Accident Details
On July 16, 2022, at 1817 Pacific daylight time, a Socata TB-21, registration N718TB, was substantially damaged during an accident near San Marcos, California. The pilot and passenger were not injured. Two occupants of a vehicle on the ground received minor injuries. The flight was conducted as a personal flight under 14 Code of Federal Regulations Part 91.
Flight History
The pilot had picked up the airplane after an annual inspection and refueled it with about 56 gallons of fuel. A pattern-only flight limited to one approach and landing was conducted due to operational constraints imposed by the local tower. After landing, the pilot shutdown, picked up a passenger, and departed for a cross-country flight to Fullerton Municipal Airport (FUL) to reposition the airplane. Approximately 30 minutes after takeoff, while cruising at 6,500 ft mean sea level above a heavy marine layer, the engine lost power. The pilot attempted to navigate toward the edge of the marine layer and divert to a nearby airport but could not reach the runway, leading to a forced landing on a road. During the landing, the airplane struck a vehicle, which resulted in the separation of the left wing from the fuselage. Two occupants of the vehicle sustained minor injuries.
Post-Accident Examination
The wreckage was recovered to a secure location, and an engine run was attempted. The engine would not start and run unless the electric fuel pump was turned on. The mechanical fuel pump was removed from the engine. During removal, it was noted that the pump’s driveshaft had sheared. Disassembly of the fuel pump revealed rotational scoring, heat discoloration, and debris on the rotor shaft and bearing bore.
Laboratory Analysis
Fuel pump components and debris were sent to the National Transportation Safety Board (NTSB) Materials Laboratory for examination. The scoring and heat discoloration were determined to have occurred due to friction between the rotor shaft and bearing bore, which resulted in the fuel pump drive coupling driveshaft shearing by torsional overstress. Mechanical damage to the affected components prevented determination of pre-event dimensions.
Scanning electron microscope (SEM) analysis with energy-dispersive X-ray spectrometer (EDS) showed the rotor material primarily consisted of iron with small alloy additions of chromium, aluminum, and silicon. The scored region exhibited primarily aluminum and silicon with sulfur and iron. Foreign debris on the carbon bearing showed iron, carbon, and sulfur peaks. Several ports on the pump had metal shavings or burrs. A shaving from a plugged port consisted primarily of aluminum and silicon, with a trace of zinc. The pump housing surfaces showed no notable mechanical damage. The rubber seal at the driven end of the rotor exhibited softening and deformation consistent with thermal distress.
Maintenance History
The mechanical fuel pump had been overhauled on May 19, 2022, and was installed on the engine during the annual inspection dated July 11, 2022. Overhaul procedures include removal and replacement of the liner and bearings, cleaning, heating the housing, pressing in new liner and bearing, honing the bearing to final size, and installing a seal. The rotor installation instructions call for a light coat of petroleum jelly on the bearing journal and grease on the drive coupling tongue and groove.