NTSB Factual Report

On September 14, 2004, approximately 1815 central daylight time, a Piper PA-31-350 twin-engine airplane, N555MC, sustained substantial damage during a forced landing to a field following a loss of engine power near Sweeny, Texas. The commercial pilot and his passenger were not injured. The airplane was registered to and operated by Mcoco, Inc., of Houston Texas. No flight plan was filed and visual meteorological conditions prevailed for the 14 Code of Federal Regulations Part 91 personal flight that originated from Calhoun County Airport (T97) near Calhoun, Texas, about 1745, and destined for William P. Hobby Airport (HOU) near Houston, Texas.

In a written statement, the pilot reported that while in cruise flight, he noticed a rattling noise coming from the left side of the airplane. The pilot checked the engine gauges and they appeared to be within the normal operating range. The sound progressively became louder, so the pilot advanced the mixtures, propeller, and throttles full forward in preparation for an engine failure. Subsequently, the left engine stopped producing power. The pilot reported that the left propeller then fell forward approximately six inches, and oil began leaking from the engine cowling. The pilot shut the left engine down and attempted to feather the propeller, but the lever would not move. The passenger then noticed a fire in the left engine nacelle. In response to the fire, the pilot pulled the mixture to an "idle cut-off position," and the fire stopped. Smoke then began to fill the cabin area, so the pilot decided to make an emergency landing to the nearest airport, but soon realized that it would be necessary to land in the nearest open field. The pilot then shut down the right engine and landed in a field with the landing gear retracted. The airplane slid to a stop, and the pilot and the passenger exited the airplane uninjured.

A review of aircraft maintenance records revealed that the airplane experienced a gear-up landing in January 2002, which resulted in a propeller strike. Both engines were removed and inspected per the manufacturers overhaul manual and returned to service. The overhaul included visual inspection, magnetic particle inspection, repolishing of the crankshaft and, among other things, replacement of all bearings. The left engine had accumulated approximately 440 hours since it was overhauled.

Examination of the left engine revealed that the entire three-bladed propeller assembly and the #1 cylinder had separated from the engine. The engine was disassembled and the crankshaft, along with an intact #3 connecting rod assembly and the fractured #2 connecting rod assembly, was sent to the Safety Board Materials Laboratory, Washington D.C. and examined by a Safety Board Materials Engineer.

According to the Safety Board Materials Laboratory Factual Report, the crankshaft was fractured in two places; one fracture was just forward of the #3 rod journal in the crank cheek, and the other fracture was in the aft crank cheek adjacent to the #2 main journal. Examination of the fracture surface adjacent to the #3 rod journal revealed that the fracture initiated in the forward transition radius and propagated forward in a plane perpendicular to the surface of the radius. The fracture was flat, contained arrest marks, and had an overall thumbnail shape with features consistent with high-stress fatigue propagation. The fatigue fracture features were present through approximately 80% of the cross-sectional thickness. The actual origin area was damaged by post-fracture smearing. The fracture initiated approximately in the middle of the rod journal radius, but no evidence of significant/abnormal grinding marks or other surface anomalies were noted.

Visual examination of the aft crank cheek thrust face adjacent to the #2 main journal revealed severe rubbing and significant radial cracking where the thrust face was contacted by the case. Examination of the fracture surface in the crank cheek revealed significant flat areas of cracking with arrest marks consistent with fatigue. The cracking initiated from radial cracks in the crank cheek itself, well removed from the radius, in the area of heavy rubbing, and propagated aft through the crank cheek. The fatigue cracking in the crank cheek propagated in multiple planes. The fact that the fatigue cracks initiated from the radial cracks was indicative that the fatigue cracking was secondary to the radial cracking and heavy rubbing. Propagation was through approximately 80 percent of the cross-section with features consistent with low-stress fatigue propagation.

The interior surface of the #1 and #2 main bearing halves exhibited significant rubbing and a portion of the bearing liner material was delaminated and missing from the forward end of the bearing. On the oil slinger face, just forward of the #1 main journal, 360 degree rubbing damage was noted just outboard of the radius. In the rub region, metallic transfer was present with a color consistent with aluminum material. No evidence of radial cracks was observed in this area.

An exact determination for the crankshaft failure could not be determined; however, evidence supported two possibilities that could have attributed to its demise. One possibility was that the crankshaft fracture initiated from rubbing damage between the crank cheek aft of the #2 main journal (thrust face) and the crankcase, with the other fatigue region being secondary. The exact cause for the rubbing was not clear, but one possibility was that the bearing liner delaminated and liberated a section causing a reduction in the clearance between the crankshaft and the crankcase as it was rubbing/exiting through the oil slinger. The other possibility was that the propeller strike caused the rubbing/cracking to occur, which was not detected during the subsequent overhaul/inspection.

At 1805, the automated weather observing station at Bay City Municipal Airport (BYY), near Bay City, Texas, located approximately 10 nautical miles southwest from the site of the accident, reported wind from 130 degrees at 14 knots, 10 statute miles visibility, scattered clouds at 3,300 feet, scattered clouds at 3,900 feet, broken clouds at 6,000 feet, temperature 88 degrees Fahrenheit, dew point 73 degrees Fahrenheit, and a barometric pressure setting of 29.83 inches of Mercury.

NTSB Probable Cause

The fracture failure of the left engine crankshaft for undetermined reasons.