What happened
On August 24, 2008, a Bell 412 helicopter, registration EC-JJE, operated by Inaer Helicópteros, S.A., was performing search and rescue duties at Fuerteventura Airport. After completing a rescue mission in Lanzarote, the crew began a training maneuver consisting of a 36-degree clockwise turn, executed in four 90-degree segments, including takeoff and landing phases.
During the second segment of the maneuver, as the aircraft's longitudinal axis pointed nearly south, the crew experienced intense vertical vibrations. In response to these vibrations, the pilot lowered the collective pitch lever, causing the helicopter to drop abruptly from an altitude of approximately 3 meters. The impact resulted in one minor injury to a crew member, while the pilot sustained a head injury against the cockpit glazing. The aircraft sustained damage to the skids, rear lower crossbar supports, and the upper right window.
The investigation
The investigation examined the mechanical condition of the aircraft, the flight environment, and the flight control systems. Investigators inspected the collective pitch lever friction and found it within normal operational limits. The four transmission mast assemblies, including all mounting bolts and spacers, were disassembled and sent to the manufacturer for analysis; all components were found to be in good condition and appropriate for their service hours. Additionally, the aircraft's weight and center of gravity were confirmed to be within established limits.
The investigation also reviewed the Automatic Flight Control System (AFCS). At the time of the incident, the AFCS was operating in SAS (Stability Augmentation System) mode with both HELIPILOT computers active. While the flight manual notes that wind from the rear-left quadrant can make longitudinal cyclic control critical during ground effect, the recorded wind speed and gusts were well below the maximum permitted limits.
Findings
- The transmission masts and associated hardware were in good working order, ruling out mechanical failure as the source of the vibrations.
- The wind conditions, while originating from the rear-left quadrant, were not severe enough to be the primary cause of the instability.
- Pilot-induced oscillation is considered a likely source of the vibrations, occurring as the pilot attempted to maintain position using longitudinal cyclic inputs.
- The primary cause of the impact was the excessive and rapid reduction of collective pitch by the pilot following the onset of vibrations.