Accident Overview
On March 18, 2013, at 1405 central daylight time, a Piper PA-25-235, registration N86AB, sustained substantial damage during a forced landing following a total loss of engine power near Panama City, Florida. The airline transport pilot, the sole occupant, reported minor injuries. Visual meteorological conditions prevailed, and no flight plan was filed for the local banner tow flight that originated from Northwest Florida Beaches International Airport (ECP) at 1355.
The pilot conducted a preflight inspection, including sampling fuel from the gascolator, and noted 75 gallons of fuel onboard. A run-up check of the engine, including carburetor heat, revealed no abnormalities. After departing and picking up a banner, about 6 minutes later at an altitude of 900 feet mean sea level (msl) and approximately 2 miles west of ECP, the engine suddenly lost all power. The pilot verified the mixture was full rich, magnetos on, fuel valve open, and applied carburetor heat with no effect. He jettisoned the banner at 500 feet msl and executed a forced landing into a pine forest.
Aircraft and Engine Background
The 1965-manufactured airplane was originally equipped with a Lycoming O-540 engine. Maintenance records indicated the engine was inspected and overhauled following a propeller strike, with an entry stating it was "Overhauled engine [in accordance with Lycoming Engine] overhaul manual." Several components, including the crankshaft and crankshaft gear, were overhauled, and the entry noted compliance with "SB 475 C excluding par 6." New parts installed included a crankshaft gear bolt, but installation of a new lockplate was not specifically called out. The entry did not document the final crankshaft gear bolt torque, verification of lockplate bending, or accomplishment of inspections and rework required by Lycoming Service Bulletin No. 475C.
Handwritten notes from the overhaul listed a "SL STD-2209 Crank gear bolt" and "67514 Crank gear torqued to 204 inch" with "18638 Lock plate folded over on bolt." The engine was modified per a supplemental type certificate for 250 horsepower and installed in April 2010 with 3,444 total hours. The most recent 100-hour inspection on February 22, 2013, recorded 8,100 airframe hours and 1,371 engine hours since overhaul; 63 additional flight hours accumulated before the accident.
Post-Accident Examination
Following recovery, one fuel tank was breached, and about 45 gallons of fuel remained in the other tank. The engine had 12 quarts of oil. Disassembly revealed the crankshaft gear was rotationally misaligned by about 45 degrees from its normal position, and the alignment dowel was sheared. The gap between the gear and crankshaft mating surfaces was less than 0.001 inch. The crankshaft, gear, and attaching hardware were sent to the NTSB Materials Laboratory.
NTSB Materials Laboratory Findings
Examination showed the gear was mounted in a counterbored pilot hole and pressed against the recess face by a bolt with a lockplate. The alignment dowel exhibited a fatigue fracture with crack arrest marks, originating on the clockwise (leading) face. The dowel fragment retained in the gear had rotated approximately 180 degrees.
Dimensions were compared to Lycoming Service Bulletin No. 475C limits. The gear pilot flange diameter measured 2.1253, 2.1254, and 2.1257 inches (original limit: 2.1250–2.1255). The counterbored pilot hole measured 2.1270, 2.1275, and 2.1275 inches (limit: 2.1262, requiring plating/regrinding above that). The reduced dowel diameter measured 0.2504 inch (original: 0.2505–0.2510). Hardness tests met requirements.
Service Bulletin Guidance
Lycoming Service Bulletin No. 475C describes procedures for attaching the crankshaft gear, noting that incorrect assembly may cause damage and complete engine stoppage. Steps include aligning the gear, torquing the bolt to 125 in.-lbs., checking seating with a 0.001-inch feeler gauge (no clearance), then final torque to 204 in.-lbs., ensuring clearance between gear flange and pilot hole no greater than 0.0005 inch. The bulletin requires a logbook entry specifying final torque, verifying lockplate bending, and documenting compliance with SB 475C inspections and rework.