What happened
On August 30, 2013, a Bell 206B helicopter, registration PT-HUE, was conducting a flight over a fish farming area near Matões do Norte, Brazil. During the flight, the pilot observed the engine chip light illuminate, followed immediately by a loss of engine power.
The pilot initiated an emergency landing via autorotation into a pasture field alongside a highway. During the landing flare, the aircraft touched the ground with the rear of the skids while in a nose-high attitude. This sudden change in attitude caused the main rotor blades to flex and strike the tail boom, severing the tail cone. Despite the substantial damage to the aircraft, the pilot and the passenger escaped without injury.
The investigation
CENIPA investigators examined the engine, a Rolls-Royce/Allison 250-C20J, which was sent to maintenance facilities in Brazil and the United States for disassembly and analysis. The investigation focused on the cause of the power loss and the mechanics of the impact.
Technical examinations revealed that a specific component, the 2½ bearing, had failed due to localized wear and concentrated load. Crucially, investigators discovered that the retaining ring for this bearing was missing from the assembly. This absence allowed the bearing to move axially, leading to misalignment, friction-induced heating, and the eventual disintegration of bearing rollers. These rollers then damaged the fuel control unit's drive gear, contributing to the engine's power loss.
Findings
- Maintenance error: The primary cause of the engine failure was the failure to install the retaining ring for the 2½ bearing during a previous maintenance service, which allowed the component to displace and fail.
- Flight control inputs: The severity of the aircraft damage was exacerbated by improper use of flight controls during the landing flare, which caused the main rotor blades to strike the tail boom.
- Organizational oversight: The investigation identified potential failures in maintenance management and supervision, as the missing component was not detected by the technician or the inspector during subsequent service intervals.
Safety action
CENIPA issued several recommendations to the National Civil Aviation Agency (ANAC), including:
- Conducting an audit of the maintenance organization responsible for the engine service to verify compliance with assembly procedures.
- Encouraging the operator to enhance autorotation training for pilots.
- Promoting the dissemination of findings to alert maintenance personnel and inspectors to the importance of strict adherence to assembly and supervision protocols.