What happened
On the morning of 11 July 2023, a Thrush S2R-H80 aircraft, registration ZS-ARC, was conducting agricultural spraying operations near Malmesbury in the Western Cape. The flight, operated by Orsmond Aviation (PTY) Ltd, began at Rosenburg Farm with the intention of spraying crops at Droeivlei Farm.
During the mission, the pilot experienced a sudden loss of engine power. The propeller blades moved into a feathered position without any input from the cockpit. Despite attempts to rectify the issue by manipulating the propeller limiter circuit breaker, the engine could not maintain altitude. The pilot subsequently executed a precautionary forced landing in a wheat field on a neighboring farm. During the landing, the aircraft's undercarriage struck soft soil and collapsed, causing the nose to pitch down and the propeller to strike the ground. The aircraft sustained substantial damage, though the pilot escaped with only minor injuries.
The investigation
An investigation by the SACAA AIID focused on the mechanical failure of the engine's propeller control system. A teardown examination of the engine revealed that the propeller governor had suffered internal damage. Specifically, contamination within the needle bearing race led to a cracked bearing and subsequent misalignment of the internal components. This misalignment caused the drive shaft and idler teeth to jam, creating a significant shock load.
This mechanical failure resulted in a loss of oil pressure within the propeller control system, which triggered the blades to advance to the feathered position. Investigators also noted that the electronic chip detector, designed to alert the pilot to anomalies in the propeller gearbox or governor, failed to illuminate. Consequently, the pilot received no prior warning of the impending failure.
Findings
- The primary cause of the accident was the loss of engine power during the spraying mission, which necessitated the unsuccessful forced landing.
- A critical contributing factor was the failure of the propeller governor, which caused the loss of oil pressure and the subsequent feathering of the propeller blades.
- The failure was driven by contamination in the needle bearing race, leading to internal misalignment and damage to the governor body and relief valve.
- The aircraft's electronic chip detector did not provide any warning to the pilot regarding the governor's malfunction.
- At the time of the accident, the aircraft had not yet undergone the replacement of the propeller governor as specified by EASA AD No. 2022-0234, though the compliance deadline was not until March 2026.