What happened
On 24 July 2019, a Sikorsky S-92A helicopter, registered C-GICB, departed Halifax/Stanfield International Airport for the Thebaud Central Facility. The flight, operated by Canadian Helicopters Offshore, carried two pilots and 11 passengers.
After two unsuccessful instrument approaches due to poor visibility and low cloud cover, the crew transitioned to a visual approach once they regained sight of the platform. During this descent, the aircraft entered a high-rate-of-descent and low-airspeed condition. The engines experienced an overtorque, reaching 146% of the maximum value. The crew managed to regain control and arrested the descent just 13 feet above the water. A subsequent departure involved a second brief, inadvertent descent that was also corrected. The aircraft returned to Halifax without further incident, and there were no injuries.
The investigation
The investigation focused on the aircraft's low-energy state, characterized by low airspeed, a nose-up pitch, and a high rate of descent. The crew's attention was fixed on the helideck, which was positioned above the fog, making it difficult to perceive the lack of a horizon.
Investigators examined the pilot's use of the cyclic trim release button during the approach. By depressing and holding this release, the pilot reduced the effectiveness of the automatic flight control system (AFCS), leading to an unstable approach with a significant left sideslip. Furthermore, the investigation looked into the failure of the Enhanced Ground Proximity Warning System (EGPWS) to alert the crew during the descent.
Findings
- The flight crew failed to detect the aircraft's low-energy state because the degraded visual environment (DVE) obscured the horizon.
- The application of power while in a steep, low-airspeed, high-rate-of-descent condition caused the helicopter to enter vortex ring state.
- The operator's standard operating procedures (SOPs) lacked specific criteria regarding energy state in stabilized approaches and did not require verbal confirmation of stability at specific progress gates.
- The use of the cyclic trim release button in DVE conditions increased pilot workload and degraded flight control stability.
- Neither the manufacturer's manual nor the operator's procedures provided warnings regarding the risks of using the trim release button in such environments.
- A known gap in the Sikorsky S-92A EGPWS prevented an alert during the descent because the aircraft was flying with landing gear extended at speeds below 50 knots.