What happened
On 29 June 2007, a Boeing Stearman (registration ZS-OVZ) was conducting a practice display at Tzaneen Aerodrome in preparation for an upcoming airshow. Shortly after departing from Runway 06, the pilot entered the aerobatic display area and began performing maneuvers. During a stall turn, witnesses heard a loud bang and observed a significant portion of the aircraft detach in mid-air. The aircraft lost its stability, descending through the air with very little forward momentum before impacting the ground in a bush-type area at a 90-degree nose-down angle. The pilot sustained fatal head injuries in the crash, while the aircraft suffered substantial damage.
The investigation
SACAA AIID investigators examined the wreckage and the components of the engine mounting system. Metallurgical analysis was performed on the four bolts used to secure the engine cradle to the airframe. The investigation focused on the integrity of the mounting hardware and the forces exerted during aerobatic flight. Investigators also reviewed the maintenance history and the specific materials used in the assembly of the engine mounts.
Findings
- The engine separated from the fuselage during flight due to the failure of the mounting bolts.
- The primary cause was the use of low carbon steel washers during the assembly of the engine mount bolts.
- These C1005 specification washers were softer than the required aircraft-grade steel, leading to an excessive reduction in thickness under the compressive loads experienced during aerobatic maneuvers.
- This reduction in thickness allowed the bolts to move under strain, which initiated fatigue cracks in the thread root areas of all four bolts.
- The right-hand top bolt had likely failed due to fatigue prior to the fatal flight, placing extreme stress on the remaining hardware.
- During the stall turn, the right-hand bottom bolt failed, followed quickly by the left-hand bolts, causing the engine to detach.
- The investigation noted that the accident might have been survivable had the pilot been wearing a full-face helmet.