What happened
On 7 February 2000, an Agusta A109C, registration G-RNLD, was conducting a private flight near Coventry Airport. While cruising with the autopilot engaged in altitude and heading modes, the pilot noticed a subtle, rhythmic noise occurring once per revolution. As the pilot attempted to assess the source of the sound, the noise intensified.
In response to the changing noise, the pilot redirected the aircraft toward Coventry Airport to perform a precautionary landing. As the airfield came into view, the helicopter began experiencing extreme vibrations. The pilot immediately disconnected the autopilot, lowered the landing gear, and reduced collective pitch. After transmitting a Mayday call to Coventry Approach, the pilot performed an emergency landing. Following the touchdown, the engines were shut down and the rotor brake applied. There were no injuries to the pilot or the three passengers, and the aircraft sustained no significant damage, though a post-flight inspection revealed that one main rotor blade tip cap had become detached.
The investigation
The investigation focused on the structural integrity of the main rotor blade. The tip caps in question consist of upper and lower aluminium alloy shells welded at the leading edge, bonded to a honeycomb core, and secured to the blade via screws and adhesive.
Records showed that the affected blade had accumulated 287.30 total flight hours. The operator, an Agusta distributor, confirmed that the tip cap had undergone the specific inspections mandated by a previous technical bulletin (BT 109-106) approximately 13 flight hours prior to the incident, with no defects identified at that time.
Upon examining the detached component, the manufacturer identified two distinct failure modes: tearing of the trailing edge fibres and adhesive rupture at the leading edge/spar interface. The manufacturer's analysis suggested that while the initial bond during manufacturing was sufficient, the failure originated from an undetected crack at the welded joint of the shells.
Findings
- The detachment of the tip cap was caused by the separation of the shell weld line, initiated by an undetected crack.
- The vibration levels increased significantly once the structural integrity of the blade tip was compromised.
- Previous inspection protocols, which relied on tapping and visual checks, had failed to identify the developing crack in this instance.