Mechanical Failure Leads to Forced Landing and Severe Injuries in Microlight Accident

Casualties unknown • Plot 25, Klipview Rand Vaal, Gauteng, ZA

A mechanical failure during takeoff caused a Skyfox microlight to lose propeller thrust, resulting in a violent forced landing in Gauteng.

What happened

On 11 September 2012, a Skyfox microlight, registration ZS-WOX, departed from a private grass airstrip at Plot 25, Klipview, Randvaal, for a local pleasure flight. The pilot had already completed one successful flight earlier that day. During the second takeoff, which included a passenger, the engine experienced a sudden and abnormal increase in revolutions. Simultaneously, the propeller lost thrust, making it impossible for the pilot to maintain level flight.

Attempting to regain control, the pilot increased the throttle, but the engine and propeller remained disconnected. Unable to return to the runway, the pilot performed a forced landing in an open area on the same property. The impact was severe; the aircraft landed heavily and flipped over, causing substantial structural damage to the airframe. The pilot sustained serious injuries, including paralysis from the waist down, while the passenger suffered strained back muscles. \n## The investigation

SACAA AIID investigators examined the wreckage and the engine assembly to determine why the propeller failed to provide thrust. While the pilot initially suspected a failure in the propeller belt drives, the investigation revealed that the coupling connecting the crankshaft to the propeller shaft had been compromised.

Upon inspecting the hardware, investigators found that the bolt and nut securing the propeller shaft to the coupling had come adrift during flight. Although the original hardware was not recovered from the site, a spare bolt from the pilot's stock was measured. The analysis showed that the bolt's grip length—the unthreaded portion of the shank—was incorrect for the application. Specifically, the shank was too short to allow the self-locking nut to engage the minimum number of threads required to maintain a secure, positive lock.

Findings

  • The engine crankshaft and propeller shaft coupling became disconnected during flight.
  • The hardware (bolt and nut) used to secure the propeller shaft to the coupling came loose.
  • The bolt used was of an incorrect length and featured an improper grip length for the coupling.
  • The coupling itself was constructed from industrial-grade aluminum.
  • The pilot was performing maintenance and servicing on the aircraft without the required regulatory approval.

Probable cause

The accident was caused by an unsuccessful forced landing following the loss of propeller thrust due to the failure of the hardware securing the propeller shaft to the engine coupling.

Frequently asked questions

What happened in the 2012-09-11 Sky fox Microlight accident near Plot 25, Klipview Rand Vaal, Gauteng, ZA?

A mechanical failure during takeoff caused a Skyfox microlight to lose propeller thrust, resulting in a violent forced landing in Gauteng.

What aircraft was involved and where did it happen?

The accident on 2012-09-11 involved a Sky fox Microlight, registration ZS-WOX, at Plot 25, Klipview Rand Vaal, Gauteng, ZA.

What was the probable cause of the accident?

The accident was caused by an unsuccessful forced landing following the loss of propeller thrust due to the failure of the hardware securing the propeller shaft to the engine coupling.

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