CEN10LA123

The pilot stated that during cruise flight he heard a “loud metallic clicking” sound, followed by a “bang” and a vibration. A few seconds later there was another “bang” and the propeller stopped rotating. He subsequently executed a forced landing to a corn field, where the airplane sustained damage to the firewall, nose landing gear, and wings. A postaccident examination on the engine revealed that the No. 3 and No. 4 engine connecting rod bolts had failed. Metallurgical examination determined that the bolts failed as a result of overstress fractures. No evidence of a fatigue fracture was observed. The teardown examination of the engine revealed that the nuts installed on the No. 1 and No. 2 connecting rod bolts were not tightened to the engine manufacturer’s specifications. Maintenance records indicated that the engine had been overhauled and the airplane returned to service about two weeks prior to the accident. At the time of the accident, the engine had been operated about 68 hours since the overhaul. The lack of proper torque on the intact rod bolts/nuts and the overstress failure of the remaining bolts indicates that the connecting rods were not installed properly during the overhaul procedure.

On February 13, 2010, about 2015 central standard time, a Cessna model 172N, N24CV, piloted by a commercial pilot, was substantially damaged during a forced landing following a loss of engine power near Genoa City, Wisconsin. The personal cross-country flight was being conducted under 14 Code of Federal Regulations Part 91 without a flight plan. Night visual meteorological conditions prevailed. The pilot sustained minor injuries. The flight departed Aurora Municipal Airport (ARR), Aurora, Illinois, about 1945, with an intended destination of Austin Straubel International Airport (GRB), Green Bay, Wisconsin. The pilot stated that during cruise flight he heard a “loud metallic clicking” sound, followed by a “bang” and a vibration. He noted that a few seconds later there was another “bang” and the propeller stopped rotating. He subsequently executed a forced landing to a corn field. The airplane sustained damage to the firewall, nose landing gear, and wings. A post-accident examination of the engine revealed crankcase fractures in the area of the #3 and #4 cylinders. The engine sump contained approximately 5 quarts of oil at the time of the exam. The oil filter contained metallic fragments. Teardown examination of the engine revealed that the #3 and #4 connecting rod bolts had failed. All four bolts (two common to each connecting rod) failed above the threads, in the shank portion of the bolts. Two of the bolt fragments included the corresponding nuts; however, they were backed off to the end of the bolts. These bolt fragments appeared to have separated from the #3 connecting rod. The remaining two nuts had separated from their corresponding rod bolts. One of the nuts was deformed and scarred consistent with mechanical damage. The other was fragmented. The remaining two sets of connecting rod bolts appeared intact. However, the nuts securing the #1 connecting rod bolts were loosened by applying minimal torque from a wrench, and the nuts securing the #2 connecting rod bolts were finger tight. The #2 connecting rod and cap were not mated, but were separated approximately 1/8 inch. Metallurgical examination of the failed connecting rod bolts revealed features consistent with overstress failure. No evidence of a fatigue fracture was observed on any of the fracture surfaces. Maintenance records indicated that the engine was overhauled on January 12, 2010. It was subsequently installed on the accident airplane on January 28, 2010, at a recording tachometer time of 419.5 hours. The records noted three engine oil changes since overhaul. The most recent oil change was completed on February 11, 2010, at 48.4 hours operation since overhaul. The recording tachometer indicated 487.8 hours (68.3 hours since overhaul) at the time of the post-accident examination. Documentation provided by the engine manufacturer noted that the connecting rod nuts should be tightened to 480 inch-pounds at the time of installation. Overhaul records included a Build Up Check Sheet.The sheet included a line item entitled “Rod Nuts Torqued & Safetied.” For this line item, initials had been placed under both the “Technician” and the “Inspector” columns. A representative of the overhaul facility stated that all connecting rod bolts are replaced and installed as new bolts on overhauled engines. He added that rod bolts are procured directly from the engine manufacturer and not from any third-party manufacturers.