S92, vicinity of Offshore Platform east southeast of Halifax Canada, 2019
S92, vicinity of Offshore Platform east southeast of Halifax Canada, 2019
On 24 July 2019, whilst a Sikorsky S92A was commencing a second missed approach at the intended destination platform, visual contact was acquired and it was decided that an immediate visual approach could be made. However control was then temporarily lost and the aircraft almost hit the sea surface before recovery involving engine overtorque and diversion back to Halifax. The Investigation concluded that the crew had failed to safely control the aircraft energy state in a degraded visual environment allowing it to enter a vortex ring condition. As context, operator procedures, Flight Manual content and regulatory requirements were all faulted.
On 24 July 2019, a Sikorsky S92A (C-GICB) being operated by Canadian Helicopters Offshore on a 155 nm passenger flight from Halifax to the Thiebaud Central Facility Platform were attempting a visual approach there in day IMC when control was lost at 250 feet agl and a recovery from a rapid descent was only achieved just before sea surface impact with significant engine overtorque. The flight then returned to Halifax without any communication in relation to possible serious airworthiness damage. No injuries were sustained by either the crew or the 11 passengers but the extent of damage caused to the helicopter could not be determined as it was immediately withdrawn from service and dismantled and the operator ceased trading as such altogether soon afterwards.
An Investigation was carried out by the Canadian Transportation Safety Board (TSB). The aircraft involved was fitted with two MPFRs from which most of the relevant FDR data were recovered but all the relevant CVR data were lost after the aircraft manufacturer provided the operators maintenance department with incorrect instructions on how to download the FDR data. Useful data were successfully recovered from the HUMS and FMS computers and crucially, all these on-aircraft data sets were usefully complemented by satellite-routed ADS-B tracking data held by the ANSP and similar data from the aircraft’s satellite-based flight-following service.
It was noted that the flight crew involved were both experienced on type and employed by the operator as Captains. The Captain in command for the investigated flight had a total of 6,713 hours flying experience of which 2,242 hours were on type. The Captain acting as First Officer had a total of 7,742 flying hours of which 3,196 hours were on type. Both pilots had commenced work as civil transport pilots flying offshore operations in the S92A after military service as S61 pilots. It was noted that both pilots “had known each other for several years” and had flown together “periodically”. In practical terms, their fitness to fly was not doubted but it was found that the Medical Certificate of the pilot acting as First Officer had expired when he had passed the age of 60 almost two months prior to the investigated event. This appeared to have occurred because of the holder’s lack of awareness of the applicable certificate validity and his employer incorrectly recording his date of birth in their systems.
It was noted that scheduled departure time of the investigated flight had been delayed due to the poor destination weather conditions (low cloud base and poor visibility) and had eventually left Halifax almost five hours late. However, this fact was not assessed to have played any part in what subsequently happened.
As the flight neared the intended destination, the crew were informed by the platform weather observer that the wind was light and from the west northwest, visibility (equivalent to) 1600 metres and the base of the overcast was 300 feet asl (126 feet above platform helideck level). However, it was also stated that both the cloud layer and visibility were “fluctuating rapidly”.
Having descended to 800 feet, the helicopter was positioned above the cloud layer for the approved Area navigation/airborne Radar Approach (ARA) procedure using an approach track of 320°. During the subsequent approach, cloud was entered between 700 and 500 feet asl and it was unsuccessful, because of a combination of low cloud and poor visibility as was a second similar approach. Both these approaches were flown with the pilot acting as First Officer as PF and discontinued approximately half a mile from the platform at 225 feet asl by commencing a climbing right hand turn to avoid the platform superstructure.
Prior to commencing the second approach, it was decided that if it was not successful, the likely fuel endurance at that point would dictate a return to Halifax. However, approximately 15 seconds after beginning the second missed approach, the helicopter exited cloud at about 300 feet and the right seat pilot (PM) informed the PF that he could see the helideck “above the ragged layer of fog beneath them”. Shortly afterwards, although there was no discernible horizon, the PF was also able to see the platform.
Having checked that they had enough fuel to make a third (abbreviated) approach by visual reference, the helicopter was levelled off at 500 feet asl and the right turn continued to roll out on a southerly track about 1.1nm from the platform. The sea surface was reported to have been visible vertically through the fog and since there was no significant wind, it was decided that the left seat pilot would take over as PF and make the approach on an inbound track just south of west since this would be more expeditious and would also provide him with a better view of the platform. During the continuing right turn, the Captain, now acting as PM advised that he would be “ready to transition to instrument flight” if visual reference with the helideck was lost.
The ground track followed during all three approaches and (the land shown north of the platform is Sable Island). [Reproduced from the Official Report]
As the descent was commenced, the PM, who had been primarily focusing on the clearly visible helideck, shifted his attention inside the helicopter and onto the flight instruments as he slowly lost sight of the helideck. As the PF increased the nose-up pitch attitude through 15°, the PM made the standard call to alert the PF to this but at this time neither pilot was aware that the airspeed was decreasing below 40 KCAS, the rate of descent was 670 fpm and increasing, the engine torque setting was only 27% or that the left sideslip was increasing.
A few seconds later, the engine torque was increased to 36% engine torque and then pitch attitude decreased to approximately 12° nose-up. The PM, by now on the flight instruments, continued to monitor the reduced pitch attitude but as the helicopter descended though 350 feet asl, the airspeed remained low - less than 30 KCAS and the rate of descent had increased to approximately 850 fpm. After a further 100 feet of descent, the pitch attitude was unchanged but all forward motion had ceased and the rate of descent was increasing though 1200 fpm. The PF reported having recognised that the helicopter was getting low and after initially responding by slowly increasing the engine torque to 45%, he then rapidly increased it to a recorded 103% engine torque. As the engine torque increased at low airspeed, the rate of descent increased from 1200 fpm to 1800 fpm and the helicopter descended below the helideck height and into the fog.
About three seconds after the engine torque had reached 100%, the rate of descent began decreasing. The PF did not announce that he had lost visual contact with the helideck but shifted his attention to the flight instruments and observed the low airspeed and high rate of descent. The PM, unaware that the helicopter had descended into the fog alerted the PF to the increasing rate of descent and reported simultaneously realising that the helicopter was now in IMC. He stated that he had been just about to call for a go around when the PF announced that he was commencing one.
The pitch attitude increased to almost 17° nose-up and the PM responded by directing the PF to use the PFD attitude indication to position the nose of the helicopter on the horizon. As the helicopter descended below 100 feet asl, the PM, able to see the water directly beneath them, reported recognising the severity of the situation and placing his hands and feet on the flight controls and making control inputs “to help the PF establish a wings-level attitude in anticipation of water impact”.
As the helicopter was descending through 70 feet asl, the collective was rapidly raised to the full up position which resulted in the engine torque increasing to 146% and the main rotor rpm decreased rapidly to a minimum of 77% which triggered a “LOW ROTOR” aural alert. The associated reduction in tail rotor thrust, as a result of the reduction in main rotor rpm meant that it was no longer possible to maintain directional control and at 40 feet asl, it yawed uncontrollably to the right, despite full left yaw pedal being applied, continuing through multiple rotations.
The low main rotor rpm caused both main generators to drop offline and the AFCS to revert to stability augmentation system SAS mode, the Attitude and Heading Reference System (AHRS) to drop offline and the left and centre Multi Function Displays (MFDs) to blank. Both pilots reported having reverted to the SBY flight instruments. Six seconds after the torque increase, the helicopter was still rotating to the right at greater than 60° per second but the main rotor speed was slowly increasing through 89% and, as both generators came back online, the descent was arrested “within 13 feet or less” of the sea surface. Water spray was visible on the helicopter’s windows and the helicopter was less than 800 feet from the closest part of the platform’s structure but drifting backwards, away from the platform complex.
The Captain in command, who reported having been able to see both the water and the platform to the right of the helicopter, then announced his takeover as PF and commenced an almost vertical visual climb, eventually emerging from the fog. The aircraft flew in a widening right turn away from the vicinity of the upset and toward the northeast with the engine torque slowly being reduced, eventually reaching 100% at 650 feet asl and then being reduced further to approximately 70% as the 1350 feet asl was reached. At this point, both pilots became distracted from properly monitoring the flight path by their unsuccessful attempts to engage altitude hold and reset the AFCS. Without their awareness, the helicopter began to descend and by the time the PF Captain noticed, the airspeed had increased to 148KCAS and the helicopter was descending through 650 feet asl. The descent was arrested at 480 feet asl which was equivalent to 463 feet agl as at this time, the helicopter was, unappreciated by the pilots, crossing Sable Island (see the illustration above) where its elevation was approximately 10 feet amsl. Given that the conditions around the platform were assessed to be not conducive to a successful visual approach, once the helicopter had been climbed to1,500 feet asl, it was decided that a return to Halifax, where conditions would permit a visual approach, would be made. Despite the potential implications of the significant engine overtorque, no advice of a possible problem during the return was communicated to ATC and in the event, the return was completed without further event.
Once the occupants had disembarked, the helicopter was immediately and permanently withdrawn from service and soon afterwards, Canadian Helicopters Offshore ceased operations at Halifax and the lessor decided that they would scrap the helicopter. Disassembly of the helicopter prevented electrical system troubleshooting and meant that a detailed examination of components from the tail and main rotor assemblies and their associated drive systems, which would have been required before return to service could be considered, was not carried out. However, Sikorsky carried out a preliminary examination of the HUMs data and concluded that “the drive train sustained torque magnitudes in excess of the system design limits and therefore many component replacements are recommended” and added that the implications of the main rotor system loading had not been fully investigated.
The Investigation found no evidence to indicate that an aircraft system malfunction contributed to the investigated event. The circumstances which led to it were therefore, by default, operational and human factors in a context of the prevailing SOPs and OFDM practice and a number of observations were made.
- It was evident from the data that the attempted visual approach had become unstable with the helicopter entering a low-energy state. This had not been recognised by either pilot as both were focused on the platform helideck which was above surface fog and within a Degraded Visual Environment (DVE). It was noted that whilst some offshore helicopter operators did have specific procedures for conducting operations in ‘Degraded Visual Environments’ (DVE), Canadian Helicopters Offshore was not one of them.
- It was found that the operator’s SOPs made no reference to the importance of energy state when conducting a stabilised approach criteria and did not establish any stabilised approach gates to be met during final. This was incompatible with industry-recommended stabilised approach guidelines.
- It was considered of particular note that during this visual approach being investigated, the PF had depressed and held the cyclic trim release, an action which reduces the overall effectiveness of the AFCS. This was considered to have created an increased workload which was a material factor in the increase in the nose-up pitch attitude to 17° concurrently with an excessive rate of descent and an increasing left sideslip. The evidence available indicated that this increased workload had made a significant contribution to control difficulties which had led to an unstable approach. When the PF instinctively increased the collective in response to the rapidly increasing rate of descent below 250 feet, it was concluded that the power applied had caused the helicopter to enter a vortex ring state. It was found that neither the manufacturer’s AFM nor the operator’s SOPs contained any warning of the potential hazard associated with the use of the trim release button when in a DVE.
- Routine Flight Data Monitoring was in place at the operator in support of its risk management processes but the offshore approach parameters being monitored did not include all the available ones needed for proper oversight of such approaches.
- The absence of any EGPWS activation as the rate of descent steepened and the sea surface neared was considered significant. The fact that the helicopter’s Mark 22 EGPWS had provided no alert to the very close proximity to the sea surface was a subject of some interest to the Investigation. It was found that at the aircraft manufacturer’s request, Mode 1 alerting in the S92A EGPWS had been disabled by the OEM prior to installation because of nuisance alert problems identified during initial type certification testing in 2002. Although this EGPWS is installed in many different helicopter types, it was noted that the S-92A is the only such helicopter which has Mode 1 inhibited. During the final stages of the attempted visual approach, recorded flight data showed that the Mode 1 “SINK RATE” Alert envelope had been entered as it descended through 230 feet asl and the subsequent “PULL UP” Warning as it descended though 180 feet asl.
The Investigation noted that a number of previous investigations into accidents and serious incidents to a number of transport helicopter types with similar or worse outcomes had involved unstabilised approaches with excessive use of the cyclic trim release facility. One of these had been a 2006 fatal accident in the UK. It was further noted that guidance contained in a recent revised HeliOffshore guidance document Flightpath Management: Recommended Practice for Oil and Gas Passenger Transport Operations included a generic approach DVE/Night approach procedure which would have been applicable to the approach under investigation. This guidance had been successfully adopted by other operators and was observed to be conducive to the conduct of a stabilised approach.
The Findings of the Investigation were formally documented as follows:
Causes and Contributing Factors
- At the time of the occurrence, instrument meteorological conditions existed. This created a degraded visual environment that was highly conducive to spatial disorientation and provided inadequate cues to permit a visual approach to the Thebaud Central Facility.
- In an attempt to complete their assigned task within self-imposed time constraints, the pilots’ decision-making process was influenced by their past experience and comfort with each other. As a result, the pilots attempted a non-standard visual approach in a degraded visual environment, without thoroughly considering the risks or alternative options.
- Canadian Helicopters Offshore’s standard operating procedures provided flight crew with insufficient guidance to ensure that approaches were being conducted in accordance with industry-recommended stabilised approach guidelines.
- Depressing and holding the cyclic trim release button, while operating in a degraded visual environment, increased pilot workload and contributed to control difficulties that resulted in an unstable approach that developed into vortex ring state.
- The helicopter’s approach became unstable, due to an inadvertent low-energy, high rate-of-descent flight profile.
- The pilots experienced attentional narrowing due to increased workload while attempting a non-standard offshore visual approach in a degraded visual environment.
- This led to a breakdown in the pilots’ instrument cross-check, which prevented the timely recognition that the approach had become unstable.
- The S-92A’s enhanced ground proximity warning system provides no warning of an inadvertent descent at airspeeds below 50 knots indicated airspeed when the landing gear is down. As a result, the enhanced ground proximity warning system did not alert the flight crew of the impending risk of controlled flight into terrain.
- The pilot flying was likely subtly incapacitated due to spatial disorientation following the helicopter’s rapid, unexpected descent into the fog layer. As a result, recovery action was initially delayed until the pilots acquired visual contact with the water and the collective was rapidly increased to arrest the descent.
- The application of power while in a steep, low-airspeed, high rate of descent condition caused the helicopter to enter a vortex ring state. This contributed to the helicopter’s rapid descent into and through the fog layer, bringing the helicopter to within 13 feet of the water.
- 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 degraded visual environment due to the sub-optimal use of the automatic flight control system.
- If pilots of a multi-crew aircraft are making control inputs at the same time during an undesired aircraft state, there is a risk of exacerbating the recovery effort and reducing safety margins.
- If pilots deviate from standard operating procedures, it can lead to an inadvertent increase of pilot workload and reduce safety margins to unacceptable levels.
- If company standard operating procedures do not clearly define and establish procedures for operating in a degraded visual environment, there is a risk that pilots will assume an unacceptable level of risk by attempting to conduct visual manoeuvres in instrument meteorological conditions and/or a degraded visual environment.
- If approach briefings lack sufficient detail, there is a risk that operational hazards will go unaddressed before attempting an approach.
- If a company’s standard operating procedures contain conflicting or ambiguous procedures, there is an increased likelihood of procedural errors that can reduce safety margins to unacceptable levels.
- If a company’s flight data monitoring program does not monitor specific parameters established in the standard operating procedures, it reduces a company’s ability to proactively identify trends that could reduce safety margins.
- Until enhanced ground proximity warning systems or 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.
- If operators do not implement adequate measures to preserve cockpit voice recorder audio and flight data recorder data following a reportable occurrence, there is a risk that potentially valuable information will be lost.
Safety Action taken as a result of the accident whilst the Investigation was in progress was noted as having included the following:
- Canadian Helicopters Offshore revised its VMC offshore approach procedure and added a definition of ‘DVE’ which stated that such an environment is considered to prevail “when flying in VMC conditions but visual cues available to the pilot are not sufficient to maintain the proper flight path during flight and/or landing” and gave “fog obscuring the water surface” and “no visible horizon” as examples of a DVE.
The Final Report of the Investigation was authorised for release on 10 February 2021 and it was officially released on 27 April 2021. No Safety Recommendations were made.
- Loss of Control
- Energy Management during Approach
- Vortex Ring
- Stabilised Approach
- Flying a Visual Approach
- Risk Management
- Helicopter Terrain Awareness and Warning System (HTAWS)
- Safety Oversight
- Pilot Workload
- Spatial Disorientation
- Flight Data Monitoring (FDM)
- AS65, vicinity North Morecambe Platform Irish Sea UK, 2006
- Flightpath Management: Recommended Practice for Oil and Gas Passenger Transport Operations - HeliOffshore, September 2020