What happened
On 3 May 2022, a Piper PA-25-235 was conducting agricultural spraying operations near Albert Falls, KwaZulu-Natal. The flight, operated under Part 137 regulations, was performing routine swath lines over a private farm. While climbing to align for a sixth spray pass at approximately 100 feet above ground level, the engine began to vibrate abnormally and lost power.
The pilot attempted to mitigate the issue by adjusting the mixture and applying carburettor heat, but the engine continued to run rough. To avoid obstacles, the pilot executed a right turn toward an open field to perform a forced landing. During the landing sequence, the aircraft traveled approximately 6 and 60 metres along the ground before the propeller struck a perimeter fence. The impact caused the aircraft to flatten the fence and ground-loop into nearby bushes. While the aircraft sustained substantial damage, the pilot escaped without injury.
The investigation
An investigation by the SACAA AIID focused on the mechanical state of the engine and the maintenance history of the aircraft, registration ZS-AWX. Investigators examined the engine following the crash and discovered that the left magneto drive gear had failed. Specifically, a tooth on the gear had sheared, which caused the engine timing to advance.
Further inspection of the hardware revealed that the left magneto drive gear shaft was significantly worn. The wear was so extreme that the gear could be removed from the shaft by hand, despite being a press-fit component. This excessive play between the drive gear and the crankshaft idler gear led to the mechanical failure during flight.
Findings
- The engine power loss was caused by the failure of the left magneto drive gear.
- The failure originated from a worn-out drive gear shaft that had reached its wear limits.
- The mechanical failure resulted in advanced engine timing, leading to the engine running rough and losing power.
- The aircraft's maintenance history showed that while the magnetos had been inspected recently, the condition of the drive gear shaft itself was not adequately assessed before being refitted to the engine.
Safety action
Following the investigation, a safety message was issued to Aircraft Maintenance Organisations (AMOs) and engineers. They are advised to specifically inspect the security and fitment of the magneto drive gear shaft and bearing before any magneto is refitted to an engine to prevent similar failures.