What happened
On June 24, 1998, at approximately 17:00, a Cessna 170A, registration PT-ADB, was performing a promotional flight near Campos do Jordão, São Paulo. The mission involved towing a banner over the city. After departing from Fazenda Vale Eldorado, the aircraft proceeded toward Pindamonhangaba to attach the banner before continuing to its destination.
Witnesses observed the aircraft flying over a valley when they heard an abnormal engine noise followed by smoke. The aircraft then banked sharply to the left and struck a hillside, which resulted in a post-impact fire. The pilot was killed in the accident, and the aircraft was destroyed.
The investigation
CENIPA investigators examined the wreckage and the engine, a Continental O-300-A. The investigation revealed that the engine showed no signs of sudden internal mechanical failure, such as broken gears or commands, suggesting the issue was related to the fuel or ignition systems. However, investigators found that the exhaust manifolds, which serve as outlets for carburetor heat, were obstructed by plugs.
Furthermore, the investigation noted that the ignition system utilized at least two different types of spark plugs. While the pilot's flight instruments and medical certificates were valid, his instrument rating had expired. Witnesses also reported that the pilot frequently used automotive gasoline, which has a lower octane rating than aviation fuel, as a substitute for proper aviation gasoline.
Findings
- Improper maintenance: The engine had undergone unauthorized modifications, specifically the blocking of the carburetor heat outlets. This, combined with the use of mismatched spark plugs, contributed to the engine's failure.
- Carburetor icing: Given the low ambient temperature (approximately 13°C) and the blocked heat outlets, there was a high probability of carburetor icing, which restricted fuel delivery.
- Inadequate supervision: As the owner and operator of the aircraft, the pilot's potential knowledge of the unauthorized engine modifications indicated a lack of proper oversight.
- Human factors: Evidence suggested a degree of overconfidence in the pilot, evidenced by the use of improper fuel and the acceptance of unauthorized engine modifications.
Safety action
CENIPA issued recommendations for flight schools, aero clubs, and maintenance workshops to inspect Cessna 170 and similar aircraft to ensure carburetor heat inlets are unobstructed, particularly in regions prone to icing. Additionally, regulatory bodies were directed to increase inspections of maintenance facilities and to promote awareness regarding the risks of unauthorized engine modifications and the use of improper fuels.