Grounded in reality: inside the Sikorsky S-92 helicopter investigation

Helicopter safety is in the spotlight again after Sikorsky recalled all S-92 aircraft worldwide for safety checks following an incident on the West Franklin platform in the North Sea in December. What can be learnt from the subsequent AAIB investigation?


At 08:44 on 28 December 2016, a Sikorsky S-92 helicopter carrying two crew and nine passengers from Total’s Elgin platform to the nearby West Franklin installation in the North Sea suffered a technical fault.

The CHC-operated helicopter left “significant gouge marks” on the platform deck after experiencing “unexpected control responses” during landing. Sikorsky immediately issued an alert service bulletin (ASB) recalling all S-92 aircraft worldwide for emergency safety checks centred on the tail rotors.

The majority of the checks were completed by operators that same day; a collaborative investigation led by the UK Air Accidents Investigation Branch (AAIB) is set to take significantly longer.

On 13 February Sikorsky issued the following update on the ongoing tail rotor pitch change shaft (TRPCS) bearing inspections being carried out on its fleet of S-92s in compliance with the initial ASB.

“Following the fleet-wide inspection notice, the great majority of operators are reporting compliance with the physical inspections outlined in the ASB, and those who are not yet in compliance are working towards completion,” the statement said.

“Additionally, Sikorsky continues to review the HUMS [health and usage monitoring systems] data as provided by the operators.  To date, we have received a small number of returned parts and all of those parts are currently being evaluated. We continue to work closely with our supply chain on replacement parts and are coordinating those activities with our operators.

“Safety remains our highest priority, and we will further communicate findings to our operators if the investigation reveals any safety or airworthiness issues that affect the S-92 helicopter fleet.”

The AAIB investigates: potential causes of the S-92 technical failure

A special bulletin released by the AAIB in January contains details of the findings of the preliminary field investigation into the incident onboard the unmanned West Franklin platform in December.

“During the descent to land, at approximately 4ft above the helideck, [the S-92] yawed rapidly to the right, reaching a maximum rate of 30° per second,” the AAIB report states. “At the same time it rolled 20° to the left, at which point the left main landing gear contacted the helideck. It continued to yaw to the right on its left main wheels and nose wheels before the right main wheels contacted the surface. The helicopter came to rest on a heading of 041° having rotated through 187°.”

The subsequent technical investigation revealed significant damage to the tail rotor servo piston, which was attributed to a “severely distressed” bearing within the S-92 helicopter’s TRPCS.

"The subsequent technical investigation revealed significant damage to the tail rotor servo piston, which was attributed to a “severely distressed” bearing."

“The initial findings suggest that the damage to the servo in this case is such that it could have imparted extreme or erratic inputs to the tail rotor at any time after the failure of the primary piston,” the AAIB report says. “Evidence suggests that the yaw which occurred on departure from the Elgin PUQ was uncommanded and may be related to the condition of the TRPCS bearing. The AAIB considers that this failure mode would seriously affect the ability of flight crews to maintain control of the helicopter.”

Just 4.5 flight hours before the accident, the aircraft’s HUMS system captured the first indication of trouble with the bearing, but detailed analysis did not reveal that until after the accident took place.

The AAIB special report also reveals two previous events – the first having occurred in 2007 − in which degradation of the tail rotor pitch change shaft led to reduced tail rotor control in flight. “These events were identified by the flight crews and resulted in immediate landings,” the report states. “The underlying causes were identified and a number of safety measures were introduced.”

The parts from the S-92 aircraft involved in the incident on 28 December in the UK North Sea are currently under forensic examination at Sikorsky’s manufacturing hub in Trumbull, Connecticut.

“At this early stage of the investigation the helicopter manufacturer is not clear whether this bearing degradation is the result of a new root cause, or a previously unidentified failure mode,” the AAIB report states.

Industry reaction: is a back-up helicopter fleet the answer?

Earlier this month, Mick Borwell, health, safety and environment policy director at industry body Oil & Gas UK, admitted the offshore sector had become heavily reliant on S-92 helicopters since the Super Puma was grounded in the wake of a crash in Norway in April 2016 that killed 13 people.

The Airbus H225 Super Puma was flying from Statoil’s Gullfaks B platform to Bergen when it crashed near the small island of Turoey. Over 27,000 people have signed a petition demanding that the helicopter be permanently removed from service.

As of August 2016, there were 56 helicopters operating on the UK Continental Shelf (UKCS), but of the five types that comprised the fleet, only two are classed as heavy – the S-92 and the Augusta Westland 189. Crucially, there were just two 189s, shifting the heavy travel responsibility on to the S-92 model.

Borwell recognised that the lack of a direct replacement for the S-92 was a problem, but also insisted that ongoing calls for a back-up or ‘resilience’ fleet of helicopters across the UKCS were unrealistic.

"It’s easy to talk about a resilience fleet, but we can’t have one."

“It’s easy to talk about a resilience fleet, but we can’t have one,” Borwell told Energy Voice. “Helicopters have to be flown to maintain safe operations. Even if you do maintain them, there is a risk. You can’t have them parked up waiting for a problem to occur.

“Each oil and gas operator has different needs in terms of business continuity and safety,” he added. “There may not be a one-size-fits-all solution. There are different ways of reacting to the problem depending on how many people you’ve got and where the installations are.”

In January, Oil & Gas UK’s board approved the establishment of the Helicopter Resilience Technical Group to discuss potential solutions to the North Sea oil sector’s ongoing transport issues.

Safety in numbers: Sikorsky strengthens S-92 HUMS capability

CHC alone carries more than a million passengers to and from ships and platforms every year. More than 62 million passengers flew to and from offshore installations in the British North Sea by helicopter between 1976 and 2012. In that time, there have been 72 accidents, twelve of them fatal.

With more than one million fleet flight hours of service, a state-of-the-art HUMS system, active vibration control and composite blades, and an accident rate less than a tenth of US civil multi-turbine engine helicopters, Sikorsky markets the S92 as a standard bearer for helicopter safety.

In the wake of the Franklin incident, the operator has introduced measures to strengthen the ability to detect impending bearing degradation, including a review of HUMS data to ensure no anomalies, fleet-wide borescope inspections, a requirement for HUMS to be serviceable before flight, the time between download/analysis reduced to a maximum of five hours and an additional assurance check.

In an era of low oil prices, and with offshore operators under increasing pressure to cut costs, such vigilance must not be allowed to wane. On 2 February, an interim report of the investigation into the fatal H225 accident in Norway in April 2016 was released, a poignant and timely reminder that it is in everyone’s interests that helicopter safety remains at the very top of oil and gas industry’s agenda.