Consolidated Aeronautics Inc LAKE MODEL 250 N522WB accident near Middle Grove, New York

Probable cause

Maintenance personnel’s application of insufficient torque to the No. 6 cylinder connecting rod bolts during overhaul, which resulted in the fatigue fracture of the connecting rod and a subsequent total loss of engine power.

NTSB analysis

The pilot stated that, shortly after takeoff, the engine experienced uncommanded rpm changes, so he turned the airplane to return to the departure airport. He then heard a "loud bang" and the engine running roughly, and it subsequently lost power. When the pilot realized that he would be unable to land at the airport, he maneuvered the airplane for a gear-up forced landing in a field. With only 1/4 of the length of the field remaining, he intentionally yawed the airplane right to avoid obstructions ahead; the airplane came to rest upright with the engine displaced to the left. Oil was covering the engine pylon, the top portion of the empennage, and the leading edges of the vertical and horizontal stabilizers.

Postaccident examination of the engine revealed a hole in the crankcase adjacent to the No. 6 cylinder. The No. 6 cylinder connecting rod was fractured and separated from the crankshaft, and one nut from the No. 6 cylinder connecting rod bolt was not present and was found inside the oil sump. Metallurgical examination of the components of the No. 6 cylinder connecting rod revealed that the connecting rod was fractured through one side of the strap at the crankshaft end of the connecting rod and that the strap segment on the other side of the connecting rod was deformed inward. The fractured strap exhibited fatigue cracks from multiple origins, and the split line face on the deformed segment of the strap exhibited fretting damage. The connecting rod likely fractured due to insufficient clamping force across the split line at the crankshaft end of the connecting rod. Although the No. 6 cylinder had been removed about 110 hours before the accident flight, the work performed at that time was not associated with the No. 6 cylinder connecting rod. Therefore, given that the engine was overhauled only about 153 hours before the accident flight, it is likely that insufficient torque was applied to the connecting rod nut during the overhaul.

Text from the NTSB. Open the NTSB docket for ERA13LA382 for the full record.