Incident Overview
On 9 November 1998 at 1550 UTC, a Bell 206B Jetranger III (registration G-OCBB) operated as a private training flight at Oxford Airport. The aircraft, manufactured in 1973 and powered by one Allison 250-C20 turboshaft engine, was crewed by two pilots: an instructor and a student. Both occupants evacuated the aircraft without injury after a successful autorotative landing.
Preceding Events
Three days before the incident, a high-frequency noise or vibration had been reported. On the day of the flight, the aircraft underwent an extensive check with no fault found. The instructor was informed by an engineer that the freewheel unit was suspected as the source of the noise. The flight involved dual training exercises, and the instructor kept the aircraft near the airfield in anticipation of possible issues.
Flight and Engine Failure
After approximately 30 minutes of flying, the first autorotation was performed from 750 feet AGL. During the autorotation, the instructor noticed rotor RPM decreasing and confirmed the student had fully lowered the collective lever. Seeing rotor RPM at 90% and suspecting freewheel unit trouble, the instructor initiated a power recovery at about 350 feet. As soon as the throttle was moved, a loud bang and shudder occurred, accompanied by an "ENGINE OUT" light and horn. The instructor took control and executed a successful autorotative landing, then called a MAYDAY, shut down the aircraft, and both occupants vacated normally.
Post-Accident Examination
The engine suffered an uncontained failure of the No 1 turbine disk. The engine was dismantled in the presence of a manufacturer's representative. Gas producer turbine components, including remaining parts of the No 1 disk, were sent to the manufacturer's metallurgical laboratory. The freewheel unit was inspected during subsequent aircraft repair but found to be fault-free.
Metallurgical Findings
The engine manufacturer reported that the first stage turbine disk failure resulted from accelerated interdendritic creep crack growth caused by exposure to extremely high temperatures—greater than 1200°C, as shown by microstructural evidence. Examination of the No 2 disk's crystalline microstructure indicated it had been heated above 1150°C. These temperatures exceeded normal operating ranges and were consistent with the engine having experienced one or more hot starts.
Aircraft Equipment Deficiency
The Turbine Outlet Temperature (TOT) indicator in the aircraft had provision for a "tell tale light" to register and record hot start or overtemperature occurrences, but the light was not operational. Early aircraft like G-OCBB were delivered without the full system required for the tell-tale light to function; only later serial numbers (3567 and subsequent) had the system operational. After the incident, the operator implemented Bell Technical Bulletin 206-82-77 to install the necessary hardware and wiring to activate the system.