Accident Overview
On January 3, 2012, at approximately 2145 central standard time, a Cessna 172P, registration N54872, executed a forced landing on a street in Conroe, Texas, following a loss of engine power. The pilot, who was the sole occupant, was not injured. The airplane, a public-use aircraft registered to and operated by the Civil Air Patrol (CAP) from Maxwell AFB, Alabama, was substantially damaged. The flight was operating as CAP flight 4272. Dark night visual meteorological conditions prevailed, and a company flight plan had been filed. The flight originated from West Houston Airport (KIWS) in Houston, Texas, at about 2120 and was en route to Lone Star Executive Airport (KCXO) in Conroe, Texas.
Engine Failure Sequence
According to the pilot's statement, the engine was set at 2300 rpm and the airplane was at 1,600 feet msl. When approximately 6 miles from the destination airport, the pilot heard a loud "boom" and the engine lost all power. Unable to glide to the airport, the pilot landed on Highway 105 and North 6th Street. During the final approach, the airplane collided with power lines. The pilot landed successfully but had to swerve right to avoid oncoming traffic. The airplane struck a curb, spun around, and the right wing struck a utility pole, resulting in substantial damage.
Engine Examination
On January 24, the engine was disassembled and examined under the direction of a Federal Aviation Administration (FAA) inspector at Fritz Aviation in Fredericksburg, Texas. CAP records indicated the engine had accrued 752 hours since its last overhaul in August 2007. When the crankshaft was turned, no movement of the intake or exhaust valves or magneto gears was observed. Upon removal of the rear accessory case, it was discovered that the rear crank gear bolt was loose and the gear dowel pin (part number STD 1065) was sheared. According to the FAA inspector, there was a line on the dowel pin, similar to a pre-existing crack. The attending mechanic reported that the sheared dowel pin would cause the camshaft and rear accessory gears to stop turning.
Metallurgical Findings
The crankshaft gear, attachment bolt, and crankshaft dowel pin were submitted to the National Transportation Safety Board’s Materials Laboratory for examination. The report indicated that the dowel pin was fractured from the aft end of the crankshaft approximately in plane with the aft face of the crankshaft where it mated to the crankshaft gear. Magnified optical examination revealed a relatively flat transverse fracture with beach markings indicative of fatigue progression initiating at two locations on opposing sides on the outer diameter of the dowel pin. The fatigue region from the primary origin area was substantially larger than that from the upper side. The cylinder faces of the dowel pin appeared relatively smooth and reflective, consistent with wear of the original surfaces. The wear was present around most of the dowel pin periphery, reducing its diameter by an estimated 0.005 inch adjacent to the fatigue origin, leaving a visible step. The crankshaft gear exhibited fretting damage on the forward face consistent with relative motion at the gear-to-crankshaft interface. The lock washer on the aft side of the crankshaft gear showed wear damage corresponding to contact with the underside of the attachment bolt head.