An Airbus Helicopters H175 parked on an oil platform had four main rotor blades snapped off in strong winds after a malfunctioning rotor brake left the crew unable to tie down the still-rotating blades, UK investigators have disclosed.
Although no-one was injured during the 17 February 2023 incident, a crew member on the oil rig was almost struck by one of the broken, but still attached, blades as the rotor continued to turn, the Air Accidents Investigation Branch (AAIB) found.
Flown by Offshore Helicopter Services (OHS), the 2018-built H175 (G-MCSH) was operating a standard crew-change flight from Aberdeen to the Elgin PUQ platform – around 135nm (250km) to the east – with six passengers and two crew aboard.
Around 60nm from the platform, an updated forecast showed strong winds – 36kt (66km/h) from the southwest, gusting to 46kt, as Storm Otto passed through the area – albeit still within the operator’s 60kt limit.
However, data derived from the helicopter’s flight-management system during the approach recorded wind of 60kt, says the AAIB.
Having landed on the platform at 08:08 following the 37min flight, the flightcrew kept the rotors turning while the six passengers disembarked and another boarded for the return flight to Aberdeen.
But during the pre-departure checks the crew was presented with a warning light from the tail rotor gearbox chip detection system. Although the H175 was still flyable, given the uncertainty and weather conditions a decision was taken for it to remain on the platform pending further investigation.
As the pilots shut down the engines and applied the rotor brake, however, “the rotor blades did not fully stop and continued to slowly turn,” says the AAIB.
Unknown to the flightcrew, ground-handling staff at the Aberdeen base had noticed the rotor head “slowly turning” despite the application of the rotor brake as the helicopter was towed out of its hangar that morning.
Although they claimed to have reported the issue to the line-maintenance crew, “when later questioned, none of the maintenance engineers could recall this and no checks were carried out on the rotor brake,” the report states.
Post-incident analysis by Airbus Helicopters revealed that part of the rotor brake mechanism had seized, resulting in the brake pads only contacting the disc over around one-third of their surface, “significantly reducing” its effectiveness.
Around 40min after shutting down the engines, the aircraft commander returned to the helicopter along with two members of the platform’s helideck team to secure the aircraft using tie-down straps.
However, this process took longer than planned; the initial straps for the fuselage were incompatible with the tie-down points on the helideck and the crew “were unable to fit the rotor tie-down straps with the rotors still turning”.
An audit conducted by the Helideck Certification Agency (HCA) – a private company — the day before the incident identified four ratchet straps stored on the helideck, but it failed to check it they would fit either the helicopter or the helideck attachment points, the report notes.
“The delay in finding straps resulted in personnel having to work in worse conditions than necessary, endangering themselves, the helicopter and the platform,” it adds.
And during the 10min in which the crew attempted to secure the aircraft, the wind direction shifted, so the H175 was “increasingly being subjected to wind from its right side”, says the AAIB, causing the blades to “sail”, or flap.
Although the team had managed to attach three straps to the helicopter, at about 09:31 while in the process of fitting a fourth to the right rear of the aircraft, “one of the blades suddenly lifted near vertical, partially breaking off near the root with the broken portion hanging down and continuing to turn.
“As it did so, it narrowly missed the single deck crew member positioned at the rear of the helicopter, who was trying to attach the final strap,” it says. The blade subsequently detached and fell into the sea.
Further efforts to secure the helicopter were then abandoned, with the team retreating into the nearby accommodation block.
But left only partially secured and with its rotor still free, the H175 faced an increasingly challenging situation.
“The four remaining blades continued to rotate, with increasingly strong wind conditions causing them to sail,” says the AAIB.
“Over the next 1h 40min three of the remaining blades lifted at different times to the point where they also failed.”
Subsequent analysis showed that all four blades had fractured in the aerofoil section slightly outboard of the transition from the root. Each had failed “in bending associated with being forced upward”.
Investigators believe the location of the helideck atop the platform’s accommodation block left the helicopter susceptible to the ‘cliff edge’ effect, where wind is funnelled over a structure causing an updraft.
“This updraft increases the wind incidence of the rotor blades causing them to lift furthermore into the updraft, exacerbating the issue,” says the AAIB.
Only partial studies had been conducted into the vertical flow component that could be generated at the helideck, the report says, and no operational reviews were carried out.
Additionally, a gap underneath the helideck meant to allow airflow and mitigate the ‘cliff edge’ effect was partially obstructed, the investigation found.
Since the incident, Airbus Helicopters has issued a safety information notice to operators, stressing the need to “move helicopters away from the edge of landing areas on vertical structures when wind speeds were at or near the published limits”, or use an appropriate means of anchoring the aircraft.
In addition, it is improving the rotor brake maintenance procedures for the type, while the operator has begun recording key parameters related to the brake’s performance.
HCA has also committed to addressing issues identified with its helideck audits.
