ERA15LA014

Maintenance personnel’s failure to apply proper torque to the cylinder flange nuts and through bolts, which resulted in shifting of the Nos. 1 and 2 main bearings, loss of lubrication, failure of the crankshaft, and a subsequent total loss of engine power.

The private pilot and passenger were conducting a cross-country flight. During the descent from cruise altitude to the destination airport, the airplane began to shake violently. The pilot heard a "loud noise," and the propeller stopped rotating then began slowly windmilling. The engine was not producing power. During the subsequent descent, the pilot realized that the airplane would not reach the airport and decided to conduct an off-airport landing. The ground was soft and sandy, and during the landing roll, the airplane nosed over and came to rest inverted. Examination of engine maintenance records revealed that the Nos. 3 and 4 cylinder assemblies had been removed and reinstalled about 150 flight hours before the accident. During a postaccident examination of the engine, the cylinder flange nuts and through bolts were checked for security and breakaway torque. One of the nuts attached to one of the hold-down studs for the No.1 cylinder was loose, with a significant gap between the nut and the cylinder flange. The breakaway torque values for each of the No. 2 cylinder through bolts was about 200 in-lbs less than that of the through bolts on the other cylinders. Disassembly of the engine revealed mechanical damage in multiple locations inside the crankcase. The No. 1 main bearings had shifted slightly out of the support and displayed deformation. The No. 1 bearing support mating surfaces also displayed fretting, as well as lock tab elongation. The No. 2 bearing support displayed damage consistent with a bearing shift and a spun bearing event. The area around the No. 2 bearing support through bolt holes displayed a significant amount of fretting between the mating surfaces. The crankshaft was fractured at the crankshaft cheek between the No. 2 main journal and the No. 2 connecting rod journal. The Nos. 2 and 3 connecting rod journals displayed incipient signatures of lubrication distress. Further examination of the crankshaft using a magnaflux booth also revealed a crack along the bend radius of the No. 1 journal. If a bearing shifts for any reason after removal of one or more cylinders, or during replacement of one or more cylinders, the oil holes in the bearing can become misaligned with the oil passages in the crankcase, partially cutting off the oil supply to the bearing. If the engine is then put back into service, the reduced oil supply to the bearing can cause increased heat and friction that can result in the bearing shifting more, further reducing its oil supply. Eventually, the bearing can shift enough to cause the oil supply to be cut off completely, resulting in a catastrophic failure. This is usually referred to as a "spun bearing." To prevent a spun bearing, proper cylinder installation requires a multiple-step torqueing process. The process requires that the through bolt nuts be torqued on both sides of the engine, even if only one cylinder is being installed. A manufacturer service bulletin warned that, "Failure to torque through bolt nuts on both sides of the engine can result in a loss of main bearing crush with main bearing shift and subsequent engine failure." Given the observed damage, it is likely that maintenance personnel did not apply sufficient torque to the cylinder flange nuts and through bolts during installation of the Nos. 3 and 4 cylinders, which resulted in shifting of the Nos. 1 and 2 main bearings, loss of lubrication, and failure of the crankshaft.

According to Continental Motors Service Bulletin SB03-3, cylinder removal regardless of compression should not be done unless a borescope inspection reveals a definite problem. Removal of a cylinder requires removal of a total of eight-cylinder hold-down nuts that secure the cylinder base flange to the crankcase. Six of these hold-down nuts are threaded onto short deck studs that are mounted in threaded holes in the crankcase. The other two hold-down nuts are threaded onto a pair of through-bolts that pass all the way through the crankcase. The left and right banks of cylinders are staggered so that each pair of through-bolts run from the front hold-down nuts on a left-bank (even-numbered) cylinder to the rear hold-down nuts on a right-bank (odd-numbered) cylinder. The engine has four pairs of these through-bolts, and they are primarily responsible for holding the two crankcase halves firmly together. The crankcase contains precisely machined semicircular main bearing saddles that hold the main bearings to support the crankshaft. The main bearings are semicircular shells consisting of steel back plates laminated to bearing surfaces made of a softer material called "babbit" to prevent scratching the crankshaft journals. The main bearing shells have oil supply holes that must line up precisely with oil passages machined into the crankcase halves. When the engine is assembled, the ends of the main bearing shells stick out slightly above the mating surfaces of the crankcase bearing saddles. As the case halves are mated during engine assembly by torqueing the through bolts, the bearing shells are forced firmly into the crank-case saddles in a process known as "crushing." If a bearing shifts for any reason, after removal of one or more cylinders, or during replacement of one or more cylinders, the oil holes in the bearing can become misaligned with the oil passages in the crankcase, partially cutting off the oil supply to the bearing. If the engine is then put back into service, the reduced oil supply to the bearing can cause increased heat and friction that can result in the bearing shifting more, further reducing its oil supply. Eventually, the bearing can shift enough to cause the oil supply to be cut off completely, resulting in a catastrophic failure. This is usually referred to as a "spun bearing." To help prevent a "spun bearing" from occurring, proper cylinder installation requires a multiple step torqueing process. According to Teledyne Continental Motors Service Bulletin SB96-7C, when doing the cylinder torque procedure, the cylinder base stud threads, through bolt threads, and nuts must be lubricated with clean 50 weight aviation oil, and through bolt nuts at cadmium plated washers will require a lower torque value to achieve the same pre-load on the through bolt since the lubricity of the cadmium plating reduces the friction in the fastener joint. The cylinder torque procedure (in summary) also requires that: 1- The torque cylinder base nuts must first be torqued to 1/2 of the specified torque value for the fastener. 2- The cylinder through bolt nuts and then the cylinder base stud nuts must then be torqued to the specified torque value for the cylinder base stud nuts. 3- The through bolt nuts, must be torqued "on both sides of the engine," to the specified value for the fastener, "even if only one cylinder is being installed." 4- Engines which incorporate the 7th cylinder deck stud, to install the 7th stud cylinder bracket and conical stud nut, and then to torque the stud nut to the value specified for the fastener. The Service Bulletin warned that: "Failure to torque through bolt nuts on both sides of the engine can result in a loss of main bearing crush with main bearing shift and subsequent engine failure." Airplane Examination Examination of the airplane revealed that it had been substantially damaged. The engine cowling was bent and dented in several places. One of the propeller blades of the 3-bladed propeller had been bent aft about 15° and a 2nd blade had a visible bend located about 12 inches outboard of the blade clamp. The left lift strut was bent at a 20° angle near the midspan location, and the left aileron was bent in two places. The upper surface of the left wing displayed wrinkling forward of the wing flap, and at the leading edge, forward of the fence which was mounted between the aileron and wing flap. The tops of the vertical stabilizer and rudder were crushed. Engine Examination The engine had remained intact and no impact damage was noted. The externally visible portion of the crankshaft was intact and appeared undamaged. The crankcase was intact; after closer examination, it was noted there was a very small crack near one of the backbone bolts. All the rear accessories remained attached and were undamaged. The exhaust system displayed normal operating signatures and was undamaged. The induction system also remained attached to the engine and there were no signs of induction leaks. The left magneto remained attached to its mounting location and was undamaged. It was noted that one of the ignition harness cap screws was missing. The magneto was placed on a test bench and it produced spark on all six posts. The right magneto remained attached to its mounting location and was also undamaged. The magneto was placed on the test bench and it also produced spark on all six posts. The ignition harness remained intact and undamaged. The spark plugs had remained installed in their cylinders and were undamaged. It was noted that the No. 5 and No. 6 top spark plugs were Autolite spark plugs. All the other spark plugs were manufactured by Champion. All the spark plugs displayed normal operating signatures when compared to Champion Aviation Service Manual AV6-R. The carburetor was undamaged, and the internal components displayed normal operating signatures. the fuel inlet screen was clear of any contaminates. The oil sump was intact and undamaged. The oil sump contained multiple metallic components and particles consistent with bearing material, piston material, and one counterweight pin retaining plate and one snap ring. The oil pick-up tube and screen was undamaged, and there was a small amount of metallic particles noted in the screen. The oil pump was intact and undamaged. The oil pump was disassembled, and it was noted that the oil pump housing displayed scoring consistent with hard particle passage. The oil filter remained secure and undamaged. The oil filter housing was cut open and the filter pleats contained a significant amount of metallic material. The oil cooler was secured to its installation point and was undamaged. The oil cooler was removed and there were no anomalies noted. All the valves were intact and undamaged, and displayed normal operating and combustion signatures. All the rocker arms also remained intact and undamaged, and displayed normal operating and lubrication signatures. All six cylinders had remained attached to their respective cylinder bays. Examination of the cylinders and pistons revealed that: - The No. 1 cylinder was undamaged. It was noted that one of the nuts attached to a hold down stud was loose and there was a significant gap between the nut and the cylinder flange. The cylinder, valve heads, and overhead components displayed normal operating signatures. The piston had remained intact and displayed minor mechanical damage to the backside of the piston and the rear piston ring was broken. The piston face displayed normal operating and combustion signatures. - The No. 2 cylinder was undamaged. It was noted that the breakaway torque on the through bolts was about 200 in-lbs. less than on the rest of the through bolts. The cylinder bore displayed a significant amount of corrosion and the cylinder skirt displayed mechanical damage. The valve heads and overhead components displayed normal operating signatures. The piston display