SOCATA MS894A near Fulton, NY — 2008-08-16

Probable cause

A loss of engine power due to fatigue failure of the crankshaft and the No. 3 engine cylinder connecting rod as a result of improper machining of the engine crankshaft during engine overhaul by maintenance personnel.

NTSB narrative

The pilot stated that he was level at 400 feet above ground level when he heard a "bang," the airplane began to vibrate, and the engine stopped. He lowered the nose for the airplane's best glide speed and initiated a forced landing to a nearby open field. While on final approach to the field the airplane collided with trees and then the ground. The crankshaft and connecting rod were forwarded to the Safety Board's Metallurgical Laboratory for further analysis. Examination of the crankshaft's ten bearing journals revealed surface finishes that were consistent with a turning operation but not with a grinding operation as specified in the overhaul manual. Examination of the engine assembly revealed that the crankshaft and the No. 3 engine cylinder connecting rod had failed. The examination revealed that the crankshaft failed in fatigue and initiated in machining marks in the rear radius of the No. 3 and 4 connecting rod bearing journals. An engine logbook entry indicated that the main and rod bearing journals had been "turned" and their diameter reduced by 0.010-inch in accordance with the Franklin overhaul manual. Journal diameters were measured and were consistent with 0.010-inch begin removed; however, the table in the overhaul manual specified a tolerance of 0.0010-inch and the dimensions indicated a tolerance of 0.0020-inch, which was contrary to the overhaul manual. The overhaul manual also recommends that the crankshafts be returned to the factory for grinding and re-nitriding. No logbook entries were located to verify that re-nitriding had been carried out.

Analysis

Primary failure mode

Mechanical failure

First missed decision gate

Pilot could have aborted the flight after hearing the initial bang.

NTSB coding

• Loss of engine power (total) · Approach-VFR pattern crosswind
• Maintenance

Evidence available

• Photos
• 10 docket documents

Docket documents10

• Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1form
• Pilot written statementinterview
• Airplane Logbooks [Excerpts]other
• Frankilin Aircraft Engine Overhaul Manual [Excerpts]other
• Franklin Aircraft Illustrated Part Catalog [Excerpts]other
• Oswego Weatherother
• Photo 1: Airplane At Rest (Courtsey of the FAA)photos
• Photo 2: Left Side of Airplane At Rest (Courtsey of the FAA)photos
• Photo 3: Hole Observed On Engine Case (Courtsey of the FAA)photos
• Materials Laboratory 15 - Factual Report 10-063report

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