ERA13LA174

A loss of preload to the crankshaft gear attachment bolt, resulting in rotation of the crankshaft gear and a subsequent total loss of engine power. The root cause for the loss of preload could not be determined during a postaccident examination of the components.

The pilot departed and picked up an aerial advertising banner for a local flight. About 6 minutes later, the airplane's engine suddenly lost all power. The pilot unsuccessfully attempted to troubleshoot the loss of engine power, jettisoned the banner at an altitude of about 500 feet, and performed a forced landing to a pine forest below, which resulted in substantial damage to the airplane. Postaccident examination of the engine revealed that the crankshaft gear alignment dowel had sheared and that the gear had rotated from its normal position. The gear's attaching hardware remained installed and undamaged. Detailed examination of the gear and alignment dowel showed that their hardness was consistent with that required by the design and that they were within or nearly within nominal dimensional tolerances described by the manufacturer. The counter-bored pilot hole at the aft of the crankshaft where the gear was seated was between 0.0008 and 0.0013-inch oversized. There were no discrepancies found with the retaining bolt, and it could be threaded into the crankshaft with minimal resistance. A definitive cause for the loss of preload to the crankshaft gear attaching bolt could not be determined during the engine examination. Review of maintenance records showed that the engine had been installed onto the accident airplane following an overhaul, which was originally precipitated by a propeller strike. The records provided that documented the overhaul and returned the airplane/engine to service lacked language specifying compliance with a manufacturer service bulletin that provided explicit instructions for the installation of the crankshaft gear to the crankshaft; however, the provided records did document part numbers and torque values consistent with those specified by the service bulletin.

On March 18, 2013, at 1405 central daylight time, a Piper PA-25-235, N86AB, was substantially damaged during a forced landing following a total loss of engine power near Panama City Florida. The airline transport pilot incurred minor injuries. Visual meteorological conditions prevailed, and no flight plan was filed for the local flight, which originated from Northwest Florida Beaches International Airport (ECP), Panama City, Florida, at 1355. The banner tow flight was conducted under the provisions of Title 14 Code of Federal Regulations Part 91. The pilot performed a preflight inspection of the airplane, including sampling fuel from the gascolator, and noted that the airplane had 75 gallons of fuel onboard. Prior to departing, the pilot performed a run-up check of the engine, which included a check of the carburetor heat, and noted no abnormalities. The pilot then departed the airport and picked up the banner. About 6 minutes later, about 2 miles west of ECP and at an altitude of 900 feet mean sea level (msl), the pilot reported that the engine suddenly experienced a total loss of power. The pilot responded by verifying that the mixture was set fully rich, that the magnetos were both on, and that the fuel valve was on. He then applied carburetor heat, which had no effect. After jettisoning the banner at an altitude of 500 feet msl, the pilot performed a forced landing to the pine forest below. According to Federal Aviation Administration airworthiness records, the airplane was manufactured in 1965 and was originally equipped with a Lycoming O-540 engine. According to maintenance records, the accident engine was inspected and overhauled following a propeller strike. The maintenance log entry detailing the overhaul of the engine stated in-part, "Overhauled engine [in accordance with Lycoming Engine] overhaul manual." The entry noted that several individual engine components were "overhauled" including the crankshaft and crankshaft gear. The portion of the entry noting overhaul of the crankshaft gear was also followed by the statement, "SB 475 C excluding par 6." The entry also noted the installation of several new parts to the engine including a crankshaft gear bolt, though the installation of a new lockplate was not specifically called out. The entry further noted, "All ADs and SBs have been complied with thru 2010-03." The entry did not explicitly state the final crankshaft gear bolt torque, note and verify the installation and bending of the lockplate against the bolt head, or that the inspections and rework required by Lycoming Service Bulletin Number 475C had been accomplished. An invoice detailing inspection and work performed to the crankshaft stated, "Inspected and/or repaired per Lyc. S.B. 475C excluding paragraph 6." In addition to maintenance log entries, the operator provided invoices and handwritten notes prepared at the time of the engine overhaul. One of the handwritten notes pages contained parts and part numbers, in an itemized list. The list stated in-part: "SL STD-2209 Crank gear bolt" "67514 Crank gear torqued to 204 inch" "18638 Lock plate folded over on bolt" During the overhaul, the engine was modified in accordance with a supplemental type certificate to produce 250 horsepower. The engine was installed onto the accident airplane in April 2010, and at that time it had accumulated 3,444 total hours of operation. On February 22, 2013, the operator completed the airframe and engine's most recent 100-hour inspection. At that time the airframe had accumulated 8,100 total hours, and the engine had accumulated 1,371 hours since the most recent overhaul. Between the time of the 100-hour inspection and the accident, the airplane had accumulated 63 additional flight hours. Following the accident, the airplane was recovered from the accident site to the operator's maintenance base. One of the fuel tanks was breached during the impact sequence and about 45 gallons of fuel was recovered from the remaining fuel tank. External examination of the engine revealed minimal impact damage and that 12 quarts of oil remained in the oil sump. The engine was subsequently disassembled. During the disassembly the crankshaft gear was found rotationally misaligned with its normal position relative to the crankshaft, and the crankshaft alignment dowel was sheared. The gap between the crankshaft gear and crankshaft mating surfaces measured less than 0.001-inch when utilizing a feeler gauge. The crankshaft, with the entirety of the crankshaft gear and its attaching hardware were forwarded to the NTSB Materials Laboratory for detailed examination. According to the Materials Laboratory Factual Report, typically the crankshaft gear was mounted inside a counterbored pilot hole in the aft end of the crankshaft. The gear was pressed against the face of the recess by a bolt that engaged a threaded hole in the end of the crankshaft. A lockplate was installed between the bolt and the gear with ears that fold down against the side of the gear and up against the head of the bolt. The gear was rotationally aligned to the crankshaft by an alignment dowel. The alignment dowel from the accident engine was fractured, one end of the dowel retained inside the alignment hole in the gear, while the other end remained press-fit and retained in the end of the crankshaft. The previously noted rotational displacement of the gear relative to the crankshaft was measured to be about 45-degrees, and the lockplate was also rotationally displaced, though to a lesser degree. The retaining bolt and the gear were removed. There were no visible discrepancies with the retaining bolt. One of the threads contained a soft dark material consistent with an anti-seize compound. The bolt was turned into the threaded hole until it nearly bottomed out. The bolt could be turned with fingers up to the thread containing the compound at which point the compound provided some resistance to further insertion (approximately 3.5 in-lbs. measured using a torque wrench). Over the area where the gear had been in contact with the face of the pilot hole recess, the surface exhibited circumferential scratch marks and had transferred metal adhered to it. The mating face of the gear exhibited corresponding scratches and gouges, consistent with galling. The dowel fracture surface was examined on the piece of the dowel that was retained in the alignment hole. The fracture surface had a comparatively flat appearance and exhibited multiple crack arrest marks. The morphology of the fracture surface was consistent with a fatigue fracture. The curvature of the crack arrest marks was consistent with the fatigue fracture originating along the surface of the dowel in the area. The fracture features were compared with the mating dowel piece retained in the crankshaft. The comparison indicated the fracture initiated along the clockwise face of the dowel (leading face in the direction of rotation) and that the dowel fragment retained in the gear alignment hole had rotated by approximated 180 degrees. Diameters on the gear, pilot hole, and alignment dowel were measured and compared with the dimensional limits in Lycoming Service Bulletin No. 475C. The diameter of the pilot flange on the gear was measured three times using a micrometer and the values were 2.1253 inch, 2.1254 inch, and 2.1257 inch. According to the service bulletin, the original dimension of the flange was 2.1250 inch – 2.1255 inch. The diameter of the counterbored pilot hole recess was measured three times using calipers and the values were 2.1270 inch, 2.1275 inch, and 2.1275 inch. According to the service bulletin, if the pilot hole diameter exceeded 2.1262 inch, the diameter should be plated and reground to 2.1250 inch – 2.1260 inch. The reduced diameter on the stepped dowel could only be measured at one location and it measured 0.2504 inch with a micrometer. According to the service bulletin, the original dimension of the dowe