs had break-away torque values that did not comply with the manufacturer's limitations.

The aircraft engine's logbook indicated that the engine received an annual maintenance inspection on February 16, 2005. The engine logbook entry stated, "Removed and overhauled all 6 cylinders. Installed new rings, ground valves, cleaned and gapped spark plugs." The tachometer time indicated 5,380.5 hours at the time of the annual inspection. The engine logbook indicated that the engine received an annual maintenance inspection on May 3, 2006. At the time of the accident on April 30, 2007, the tachometer time was 5,662.24 hours, 281.74 hours since the cylinders had been replaced.

NTSB Probable Cause

Improper maintenance by maintenance personnel by not properly torquing cylinder thru-bolts. The loose stud nuts which resulted in the separation of the cylinder and the crop and ditch were factors.