Accident Overview
On October 8, 2021, at approximately 1904 Pacific daylight time, a Beech B36TC airplane, registration N7220B, was substantially damaged during an accident near Los Banos, California. The pilot, who was the sole occupant, reported no injuries. The flight was conducted under Title 14 Code of Federal Regulations Part 91 as a personal flight.
Flight History
The pilot had flown the airplane to Paso Robles Municipal Airport (PRB) in Paso Robles, California, for refueling and was returning to Gnoss Field Airport (DVO) in Novato, California, when the accident occurred. While cruising at about 10,000 ft mean sea level, the engine lost oil pressure and manifold pressure dropped. The pilot immediately diverted to Los Banos Municipal Airport (LSN). During the descent, the engine initially produced partial power but then exhibited severe vibrations and lost all power. The cockpit filled with smoke, and fire and sparks were observed coming from the engine cowling.
Forced Landing
Due to the fire, the pilot elected to conduct a forced landing on a road near the approach end of the runway. During the landing roll, the airplane struck an object, causing it to spin around before coming to rest.
Engine Data Analysis
Review of engine data from a Garmin G500TXi device revealed that the airplane had leveled off with the engine set to about 2,500 rpm. Approximately 9 minutes later, engine oil pressure began to decrease while engine rpm remained unchanged. Over the next 12 minutes, oil pressure continued to decrease, culminating with engine rpm dropping from about 2,371 to zero in about 5 seconds while the airplane was descending.
Postaccident Examination
Examination of the engine revealed external damage, including an exit hole on the crankcase near the top of the No. 2 cylinder, measuring about 2 inches by 3 inches, and a second hole on the left back side with a diameter of about 1 inch. Large cracks were observed on both crankcase halves. All engine accessories on the back side remained attached except for the left magneto. Rocker covers were removed and showed evidence of heat distress on all cylinders. Rotational continuity could not be established due to crankshaft damage.
Disassembly found a missing No. 4 connecting rod bolt nut. Thermal distress was observed where the No. 4 connecting rod connected to the crankshaft, consistent with oil starvation; this area showed the most prominent thermal distress. Considerable metal debris was noted in the oil sump and oil filter.
Maintenance History
The engine was installed in July 2012 and had logged about 671 hours of time before the accident. In June 2020, the engine was disassembled for a propeller strike inspection, about 27.3 hours before the accident. During that inspection, the connecting rods were overhauled by a third-party vendor, who reinstalled them on the crankshaft using new bearings, rod bolts, and nuts. The most recent annual inspection was completed in September 2021, about 2.3 hours before the accident.