Air transportation safety investigation A19A0055

The occurrence

On 24 July 2019, at 1154 Atlantic Daylight Time, a Canadian Helicopters Offshore (CHO) Sikorsky S-92A helicopter (registration C-GICB, serial number 920121), departed from Halifax/Stanfield International Airport, Nova Scotia, on an instrument flight rules flight to the Thebaud Central Facility, approximately 155 nautical miles to the east-southeast. On board were 2 pilots and 11 passengers.

Two instrument approaches were attempted at the platform but both were unsuccessful due to low clouds and poor visibility. During the second missed approach, the flight crew acquired visual contact with the platform and elected to carry out a visual approach. Shortly after they commenced the visual approach, a high-rate-of-descent and low-airspeed condition developed in low-visibility conditions. During the descent, the helicopter’s engines were overtorqued, reaching a maximum value of 146%. The crew regained control of the aircraft and arrested the descent at approximately 13 feet above the water.

During the subsequent hand-flown departure, a second inadvertent descent occurred but was rectified in a timely manner. The aircraft then returned to Halifax/Stanfield International Airport without further incident. The extent of the helicopter’s damage is unknown, as the helicopter has been removed from service. There were no injuries.

The investigation determined that during the final visual approach, the helicopter entered a low-energy state: it was flying at low airspeed with a high rate of descent, a nose-up pitch attitude, and at a low power setting. The helicopter’s low-energy state went undetected by the flight crew, who were focused on the helideck, which was sitting above the fog and in an area without a discernible horizon. The degraded visual environment (DVE) made it difficult for the pilots to recognize the unstable approach.

Contributing to the difficulties encountered, CHO standard operating procedures (SOPs) made no reference to energy state in its stabilized approach criteria, increasing the risk of a low-energy state developing and going undetected. Additionally, CHO had not adopted the recommended practice of requiring crews to check and verbally confirm that the approach was stable at specific intermediate progress targets (typically referred to as gates) on final approach. As a result, the SOPs provided flight crew with insufficient guidance to ensure that approaches were being conducted in accordance with industry-recommended stabilized approach guidelines.

The investigation also determined that while on final approach in a DVE, the pilot flying depressed and held the cyclic trim release. As seen in previous occurrences, this technique reduces the overall effectiveness of the automatic flight control system (AFCS). In this occurrence, the helicopter reached a nose-up attitude of 17°, an excessive rate of descent, and an increasing left sideslip while on final approach. Flying the visual approach in a DVE while depressing and holding the cyclic trim release button increased pilot workload and contributed to control difficulties that resulted in an unstable approach. As the helicopter descended below 250 feet radar altitude, it was in a steep, 800 fpm descent, at very low airspeed, with power being applied. When the pilot flying instinctively increased the collective, the helicopter’s rate of descent rapidly increased to 1800 fpm. The application of power while in a steep, low-airspeed, high-rate-of-descent condition caused the helicopter to enter vortex ring state.

Neither the manufacturer’s flight manual nor the operator’s SOPs warned of the potential hazards associated with the use of the trim release button under conditions such as a DVE. If manufacturers’ flight manuals and operators’ standard operating procedures do not include guidelines for the use of the cyclic trim release button, it could lead to aircraft control problems in a DVE due to the sub-optimal use of the AFCS.

The helicopter inadvertently descended with a very high rate of descent into the fog bank at low airspeed with the landing gear extended. Despite this, the helicopter’s enhanced ground proximity warning system (EGPWS) did not alert the crew to the situation. This is the result of a gap, previously identified by the TSB, in the coverage provided by the S-92’s EGPWS. If an inadvertent descent occurs with the gear down at airspeeds below 50 knots indicated airspeed, the EGPWS will provide no warning against controlled flight into terrain.

In 2016, the TSB issued a recommendation calling for terrain awareness and warning systems for commercial helicopters that operate at night or in instrument meteorological conditions. At the time of report writing, it is still not required by regulation. As a result, helicopter manufacturers and operators are free to disable EGPWS modes, as seen on the S-92A. Until EGPWS / helicopter terrain awareness and warning systems become mandatory for Canadian commercial helicopters that operate at night or in instrument meteorological conditions, flight crew and passengers aboard these flights are at increased risk of controlled flight into terrain.

Media materials

News release