What happened
On 20 November 2019, a Zenith Air, Zodiac CH-601 XL, registered as ZU-CUN, was conducting a private flight from Kimberley Aerodrome toward Vryburg Aerodrome. While cruising at 7,500 feet, the pilot experienced a sudden, violent shudder throughout the airframe, followed by a brief loss of control. The pilot subsequently discovered that the propeller had separated from the crankshaft flange, causing the engine to overspeed.
Recognizing the emergency, the pilot shut down the engine and prepared for a forced landing. After broadcasting a Mayday on the Johannesburg radar frequency, the pilot successfully landed the aircraft in an open field near Spits andkop Dam. While the pilot escaped without injury, the aircraft sustained minor damage after the left wing struck a perimeter fence post during the landing sequence.
The investigation
SACAA AIID investigators examined the aircraft's maintenance history and conducted a microscopic analysis of the engine components. The investigation focused on the condition of the drive bushes and the attachment bolts. Laboratory testing of the flywheel and ring gear revealed significant elongation damage in the bolt holes. This damage indicated that the attachment bolts had been under-torqued, allowing the drive bushes to move within the rotational plane while the engine was under load. This movement increased propeller vibration, which accelerated the fatigue and eventual failure of the components.
Findings
- The primary cause of the propeller separation was the failure to comply with the propeller maintenance manual inspection requirements.
- The propeller had been in service for 12 years, during which time it had only accumulated approximately 203.7 hours of flight time.
- The manufacturer's manual mandates that the propeller be removed for inspection every 5 years or 1,000 flight hours, whichever comes first; however, no evidence was found that such an inspection had occurred since its installation in 2007.
- Maintenance records also indicated that several required annual inspections for the airframe had been missed in previous years, impacting the aircraft's continuous airworthiness.
Safety action
- The investigation issued a safety message reminding aircraft owners and operators that strict adherence to manufacturer maintenance manuals is essential to prevent such incidents.
- A secondary safety message addressed the need for the SACAA to ensure more rigorous oversight of aircraft compliance with maintenance instructions during the annual renewal of Authorities to Fly.