What happened
On 17 February 2018, an Agusta Westland AW189, registration G-MCGR, was conducting a search and rescue mission near Beinn Narnain, Scotland. The crew was attempting to locate three climbers in challenging night conditions characterized by low cloud, snow, and freezing temperatures. Using Night Vision Imaging Systems (NVIS), the crew made several attempts to reach the climbers from different directions.
During a third attempt to enter a re-entrant, the helicopter entered thick cloud, causing the pilot to lose all external visual references. While attempting to execute a valley turn to escape the terrain, the pilot became distracted by the loss of visibility and inadvertently missed the required escape heading. Believing the aircraft was heading toward a rock face, the pilot initiated a right-sided turn. This maneuver caused the airspeed to drop rapidly to approximately 11 KIAS.
As the aircraft's speed plummeted, the helicopter began to yaw aggressively to the right. The pilot attempted to use the Automatic Flight Control System (AFCS) to assist with the recovery, but the engagement of certain modes caused the collective lever to lower, which the pilot disengaged to maintain a climb. The aircraft eventually rotated through approximately 370° before the crew regained visual meteorological conditions (VMC) above the cloud layer and stabilized the flight.
The investigation
The AAIB investigation examined the flight path, the performance of the AFCS, and the cockpit environment. Investigators reviewed the aircraft's recorded data, which showed the rapid reduction in airspeed and the significant yawing motion. The investigation also looked into the use of the aircraft's terrain alerting systems and the impact of the weather on the crew's situational awareness. The crew had inhibited the Terrain Alerting and Warning System (TAWS) to prevent nuisance warnings while flying close to the mountains, which meant they lacked automated terrain proximity alerts during the critical phase of the maneuver.
Findings
- The primary cause of the incident was the loss of visual references due to the aircraft entering cloud during a low-speed maneuver.
- The pilot's concentration on regaining visual cues led to a failure to monitor the aircraft's heading, resulting in the missed escape track.
- The use of the AFCS 'Go Around' and 'Transition Up' modes contributed to pilot confusion, as the system's automated collective adjustments were contrary to the pilot's immediate need to maintain a climb.
- The inhibition of the HTAWS meant the crew had no automated terrain warnings during the period of disorientation.
- The aircraft's rapid yaw was a result of low airspeed and insufficient anti-torque control during the high-power climb.