What happened
On 9 July 2010, a Sikorsky S-92A, registration G-SARC, was performing a manually flown Search and Rescue (SAR) mission over the mountainous terrain of Harris Hills, Isle of Harris, Scotland. During the operation, the aircraft entered instrument meteorological conditions (IMC).
While the pilot flying attempted to exit the clouds using a pre-briefed escape heading, the automatic flight control system (AFCS) unexpectedly entered a hover mode. This uncommanded mode change halted the aircraft's climb and caused an increase in airspeed. The helicopter subsequently adopted a 15-degree nose-up attitude with a slight bank and began to descend. During this period, a gap in the cloud cover revealed a hill positioned directly in front of and below the aircraft.
In response to the terrain threat, the co-pilot called for an immediate climb. The commander disconnected the autopilot and initiated a manual climb. The crew attempted to re-engage heading and altitude hold modes, but the co-pilot was unable to successfully engage the requested altitude mode. The aircraft eventually transitioned into visual meteorological conditions (VMC) over lower terrain, and the mission was completed without further incident.
The investigation
The investigation examined the behavior of the AFCS and the crew's response to the unexpected mode change. It was noted that the crew observed the 'hov' (automatic hover velocity) mode annunciated on the Primary Flight Display (PFD). The investigation also looked into the difficulty the co-pilot experienced when attempting to select the altitude pre-select mode.
Findings
- The autopilot entered an unexpected mode that resulted in the aircraft not responding as intended.
- The unexpected engagement of the automatic hover mode caused the aircraft to stop climbing and begin a descent.
- The difficulty in engaging the altitude mode was linked to the aircraft's airspeed being below the 50-knot lower capture limit during the event.
- The operator identified that switch selection in the cockpit could be prone to error during periods of high workload.
Safety action
Following the incident, the operator implemented several measures to standardize cockpit switch operations, including:
- Standardizing autopilot engagement procedures to include clear commands and execution instructions within the Operations Manual and simulator training.
- Adopting a "Locate, Mark, Select" principle for switch operation to ensure positive identification of any switch before activation.