What happened
On March 2, 1977, a Cessna 188 AgTruck, registration CS-AOG, was performing agricultural spraying operations at Herdade do Peso, near Vidigueira, Portugal. The aircraft, operated by SANAGRI, was executing a turn to begin a second spray pass when the engine experienced a partial power failure.
As the aircraft lost altitude, the pilot attempted emergency procedures, including activating the emergency switch, enriching the fuel mixture, and applying full throttle. Despite these efforts, the aircraft struck a mound of rocks on the ground. The impact caused significant damage to the left wing, the horizontal stabilizer, and the elevator. The spray boom on the left side detached during the impact and was left at the site. Following the strike, the pilot managed to maintain enough control to continue the flight and land safely at the nearby operating strip. There were no fatalities and the pilot was uninjured.
The investigation
The investigation focused on identifying the cause of the engine's power loss. Maintenance technicians performed a ground run at the accident site, which revealed significant fluctuations in engine RPM and a low fuel flow rate. Specifically, under normal flight settings, the fuel flow failed to reach the expected 16–17 gallons per hour, instead peaking at only 12 gallons.
Investigators examined the fuel supply and found that the AVGAS 100LL stored in drums at the Évora airfield contained a significant amount of ferrous deposits. While fuel filters were inspected and found to be normal, the presence of contaminants was noted. Additionally, laboratory testing of the fuel components was conducted by OGMA (Oficinas Gerais de Material Aeronáutico). While some components of the fuel injection system, such as the distributor and control unit, were cleared of involvement, there was an investigation into the fuel pump's pressure regulation screw, which showed damage, though it could not be definitively proven to be the primary cause.