CHC Scotia Rollover on West Navion Helideck after Loss of DP (AS332L Super Puma G-BKZE)
On 10 November 2001, CHC Scotia Airbus AS332L G-BKZE, chartered by BP, rolled over while being rotors running refueled on the helideck of the Norwegian drillship West Navion, 80 nm west of Shetland.
Wreckage of CHC Scotia AS332L G-BKZE on Helideck of West Navion Drillship West of Shetland (Credit: via AAIB)
The copilot, who was on the helideck, suffered serious leg injuries from flying main rotor debris.
The UK Air Accidents Investigation Branch (AAIB) published their safety investigation report in May 2004. While their report was issued over 20 years ago this accident and the AAIB recommendations remains highly relevant for the safety of offshore helicopter operations.
The Accident
The helicopter departed on a routine crew change flight from Aberdeen, refuelling en route at Wick, before its approach to the Smedvig operated drillship.
The helicopter touched down at 1242 hrs toward the forward edge of the deck on a heading of about 296°M (289°T).
As used passenger disembarkation occurred while rotors running. The co-pilot left the aircraft to assist the ship’s helideck crew with the refuelling.
The drillship’s position over the seabed well head was maintained by a simplex computer controlled Dynamic Positioning (DP) system. This determined position from navigation satellites and seabed transponders. Electrically powered thrusters were used to maintain station.
The DP system was programmed to take account of the prevailing sea current and wind to maintain station. The DP system is normally operated in AUTOMATIC mode but has a manual reversion capability. MANUAL control is selected via a ‘one-touch’ switch.
In AUTOMATIC mode, the DP system gives the operator a warning if the ship’s heading deviates more than ± 2°, and an alarm when it deviates by more than ± 3°.
In MANUAL mode no warnings are given and the only indication to the DP operator that the system is operating in MANUAL mode is the absence of a heading window on one of his DP screens.
AAIB explain that:
At about 1245 hrs the commander became concerned about the ship’s movement and requested the pitch, roll and heave readings from the ship’s Radio Operator (RO).
Vessel Motion (Credit: UK CAA)
The RO was on the bridge below the helideck and only had a partial CCTV view of the helideck.
After some confusion over exactly what the commander had requested the RO provided the readings, all of which were within the helicopter’s operating limits.
Noticeably:
The RO’s position on the bridge was between, and to the right of, two parallel consoles from where the ship is controlled when underway and when drilling. Weather, pitch, roll and heave information were normally available from a computer screen on the forward console. However, on the day of the accident, this screen was out of service and the RO had to use the information from the rear consoles, which were in use by the DP operator.
Neither the forward computer screen nor the DP consoles were easily visible from the RO’s position and…much of the information on the DP screens is factored or averaged for use by the DP computers, [so] the basis of the data presented on the screen was not immediately evident. [Therefore]…the RO had to leave his station, walk to the DP operator’s position and ask for the requested data.
The aircraft commander remarked that his instruments appeared to be showing greater movements. A little later, the helideck crew noticed a shift in wind direction. Crucially:
At 1247 hrs, unknown to the helicopter commander and unnoticed by the ship’s DP operator, the ship’s DP system reverted from AUTOMATIC to MANUAL heading control, and after about two minutes the ship’s heading started to drift to the right.
No external visual cues, such as land or a stationary ship, were available to draw the pilot’s attention to this change of heading and the windsock, which would have been indicating the change in the relative wind, was not within the commander’s field of vision. However, some of the helideck crew had noticed the change in wind direction.
As the ship’s heading increased, a list of approximately 1° to the right developed and, at about 1254:14 hrs, the DP system provided an alarm that the ship’s position was out of limits.
This alarm was set to provide warning that the vessel has moved off station to the point where there is a risk of problems with the drilling riser (the connection from the wellhead to the drillship) and was not designed around the effect on a helicopter on deck. A similar loss of heading control had occurred less than a month earlier on 12 October 2001.
When the ship’s crew received the warning of the DP system’s position error, their primary concern was to proceed with the laid down procedure for dealing with the warning. …there was no appreciation on the bridge that the ship’s loss of heading might cause a problem for the helicopter.
Twelve seconds later, at 1254:26, the helicopter toppled over to the right.
DP System Screenshot (Credit: via AAIB)
Fortuitously the refuelling was complete and three of the helideck crew were stowing the refueling hose off the helideck, leaving a fourth as fire guard at the foam monitor controls in the ‘foam shack’ next to the helideck. The co-pilot was this the only person outside the aircraft on the helideck. He…
…noticed out of his peripheral vision the tail rotor starting to move towards him. He threw himself on the deck and began crawling towards a gangway that connected the helideck with the upper bridge and ‘foam shack’ area. As he crawled he became aware of the main rotor blades striking the deck, the engines screaming and the deck vibrating and of high velocity flying debris. At some point he felt a heavy, immobilising blow to his left leg.
The fire guard activated the foam system.
The helicopter finally came to rest on its right side very close to the forward edge of the helideck having turned to the left by about 80°.
In the cockpit, the commander recalled seeing the aircraft attitude indicator reading 5° of right bank and very soon after the aircraft toppled on to its right side.
Almost immediately the commander was aware of fire-fighting foam being applied to the aircraft and foam entering the cockpit. He…operated the two Fuel Shut-Off levers situated on the overhead panel and vacated the aircraft, with some difficulty [due to foam that had entered the cockpit], through the left cockpit door.
The deck netting had become tightly entangled around the rotor head as the drivetrain wound down, and it was [subsequently] evident [to investigators] that this had caused the helicopter to winch itself anticlockwise along a circular path.
Approximately 5 minutes later the West Navion also disconnected the wellhead riser to prevent damage as the vessel continued to yaw.
The AAIB & Duty Holder Investigations
The installation Duty Holder investigated the two loss of DP events. This concluded that both were similar…
…in that they both started with a loss of automatic heading control, followed by a change of heading to the right.
No DP system data recording was available for download so trend analysis was therefore restricted to screen shots from the DP console. AAIB comments that:
Nevertheless, their report indicated that on both occasions, at about the time that directional control was lost, the DP system was applying a turning moment in the wrong direction relative to the selected heading. This gave rise to the possibility of a software failure having occurred, although the investigation could not decide whether this, or operator intervention was responsible for the loss of control.
The AAIB note that tests conducted after the accident did not find any evidence of a system failure. It is perhaps significant that MANUAL control is selected via a ‘one-touch’ switch, so the system is vulnerable to an unintended mode change.
In 2016 we wrote about an incident in Australia were DP was deselected by a notebook contacting a switch. Also see: The US Navy installed touch-screen steering systems to save money. Ten sailors paid with their lives.
The Duty Holder DP investigation concluded that prompt action after first DP incident to require a dual action to change mode and an independent loss of heading alarm may have prevented the second incident.
AAIB note that:
Since a change in wind direction, caused possibly by a line squall or other meteorological feature, could have a similar effect on helicopters sitting on helidecks, it is important that any changes in the prevailing environmental conditions are also conveyed to the helicopter pilot.
That was a prescient observation as within 38 months of the West Navion accident two CHC helicopters had made accelerated departures after being affected by weather phenomenon on fixed installations. In one case this resulted in a minor overtorque, the other occurring with an open baggage bay, an unrestrained passenger holding on in the doorway and one pilot left on the helideck.
The CHC Scotia After Landing checklist required the autopilot to be disengaged when on deck. The CHC Scotia Operations Manual (OM) stated that on a helideck “PF [Pilot Flying] must physically monitor the controls at all times and maintain the cyclic and yaw pedals in a central position…”.
AAIB explain that:
The autopilot is disengaged to prevent it from applying cyclic or yaw control to counter deck movement and the cyclic should remain neutral, for several reasons.
Firstly, any movement of the rotor disc from a position horizontal to the helideck has fatigue implications for the main rotor mast.
Secondly, and more importantly, if the main rotor tips it can become a hazard to passengers and crew on the helideck.
Blade strike fatalities had occurred with an S-76A+ in 1992 and an AS365 a few months later.
Finally, if the pilot [or autopilot] tries to ‘fly’ the rotor disc to counter ship movement, and subsequently needs to apply collective control quickly for an emergency lift off, there is a risk of dynamic rollover.
The OM also states:
Commanders are to ensure that while the helicopter is on a helideck rotors running, one pilot is looking out at all times, so that the HLO can attract the attention of the pilot in the cockpit, and that any helicopter movement may be perceived and corrected immediately.
AAIB note that the ‘movement’ the OM intended to refer to was of the helicopter sliding not rolling over. While not commented on by AAIB, this clearly assumes both crew remain onboard while on deck, when it was common for one to disembark to supervise loading and refuelling.
The above requirement ensures that much of the pilot’s attention is focussed outside the cockpit whilst the helicopter is on deck. Inside the cockpit, it might normally be expected that the compass should indicate a change of heading and alert the pilot, in this case, to a change in the ship’s heading.
However, prior to landing on an offshore installation or vessel, the helicopter gyro compass is selected to ‘Direct Gyro’ (DG or Free) mode, as opposed to ‘Slaved’ (Magnetic) mode. This is to prevent the mass of steel of the installation structure from inducing a deviation error on the compass reading. Whilst on deck with the compass in DG mode, the compass will be subject to precession and other errors, and thus will not necessarily indicate an accurate heading.
If there are no external features, like land or another vessel in view, it can be difficult for a pilot to perceived any change of heading.
The West Navion had no procedures for what to do if there was a heading change while an helicopter was on deck.
An Aftercast issued by the Met Office for the period covering the time of the accident, gave a surface wind for the West Navion of 270°T/32-42 kt. The Aftercast observed that it was difficult to give an accurate indication of gusts aboard a ship since the wind tends to gust around some parts of the superstructure differently from others. The Aftercast further stated that the maximum gust of 42 kt should be regarded as a guide to that which was possible in the airmass.
West Navion’s own wind measurements indicated an average of 26 kt, but filtered out gust data. Standby vessel Faroe Connector recorded 37 knots on average, again filtering out gusts. The Foinaven fixed installation (c 10 nm SSE) recorded an average of c 37 kt, with gusts up to 45 kt.
AAIB Conclusions
The AAIB identified the following causal factors:
(i) Unknown to the crew on the bridge, the ship’s DP system reverted to manual heading control and the ship’s heading began to drift to the right.
(ii) The increased lateral wind component to which the helicopter was consequently subjected, generated increasing aerodynamic forces to the right due to the change in the relative wind, and these forces provided the most significant toppling moments of all the forces acting on the helicopter.
(iii) The ‘static’ roll attitude of the helicopter adopted after landing, relative to the helideck, of 2.5° to the right, together with the lift force generated by the main rotor in the prevailing wind, the 1° list of the ship to the right at the time of the accident and the natural motion of the ship, contributed to the de-stabilisation of the helicopter.
(iv) The lack of procedures on the ship to transmit the change in the alert status to the crew of the helicopter, and of any specified procedure available to flight crews concerning action to be taken if control of the ship is lost or degraded whilst on the helideck, denied the pilot an appropriate course of action to ensure the safety of the helicopter.
Four safety recommendations were raised as a result of this investigation.
Helideck Motion Systems (HMS)
Since 1 April 2021 it has been regulatory requirement in the UK that helidecks are either fitted with an HMS (including visual warning to the flight crew) or are limited to stable deck conditions.
a) Measurement/monitoring of helideck ROLL, PITCH, INCLINATION and HEAVE RATE (as previously).
b) Measurement/monitoring of the new helideck Motion Severity Index (MSI) and Wind Severity Index (WSI).
c) Measurement/monitoring of relative wind direction (RWD) and 2min averaged wind speed.
d) New HMS repeater lights are installed on the helideck. The status of the lights is determined automatically by the HMS, as explained below.
The procedures are divided into two phases, and the HMS operates in two corresponding, distinct modes with the following repeater light indications:
HMS Pre Landing Mode (Credit: HCA)
After touchdown, the pilot reports the helicopter’s heading to the RO who enters this into the HMS. This switches the HMS to ‘ondeck’ mode. The HMS then calculates the relative wind direction using helicopter heading at touchdown together with the real time vessel heading and wind direction, and monitors this against the wind speed-related safe operating limit.
HMS On Deck Mode (Credit: HCA)
Safety Resources
The European Safety Promotion Network Rotorcraft (ESPN-R) has a helicopter safety discussion group on LinkedIn. You may also find these Aerossurance articles of interest:
- Helideck Heave Ho!
- S-92A Offshore Landing Obstacle Strike: CENIPA Report
- Urgent Exit Required: A Helideck Incident (Omni Sikorsky S-76C+ PR-SEC)
- Helideck Safety Alerts: Refuelling Hoses and Obstructions
- NTSB Recommendations on Offshore Gas Venting
- Mind the Handrail! – Walk-to-Work Helideck Hazard
- US BSEE Helideck A-NPR / Bell 430 Tail Strike
- Troublesome Tiedowns
- Wrong Deck Landings
- FOD and an AS350B3 Accident Landing on a Yacht in Bergen
- Helideck Lightning Strike: Damage Missed
- Pedestrian Seriously Injured by Air Ambulance Landing at Melbourne Hospital
- Air Methods AS350B3 Air Ambulance Tucson Tail Strike
- Air Ambulance B407 Hospital Helipad Deck Edge Tail Strike During Shallow Approach
- NTSB on LA A109S Rooftop Hospital Helipad Landing Accident
- Air Ambulance Helicopter Fell From Kathmandu Hospital Helipad (Video)
- AAIB Report on 2013 Sumburgh Helicopter Accident
- AAIB Report on the Ditchings of EC225 G-REDW 10 May 2012 & G-CHCN 22 Oct 2012
- EC225 LN-OJF Norway Accident Investigation
- Sikorsky S-92A Loss of Tail Rotor Control Events
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