NTSB Factual Report

On July 17, 2013, about 1550 eastern daylight time, a Univair Aircraft Corporation (Stinson) 108-1, N8721K, registered to and operated by a private individual, was substantially damaged during a forced landing near Tuftonboro, New Hampshire. Visual meteorological conditions prevailed at the time and no flight plan was filed for the 14 Code of Federal Regulations (CFR) Part 91 personal flight from Moultonboro Airport (5M3), Moultonboro, New Hampshire, to Mountain View Field Airport (NH31), Wolfeboro, New Hampshire. The airline transport pilot and pilot-rated passenger sustained minor injuries. The flight originated about 1543, from 5M3.

The pilot-in-command (PIC) stated that he was in the right seat and his daughter (a recently licensed pilot) was in the left seat. She initiated the takeoff and climbed to 1,800 feet while proceeding towards the destination airport only 12 minutes away. After leveling off she began to lean the fuel to air ratio and the engine immediately began to run rough. He (PIC) took the controls and moved the mixture control to full rich, but the engine began to run really rough. He switched tanks, and manipulated the throttle control but the engine quit. The propeller continued to rotate about 6 times then the engine seized and the propeller stopped. He maneuvered the airplane for a forced landing in a field and just before touchdown he noticed a 1 foot high by 3 foot wide raised concrete item that was obscured by tall weeds. He banked left in an attempt to avoid it but the left wing contacted the ground and the gear hit the concrete. The airplane cartwheeled and came to rest upright. The airplane was recovered for further examination.

Following recovery of the airplane, the mechanic removed the top engine cowling, and noted the rear portion of the accessory case was cracked, and the right side of the engine crankcase was also cracked. A plate was removed from the engine, and the crankshaft was noted to be fractured. No connecting rods were separated. The crankshaft was removed from the engine and retained for further examination by the NTSB Materials Laboratory located in Washington, D.C.

According to the NTSB Materials Laboratory Factual Report, the crankshaft fractured through the cheek, between rod journals "R1" and "R2." The surface of the rod and main bearing journals whose dimensions were consistent with 0.010 inch undersize contained no evidence of blue or black tint. Bench binocular microscope examination of the crankshaft revealed crack arrest marks typical of fatigue cracking emanating from the aft radius of rod journal "R2". Fatigue crack propagation was aft and through the thickness of the crank cheek. Scanning electron microscope (SEM) examination of the excised origin of the fatigue crack revealed the surface of the aft relief radius in the area of the fatigue origin contained evidence of circumferential gouge marks that were more severe when compared to the circumferential marks in the adjoining flat portion of the rod journal. The relief radius showed no evidence of ladder cracks. A copy of the NTSB Materials Laboratory Factual Report is contained in the NTSB public docket.

A review of the engine logbooks and records dating to October 1951, revealed an entry dated June 1, 1968, indicating a major overhaul of the engine was performed. The entry further indicated, "Crankshaft replaced (ground .010/.010 undersize)." The engine was installed in an airframe other than the accident airframe, and a maintenance release yellow tag associated with the crankshaft dated August 30, 2005, indicates non-destructive testing was performed and no cracks were detected. A subsequent entry dated August 28, 2006, indicated the engine was disassembled for inspection, and the crankshaft was polished, ".010 mains .010 rods." The entry further indicates all work was performed in accordance with the Franklin engine overhaul manual. No determination could be made as to what facility polished the crankshaft. The engine was installed in another airplane on September 30, 2006, and the next entry dated May 23, 2007, indicates performing engine "break-in ground runs." The engine remained installed in that airplane until April 22, 2010, when it was removed for installation into the accident airplane. The engine at that time had accrued approximately 56 hours since major overhaul. The engine was installed into the accident airplane, and routine inspection entries were noted. The engine had accrued approximately 115 hours since overhaul in 2006 at the time of the accident. Excerpts from the engine maintenance records are contained in the NTSB public docket.

NTSB Probable Cause

Maintenance personnel’s failure to remove circumferential gouges in the aft radius of the R2 rod journal on the crankshaft during its last engine overhaul, which resulted in the fatigue failure of the crankshaft and a subsequent total loss of engine power.