What happened
On September 7, 1996, a Cessna 172, registration PT-JLV, departed from the Santa Cruz do Sul Aerodrome for a local flight. The aircraft was carrying four occupants. Shortly after takeoff, while the pilot was performing a left turn to enter the downwind leg at approximately 500 feet, the engine began to run irregularly and subsequently failed.
The pilot attempted to restart the engine but was unsuccessful. In an effort to perform a forced landing, the pilot targeted a plowed field; however, during the maneuver, the aircraft struck a tree with its wing and hit the ground, resulting in a nose-over. All four occupants sustained light injuries, but the aircraft suffered severe damage.
The investigation
CENIPA investigators examined the engine's magneto plugs and the fuel system. The plugs for cylinder number one showed heavy carbonization, a condition likely exacerbated by the aircraft idling at the holding point for ten minutes prior to takeoff with a rich mixture.
Crucially, the investigation focused on the fuel selector valve, which was found to be stuck. Upon disassembly, investigators discovered that the spheres (P/N S272-209) within the valve were corroded and had an altered profile. The diameter of the actuating cam was 2.72% smaller than standard, which allowed the spheres to displace from their housings and obstruct the movement of the valve. Laboratory analysis revealed that these spheres contained elements (chlorine and sulfur) that made them highly susceptible to corrosion, even when in contact with fuel. Furthermore, the investigation determined that these specific spheres were not standard aeronautical components and were likely sourced from a non-certified machine shop.
Findings
- The use of non-certified components in the fuel selector valve caused the fuel flow to the engine to be interrupted.
- The fuel selector valve was prone to random locking due to the presence of corroded, non-standard spheres.
- The engine's spark plugs for cylinder number one were heavily carbonized due to prolonged idling at a rich mixture setting before takeoff.
- The pilot's decision-making and control during the emergency were adequate, and no human factors contributed to the accident.