N4133SBeech F33A · 1975

On August 20, 2022, about 1800 mountain daylight time, a Beech F33A, N4133S was substantially damaged when it was involved in an accident near Wilder, Idaho. The pilot and passenger sustained minor injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

The pilot and a mechanic had been performing maintenance, attempting to resolve a problem where the engine would intermittently shudder during flight. After checking the cylinder compression, replacing a set of spark plugs, cleaning the fuel injectors, and confirming the engine timing, they decided to perform a test flight.

The engine runup and takeoff were uneventful, and during the initial climb they monitored the engine instrument gauges, and all indicated nominal values. A short time later, the engine shook violently, and stopped producing power. They performed a forced landing into a field, and the airplane sustained damage to the lower forward fuselage and both wings.

The airplane was powered by an IO-520-BA engine manufactured by Continental Motors, and overhauled by T.W. Smith Engine Company in 1998, about 950 flight hours before the accident. During the 14-year period leading up to the accident, routine maintenance was performed on the engine, with no work that would have required opening the crankcase or removing any cylinders.

The engine was equipped with two sets of moving counterweights mounted to crankshaft blades adjacent to the journals of cylinder Nos. 1 and 2, and 3 and 4. Each counterweight was attached to their respective crankshaft blade by two pins housed within bushings and held in place at their ends with retaining plates and internal retaining “snap” rings.

Examination of the engine revealed a large hole in the upper rear section of the crankcase, next to the Nos. 1 and 2 counterweights. Both counterweights had separated from their respective blades and showed similar damage signatures, with one counterweight mounting hole intact with its bushing in place, and the other split open with its material formed outwards. The Nos. 1 and 2 connecting rod caps had detached, and the rods remained attached to their respective pistons. Remnants of the connecting rod caps and bolts, along with two counterweights, fragments of counterweight bushings, a single counterweight pin, and four retaining rings and plates were found in the oil sump. Examination of the fracture surfaces of the remnants revealed features consistent with ductile overload, including granular and rough separation areas, bolt necking along with smear and bending damage consistent with impact. There was no evidence of scoring or heat distress to the rod journals or rod bearings contact surfaces for cylinder Nos. 1 and 2.

The engine was then completely disassembled, and although the two counterweights for cylinder Nos. 3 and 4 were intact and essentially undamaged, one retaining ring, retainer plate, and pin were missing.

The Continental Maintenance Manual applicable to the engine gives specific instructions for the installation of the counterweight assembly. It states that the retaining rings, which are stamp-cut, have a sharp side and a beveled side, and should be installed with the sharp side out. The retaining ring ears should be installed towards the crankshaft centerline, and to confirm proper fitment after installation the gap between the ears should be no less than 0.179 inches (Figure 1).

Figure 1 – One of the counterweights with its retaining plates and rings, showing the incorrectly installed ring on the right.

Examination of the engine revealed that one retaining ring on each of the remaining counterweights was installed incorrectly with the sharp side in, and both had ear-gaps of 0.11 (Figure 1) and 0.14 inches respectively.

Continental Service Bulletin (SB) M93-4, issued in February 1993, reiterates the importance of thorough inspection and proper assembly of crankshaft counterweights during engine overhaul, or any time the crankshaft is removed from the engine. It warns that improper installation of the bushings, retaining plates, or retaining rings will cause engine failure. The SB also states that the retaining rings should be fully seated in the bushing bore grooves with the sharp edge facing out.