What happened
On 24 August 2016, a Grob G115E Tutor, registration G-BYVE, was conducting a training flight at RAF Wittering, Cambridgeshire. During a formation takeoff, when the aircraft had reached an altitude of approximately 400 ft agl, the pilot heard a loud thud followed by intense vibrations throughout the airframe.
In response to the vibration, the commander immediately turned the aircraft downwind and declared a PAN. After gaining sufficient runway length to execute a safe approach, the pilot transitioned to the final approach. The aircraft was able to complete a successful glide approach and landed safely without any injuries to the two crew members.
The investigation
Following the incident, an examination of the aircraft's propeller revealed that the anti-erosion sheath had separated from one of the propeller blades. The investigation into the failure was conducted by the propeller manufacturer, who identified a combination of three specific manufacturing defects. These included an insufficient amount of adhesive between the blade and the sheath, excessive sanding of the glass fibre layers on the inner part of the sheath, and a failure to thoroughly clean the sheath before it was refitted to the blade.
While the investigation found that the other two blades on the affected propeller and blades on another propeller overhauled by the same UK subcontractor did not exhibit these specific defects, the operator took the precautionary measure of removing all propellers overhauled by that subcontractor from service.