What happened
On 2 August 2010, a Thruster T600N microlight, registration G-CBWJ, was performing a training flight near Bradley Lawn Farm in East Sussex. Shortly after takeoff, while climbing through approximately 300 ft, the pilot experienced heavy vibrations throughout the airframe. The pilot took control of the aircraft from the student and reduced engine speed in an attempt to stabilize the flight. However, the propeller hub and blades subsequently detached from the engine, narrowly avoiding the aircraft's wing.
The pilot successfully executed a forced landing in a nearby field. While the airframe sustained no damage, the propeller assembly could not be recovered from the site. A subsequent flight conducted by the pilot on the same day, intended as a cross-country navigation exercise, proceeded without further incident.
The investigation
The AAIB conducted a field investigation and performed a forensic examination of the engine's flange mounting screws. The investigation focused on the remains of the six screws that had secured the flange to the crankshaft. Metallurgical analysis, including Scanning Electron Microscope (SEM) examination, was used to identify the fracture mechanisms.
Investigators also reviewed the aircraft's maintenance history. It was noted that the propeller blades had been replaced approximately 12 flying hours prior to the incident. While documentation showed that a manufacturer-approved repair organisation had previously worked on the engine, there was no specific record of the flange mounting screws being replaced during recent maintenance. Furthermore, there was no evidence that the manufacturer's service bulletin JSB 022-1, which addresses propeller loss events related to mounting screw failure, had been implemented on this engine.
Findings
- The forensic analysis of the mounting screws revealed that five of the six screws failed due to high cycle fatigue.
- The fatigue cracks initiated at the thread roots, a known area of stress concentration.
- The pattern of the fatigue cracks suggested a loss of clamping load on the flange mounting screws.
- This loss of load likely allowed relative movement between the flange and the crankshaft, creating additional load cycles related to engine RPM.
- The material properties and hardness of the screws were found to be consistent with manufacturer specifications, indicating no manufacturing defects in the hardware itself.