What happened
On 8 June 2019, a Sportscruiser X320 with registration ZU-LSA departed Bethlehem (FABM) Aerodrome to participate in a speed rally competition organized by the Bethlehem Aero Club. While cruising at 6,500 feet above mean sea level with an indicated airspeed of 105 knots, the pilot experienced intense vibrations from the propeller. Within seconds of the pilot attempting to troubleshoot the issue using checklists, one propeller blade (blade C) detached from the hub.
In response to the emergency, the pilot shut down the engine and transitioned to a glide, searching for a suitable landing site. The pilot identified a narrow dirt road for an emergency landing. During the landing roll, the aircraft struck a wooden fence post with its right wing, causing the aircraft to spin and come to a halt facing the opposite direction. The landing gear collapsed, and the engine carburettors were torn from the airframe. While the two occupants sustained no injuries, the aircraft suffered substantial damage.
The investigation
SACAA AIID investigators performed a teardown inspection of the Woodcomp three-bladed electric variable pitch propeller at Brits (FABS) Airfield. The examination of the propeller hub revealed that the grease on all components, including the main bearing, had dried out. While no internal corrosion or chafing was found, the investigation focused on the aluminum sleeve of blade C.
Review of the maintenance logs showed that the propeller had accumulated 476 hours since its last recorded overhaul in May 2014. According to the manufacturer's service bulletin, propeller overhauls must be conducted every 300 operating hours or every five years, whichever comes first. The investigation established that the propeller had exceeded the required maintenance interval by 176 hours.
Findings
- The primary cause of the blade separation was a fatigue crack in the aluminum sleeve of blade C.
- This crack was initiated by vibration stresses caused by the blades failing to operate in a synchronized movement.
- The failure was driven by non-compliance with manufacturer maintenance intervals, specifically the failure to overhaul the propeller after the 300-hour limit.
- Centrifugal and aerodynamic forces acted upon the unbalanced blades, propagating the crack until the blade separated in-flight.