Socata TB21 N718TB accident near San Marcos, California

Probable cause

The failure of the mechanical fuel pump due to internal friction for reasons that could not be determined based on available evidence, which resulted in a total loss of engine power.

NTSB analysis

According to the pilot, he had just picked up the airplane after an annual inspection. He conducted a pattern-only flight before departing for a cross-country flight to reposition the airplane. About 30 minutes after takeoff, there was a total loss of engine power and the pilot performed a forced landing on a road. The airplane struck a vehicle during landing, which separated the left wing from the fuselage.

Postaccident examination of the engine revealed the mechanical fuel pump shaft had sheared. Further disassembly revealed rotational scoring, heat discoloration, and foreign (aluminum) debris on the pump’s rotor shaft and bearing bore. The scoring and heat discoloration were determined to have occurred due to friction occurring between the rotor shaft and bearing bore. Mechanical damage to the affected components did not allow for the determination of pre-event component measurements, including the rotor shaft or the bearing dimensions.

The mechanical fuel pump was a recently rebuilt unit and was installed on the engine during the annual inspection that was completed five days before the accident. The overhaul procedures for the fuel pump specify the bearing to be honed and the rotor to be lubricated during reassembly. Honing to incorrect dimensions or improper lubrication would likely result in increased friction between the two components. Debris between the rotor and bearing could also increase friction. The rubber seal installed at the driven end of the rotor exhibited softening and deformation consistent with thermal distress, which could have allowed debris to enter the fuel pump after the pump began to overheat. Additionally, debris could have been introduced during the rebuild process or during the installation process through the fuel line ports and fittings, but due to the friction damage the source of the debris could not be identified. The fuel pump shaft sheared due to the friction between the rotor shaft and bearing bore; however, damage prevented determination of the source of the friction.

Text from the NTSB. Open the NTSB docket for WPR22LA259 for the full record.