What happened
On April 26, 1979, an Aero Commander 680 FL, registration 7T-VBA, was conducting an IFR flight from Zurich to Marseille when the crew encountered significant engine issues. During the initial climb, the pilot noticed fluctuating fuel pressure in the right engine. While cruising at FL 110, the pilot deactivated the right booster pump, which caused the fuel pressure to drop to zero and resulted in a loss of power in the right engine. The pilot immediately reactivated the pump to stabilize the engine and decided to return to Zurich.
During the subsequent descent through icing conditions, the right engine failed completely. In an attempt to manage the situation, the pilot activated the left booster pump as a precaution, but shortly thereafter, the left engine also began experiencing severe fuel pressure fluctuations and power loss. Unable to maintain the assigned altitude of 4000 ft due to insufficient power, the pilot performed an emergency landing with the landing gear retracted in a soft field near Schneisingen. There were no fatalities among the three crew members, though the aircraft sustained damage to the fuselage and the landing site was impacted.
The investigation
SUST examined the aircraft's maintenance history and the condition of the fuel system. The investigation revealed that a 100-hour inspection had recently been performed by Jet Aviation AG in Zurich, which included replacing and sealing rubber fuel tanks. Investigators analyzed the fuel filters and found significant contamination. The left filter contained a much higher concentration of coarse debris than the right, including textile fibers (wool, cotton, and synthetic), wood particles, paint flakes, and fine metallic particles, as well as dust and quartz sand.
Chemical analysis of the fuel samples from the central tanks revealed a fuel-water emulsion containing residues of detergents, silicates, and various mineral particulates. Furthermore, the investigation found that the fuel filters and the central tank sump had not been cleaned following the recent maintenance work. The investigation also noted that a suction strainer was missing from the right auxiliary tanks.
Findings
- The primary cause of the engine failures was contamination of the fuel filters resulting from inadequate cleaning of the fuel system following recent maintenance.
- The right engine failure and the subsequent power loss in the left engine were both caused by fuel flow obstructions due to the debris in the filters.
- Maintenance performed during the 100-hour inspection was found to be substandard, as the fuel system was not properly cleaned after the replacement and sealing of the rubber tanks.
- The pilot's decision to return to Zurich and perform an emergency landing with the gear retracted was deemed appropriate given the circumstances.