What happened
On 23 July 2019, a Bell UH-1H helicopter, registration ZS-HLJ, was part of a multi-aircraft deployment for a fire-fighting mission at the Woodbush Forest Reserve. The aircraft departed from Letaba Air Base in Politsi alongside another helicopter and a spotter plane.
Shortly after departing, while climbing through approximately 200 feet, the pilot experienced a noticeable loss of engine power. This was immediately followed by a loud bang originating from the engine compartment. The pilot observed that the engine and rotor RPM were dropping rapidly and the master warning light had activated. After declaring a Mayday, the pilot identified a suitable landing site at the Bosveld Citrus Farm and performed a forced landing. The impact was severe, causing the main rotor blades to sever the upper section of the tail boom and breaking both skids. The pilot escaped the aircraft without injury.
The investigation
SACAA AIID investigators examined the wreckage and engine components to determine the cause of the power loss. Evidence from eyewitnesses noted black smoke emitting from the engine exhaust during the initial takeoff and climb phases.
Technical analysis of the Honeywell T53 engine revealed that a leak had developed in the fuel manifold due to a damaged seal. This leak allowed both the primary and secondary fuel systems to engage simultaneously during the engine start sequence. This malfunction resulted in excessive fuel entering the combustion chamber, creating extreme temperatures during the starting phase.
Findings
- The engine lost power due to a fatigue failure of the first gas producer inner transition liner.
- This failure was driven by a non-uniform interference fit between the transition liner and the combustion chamber liner.
- Excessive temperatures during engine start cycles caused the development of fatigue cracks and oxidation in the first stage nozzle assembly.
- The fuel manifold leak was a primary contributing factor to the thermal damage.
- The pilot successfully executed an emergency landing despite the significant damage to the airframe.