What happened
On November 8, 2008, a Robinson R44 helicopter, registration PP-MSD, was performing a local flight departing from the Paraíso do Tocantins (SWTO) aerodrome under visual flight rules. During the return leg of the flight, the engine experienced a sudden loss of power. The pilot was forced to execute an emergency landing via autorotation in an open field approximately five kilometers from the aerodrome.
Immediately following the touchdown, a fire broke out on the left side of the aircraft. The pilot successfully evacuated the helicopter before the flames spread, escaping without injury. However, the aircraft was completely consumed by the fire.
The investigation
CENIPA investigators examined the wreckage and the engine, a Lycoming O-540-F1B5. Although the intense heat caused significant melting and combustion marks on the engine and its components, the engine was found to be capable of rotating freely on its shaft, with no evidence of internal seizure.
Detailed inspection of the Marvel-Schebler MA-4-asc model carburetor revealed that the float and acceleration valve had melted. Investigators also consulted maintenance facilities regarding the use of specific carburetor components. It was noted that while airworthiness directives allowed for the use of advanced polymer floats (P/N 30-802 or 30-804), many maintenance shops were proactively replacing them with metal floats due to reliability concerns.
Findings
- The engine suffered a loss of power during flight, necessitating the forced landing.
- The fire, which destroyed the aircraft, may have been ignited by contact between hot engine components and dry grass, or by a fuel leak caused by the impact of the landing.
- The potential jamming of the advanced polymer float within the Marvel-Schebler MA-4-5 carburetor was identified as the most plausible cause of the engine power loss. Evidence suggested that the flexibility of the polymer float's connecting rod and its asymmetric shape could cause the float to strike the internal wall of the carburetor bowl, leading to improper fuel regulation.