What happened
On 9 November 1998, a Bell 20cal 206B Jetranger III, registration G-OCBB, was conducting a private training flight at Oxford Airport. The flight involved an instructor and a student performing various exercises and emergency procedures. During the session, the crew performed an autorotation from 750 feet. While in the autorotative state, the instructor observed a decline in rotor RPM to 90% and, suspecting a potential issue with the freewheel unit, attempted a power recovery at approximately 350 feet.
As the throttle was adjusted, a loud bang and significant vibration occurred, followed by the activation of the engine failure warning system. The instructor took control of the aircraft and performed a successful autorotative landing. There were no injuries to the two crew members on board.
The investigation
The investigation involved a review of the pilot's report and a metallurgical analysis of the engine components conducted by the manufacturer. The engine, powered by an Allison 250-C20 turboshaft, had suffered an uncontained failure of the No 1 turbine disk, which caused damage to the compressor discharge tube, engine ancillaries, and the cowling.
Investigators examined the turbine components in a laboratory setting. Additionally, the aircraft's freewheel unit was inspected during a scheduled maintenance procedure, which revealed no faults. The investigation also looked into the aircraft's Turbine Outlet Temperature (TOT) indicator, which was intended to record overtemperature events via a tell-tale light.
Findings
- The primary cause of the engine failure was an uncontained failure of the first stage turbine disk.
- Metallurgical examination revealed that the failure was caused by accelerated interdendritic creep crack growth.
- This damage resulted from the engine being exposed to extreme temperatures exceeding 1200°C, which is above normal operating limits.
- Evidence suggested the engine had experienced one or more hot starts.
- The aircraft's tell-tale light system was non-functional because the specific hardware and wiring required for the indicator to operate were not installed on this older model.
Safety action
Following the incident, the operator implemented Bell Technical Bulletin 206-82-77 to install the necessary hardware and wiring to ensure the overtemperature recording system was fully operational.