Accident Overview
On August 20, 2022, at about 1800 mountain daylight time, a Beech F33A (registration N4133S) was substantially damaged during a forced landing near Wilder, Idaho. The pilot and passenger sustained minor injuries. The flight was conducted as a personal flight under Title 14 Code of Federal Regulations Part 91.
Maintenance History
Prior to the flight, the pilot and a mechanic had performed maintenance to address an intermittent engine shudder. They checked cylinder compression, replaced spark plugs, cleaned fuel injectors, and confirmed engine timing before deciding on a test flight.
Engine Failure and Forced Landing
The engine runup and takeoff were uneventful, with all engine instrument gauges indicating nominal values during initial climb. Shortly afterward, the engine shook violently and stopped producing power. The pilot executed a forced landing into a field, during which the airplane sustained damage to the lower forward fuselage and both wings.
Engine Examination
The engine was a Continental Motors IO-520-BA, overhauled by T.W. Smith Engine Company in 1998 (about 950 flight hours before the accident). No maintenance requiring crankcase opening or cylinder removal had been performed since the overhaul. Examination revealed a large hole in the upper rear crankcase adjacent to the Nos. 1 and 2 counterweights. Both counterweights had separated from their crankshaft blades. One counterweight mounting hole was intact with its bushing in place; the other was split open with material formed outward. The Nos. 1 and 2 connecting rod caps had detached, but the rods remained attached to their pistons. Fragments of connecting rod caps, bolts, counterweights, bushings, a single pin, and four retaining rings and plates were found in the oil sump. Fracture surfaces showed features consistent with ductile overload, including granular and rough separation areas, bolt necking, and impact damage. No scoring or heat distress was observed on the rod journals or bearings for cylinders 1 and 2.
Upon complete disassembly, the counterweights for cylinders 3 and 4 were intact and undamaged, but one retaining ring, retainer plate, and pin were missing.
Counterweight Installation Findings
The Continental Maintenance Manual specifies that retaining rings, which are stamp-cut with a sharp side and a beveled side, should be installed with the sharp side out. The retaining ring ears should be oriented toward the crankshaft centerline, and the gap between the ears should be no less than 0.179 inches. Examination of the remaining counterweights revealed that one retaining ring on each was installed incorrectly with the sharp side in, and the ear gaps measured 0.11 and 0.14 inches, both below the specified minimum.
Continental Service Bulletin M93-4, issued in February 1993, emphasizes the importance of thorough inspection and proper assembly of crankshaft counterweights during engine overhaul or anytime the crankshaft is removed. It warns that improper installation of bushings, retaining plates, or retaining rings will cause engine failure, and states that retaining rings should be fully seated with the sharp edge facing out.