What happened
On 3 November 2000, a Cessna 425, registration ZS-KSU, was conducting a private flight from Zimbabwe to Lanseria. While the aircraft was on its base leg for runway 06 Left at Lanseria aerodrome, the left-hand engine suffered a catastrophic failure. This mechanical breakdown triggered an in-flight engine fire. Despite the emergency, the pilot managed to land the aircraft safely at the aerodrome. Ground fire services arrived promptly to extinguish the flames, and no injuries were reported among the pilot or the three passengers on board.
The investigation
Following the incident, the engine was transported to an approved maintenance facility, where it underwent a complete disassembly on 6 November 2000. An external inspection of the unit revealed that the power turbine (free turbine) had partially shed several blades, which were then ejected through the exhaust system. This process caused physical damage to the engine, specifically puncturing one of the scavenge pump oil return lines. Further internal examination of the engine's components identified that a failure occurred in one of the three planet gears within the first-stage reduction gearbox, which ultimately led to the gearbox seizing.
Findings
- The primary mechanical failure was the seizure of the first-stage reduction gearbox due to a failed planet gear.
- The loss of turbine blades caused an in-flight fire and subsequent damage to the oil return lines.
- Investigators noted uncertainty regarding whether cavitation erosion played a primary role in the bearing failure or acted merely as a contributing factor.
- There is a possibility that a sleeve bearing became dislodged from its press-fitted position, leading to rotation with the shaft and subsequent failure.