Limited night-time hours on type and workload contributed to an autopilot mode selection error that resulted in an S-92 helicopter inadvertently descending below 200 ft above the water at night, triggering an EGPWS terrain warning, an ATSB investigation has found.
The investigation’s final report details that three pilots were onboard the PHI International Australia operated Sikorsky S-92A when it took off from Exmouth, WA just prior to last light on 1 February 2025.
The helicopter tracked towards the drilling rig Transocean Endurance, about 71 km to the north-north-west, where each of the pilots was to conduct three take-offs and landings to maintain recency in case of an emergency night evacuation from an offshore rig.
The captain was the pilot flying for the outbound flight to the rig, with the copilot as pilot monitoring. The third pilot was seated in a passenger seat in the helicopter’s main cabin.
A short time after arriving in the vicinity of the Transocean Endurance, the helicopter continued descent below the intended pre-set circuit height, which was not recognised by the flight crew.
As it descended to 220 ft above the ocean, the helicopter’s enhanced ground proximity warning system (EGPWS) indicated a terrain caution alert, which was followed by a terrain warning about 4 seconds later, as the S-92 descended through 181 ft.
The crew responded and initiated a climb, stabilised, and debriefed the incident before continuing with the planned exercise.
The lowest recorded altitude of the helicopter during the incident was 152 ft.
The ATSB investigation final report notes that, when the helicopter arrived near the rig, the captain had set the radar altitude hold mode to the circuit height of 660 ft.
“The intent of this setting was for the helicopter to descend from its 1,500 ft cruise altitude to the circuit height, and then for it to automatically hold there,” ATSB Director of Transport Safety Dr Stuart Godley said.
“However, during the descent the captain inadvertently mis-selected the vertical speed mode while attempting to select the helicopter’s autopilot heading hold mode.
“This selection error, which was not recognised by either flight crew member, cancelled the original radar altitude hold order, and instead inadvertently commanded the helicopter to descend at a continuous 500 ft per minute.”
The ATSB found both the pilot flying and pilot monitoring at the time of the incident had limited experience on the S-92A at night, and were experiencing a higher than normal workload.
“The vast majority of the flight crew’s experience in the S-92A was from daytime passenger transfer flights, which involved flying first from Exmouth to Learmonth, to collect passengers before flying out to an offshore rig,” Dr Godley explained.
“The recency flight, which departed from Exmouth directly to the rig, was therefore shorter than usual, reducing the copilot’s time to complete the required cockpit administration and plan the approach, and resulting in the flight crew experiencing a higher than normal workload, at night.”
In the shorter flight time available, the pilots had been unable to accurately determine the take-off safety speed prior to their descent for landing. While in the circuit area the captain requested the copilot calculate it, increasing their workload and focusing them away from their monitoring duties.
At about the same time, the captain elected to activate the helicopter’s moveable searchlight, shifting their focus outside to adjust its position.
“This meant both flight crew members were preoccupied with additional tasks and therefore not monitoring the helicopter’s altitude as it descended towards the water,” Dr Godley said.
“In this case, the EGPWS performed its role, and alerted the flight crew to the undesired low altitude.”
In response to this occurrence, PHI International Australia made several procedural improvements, and adjusted its operational risk assessment for pilots with less than 500 hours on type, or for night flights where both pilots have less than 500 hours at night on type.
“This incident highlights the importance of disciplined and effective multi-crew cooperation, and is a reminder of the risks associated with divided attention in the cockpit.”
Dr Godley said the incident also highlights to S-92A operators of a potential hazard that exists with differing display versions of the automatic flight control system mode select panel.
“Due to the lighting of the panel, distinguishing between hard and soft keys is more difficult at night,” he explained.
“More recent versions of the mode select panel include a tactile white finger barrier installed between the rows of hard and soft keys, reducing – but not eliminating – the risk of a mode selection error.”
Read the final report: Ground proximity alerts involving Sikorsky S-92A, VH-IPE, 71 km north-north-west of Exmouth Aerodrome, Western Australia, on 1 February 2025