AAIB: AW189 Incident Crew Did Not Understand Its AFCS

The crew of a search-and-rescue Leonardo AW189 narrowly avoided an accident due to an incomplete understanding of the helicopter’s automatic flight control system (AFCS) modes. So concluded the UK’s Air Accidents Investigation Branch (AAIB) in a recently released report.

The incident occurred on July 26, 2021, at Ballintoy Harbour in Northern Ireland. The helicopter was on its third approach to collect a casualty from a site adjacent to high ground in poor visibility. A mismatch between the previously selected AFCS heading reference and the heading flown by the pilot flying (PF) triggered uncommanded yaw oscillations. According to the AAIB, the pilot selected AFCS hover mode, which resulted in yaw toward the high ground. The PF then selected AFCS go-around and the helicopter accelerated toward the terrain. The crew made a climbing turn to its planned escape heading, recovered to a safe height, and returned to base.

The AAIB concluded that "the unexpected yaw occurred as a result of flying out-of-detent, thereby overriding the engaged HDG [heading] mode. Consequently, the reference heading used by the AFCS did not capture the helicopter’s current heading, but instead remained at the heading selected when HDG was last engaged. When the PF stopped flying out-of-detent, the AFCS yawed the helicopter towards the previously captured reference heading, which resulted in the helicopter tracking towards high ground.”

The AAIB also noted that, in this incident, standard operating procedures were not always followed, and lack of communication between the pilots meant that it might not have been obvious to the pilot monitoring that the PF was flying out-of-detent. The AAIB counseled, “In mixed mode flying it is important that both pilots have a clear understanding as to which axis the AFCS and pilot are controlling, and the effect this might have on parameters such as heading reference.”