What happened
On July 17, 2017, at approximately 16:30 UTC, a flight crew consisting of an instructor and a student pilot was conducting training flights at the Bukowina Tatrzańska landing field. Following a standard pre-flight inspection, the crew proceeded to perform an engine start on a Robinson R44 Raven I.
Immediately after engine startup, but prior to engaging the clutch, the crew heard a cyclic, metallic grinding sound originating from the rear section of the helicopter fuselage. The instructor promptly shut down the engine. The crew performed a second pre-flight inspection and attempted another engine start; however, the same metallic noise recurred, leading to a second immediate engine shutdown. The incident was recorded in the aircraft's technical logs, and the maintenance organization was notified.
The investigation
An investigation conducted by the operator's internal committee focused on the source of the noise. Upon disassembly of the fan assembly, technicians discovered that the bolts securing the cone to the fan assembly were loose. Despite the loosening, the bolts retained traces of factory paint, suggesting that no unauthorized interference or improper maintenance had occurred by third parties or the maintenance organization.
Additionally, technical personnel identified traces of corrosion on the fan where it attaches to the cone shaft, as well as on the rear wall of the fan. The disassembled fan assembly was sent to the manufacturer for further analysis.
Findings
On November 21, 2017, the manufacturer reported that a crack had been detected in the fan cone. The manufacturer determined that this failure was the direct result of the loose mounting bolts operating under conditions of permanent vibration. The manufacturer subsequently provided a replacement fan assembly under warranty, and the aircraft was returned to service.