According to the pilot, while in cruise flight, he noticed an increase in cylinder head temperature on two cylinders in the Mooney M20K. He attempted to reduce the temperatures, however, he soon heard “an explosion,” and noted a hole in the engine cowling.
He made a forced landing to a golf course near Paramus, N.J. During the landing roll, the airplane hit a berm, became airborne again, and hit the ground, resulting in the landing gear collapsing and substantial damage to the right wing. The pilot and one of his passengers received minor injuries.
The engine was examined at the manufacturer’s facility under NTSB supervision. The examination revealed that the engine case was breached over the No. 2 cylinder. The oil sump was removed and metallic debris was noted. The oil filter was removed and disassembled, and metal was noted in the filter. The oil pump was examined and scoring was noted inside the pump housing. The crankcase was split and the No. 1 connecting rod was separated from the crankshaft, and the Nos. 1 and 2 connecting rods exhibited thermal discoloration. Further examination of the mating surfaces of the main bearing saddles had remnants of silk thread around the through-bolt holes.
The break-away torque was measured for the through-bolts on the engine. Eight out of the 12 through bolt torque values were below the manufacturer’s recommended torque values. The through bolts associated with the Nos. 2 and 3 main bearings and measured between 172 in-lbs and 395 in-lbs, which was below the manufacturer specification of 590-610 in-lbs of torque. The No. 2 main bearing saddles exhibited impingement damage and the No. 2 bearing was partially rotated clockwise, which blocked the oil passage to the connecting rod bearings.
The engine manufacturer published a Service Information Letter (SIL), “Sealants, Lubricants, and Adhesives Authorized by CMI” in 1999 (revised Sept. 16, 2014). This guidance, which was applicable at the time of the engine’s overhaul, stated that when reassembling the crankcase halves, maintenance personnel should “…apply and position P/N 641543 (a continuous, single piece of grade ‘D’ silk thread on the 2-4-6 case half as specified (see Figures 5 through 8…). Ensure the free ends of your thread are covered by gaskets (except the nose oil seal).”
The Continental Aircraft Engine Maintenance Manual, “Six Cylinder Engine Crankcase Threading,” which was released in April 2016, just after the engine overhaul, stated, “Do not apply any form of sealant to the crankcase cylinder deck, chamfer, cylinder mounting flange, cylinder base O-ring, or cylinder fastener threads. The use of RTV, silicone, Gasket Maker or any other sealant on the areas listed above during engine assembly will cause a loss of cylinder deck stud or through-bolt torque. Subsequent loss of cylinder attachment load, loss of main bearing crush and/or fretting of the crankcase parting surfaces will occur. The result will be cylinder separation, main bearing movement, oil starvation and catastrophic engine failure.”
Probable Cause: Maintenance personnel’s improper assembly of the engine during overhaul, which resulted in lubrication restriction due to a bearing shift and the subsequent engine failure.
This December 2018 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.
I wouldn’t call this a maintenance error. It’s actually an incorrectly assembled engine.
The materials used caused a loss of torque on the through bolts and caused 1 or 2 spun main bearings, assuming that they were properly torqued originally.
I am surprised that the torques were not checked after the first few hours; the through bolts and the base nuts.