NTSB Identification: LAX99FA270.
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Accident occurred Tuesday, August 17, 1999 in MARYSVILLE, CA
Probable Cause Approval Date: 05/08/2001
Aircraft: Beech J35, registration: N4932B
Injuries: 2 Serious.

NTSB investigators either traveled in support of this investigation or conducted a significant amount of investigative work without any travel, and used data obtained from various sources to prepare this aircraft accident report.

The pilot radioed that the aircraft's engine was failing and he was altering his course to land at the nearest airport. While on left base leg for landing, the cowling flew open, oil filled the windshield, and the engine ceased delivering power. The aircraft impacted a tree and came to rest in a city park. At the scene, the No. 6 engine cylinder was separated from the crankcase and engine components associated with that cylinder were found over a 40-foot radius around the aircraft. The engine crankcase where the No. 6 cylinder attached was broken away at the 12 o'clock position; the upper forward, upper rear, and lower rear mounting studs were broken; and the lower forward stud and the two lower center studs were present. The skirt of the No. 6 cylinder exhibited a pattern of longitudinal striations where it mated into the crankcase opening. At the lower left of the cylinder mount location was a small ledge of material, which approximated the shape of the circumference of the cylinder base flange. Opposite this location, on the upper right of the cylinder mounting location, was a brightly polished wear area. The engine received a top overhaul 7 months, 41.8 hours prior to the accident. Breakaway torque was measured on the remaining cylinder base nuts and found to range between 25 and 40 foot-pounds; the specification torque values are between 40.8 and 42.5.

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

A loss of engine power due to improperly torque cylinder base nuts by maintenance personnel during a top overhaul.

Full narrative available

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