A Eurocopter EC225 operated by Bond Offshore Helicopters with 14 on board ditched safely into the North Sea on May 10. At 12:13 p.m., G-REDW made “a controlled descent 24 nm offshore,” according to Bond. The investigation is focusing on the failures of two main-gearbox lubrication systems–the standard one and the back-up one.

The helicopter was en route from Aberdeen to the Ensco102 Maersk Resilient platform. The captain and the copilot followed “standard operating procedures” after the helicopter suffered “a loss of gearbox oil pressure,” Bond said. The aircraft remained afloat and upright thanks to its emergency floats–a major contributor to the absence of fatalities. Most fatalities in ditching events happen when occupants drown while trying to escape an inverted cabin.

The Aberdeen Coastguard found the 12 passengers and two crewmembers in a liferaft. After a precautionary medical examination–two of them suffered minor injuries–they returned home. Immediately after the accident, Bond suspended all EC225 flights.

“This was a challenging maneuver but one that our pilots regularly practice in our simulator,” said Bill Munro, Bond’s managing director. British pilot union Balpa also praised the airmanship of the pilots. “This looks like a terrific piece of airmanship from very skilled pilots; a helicopter ditching is one of the most difficult maneuvers in commercial aviation,” said general secretary Jim McAuslan.

Main Gearbox Oil Pressure Slips to Zero

Efforts then shifted to recovering the helicopter with as little damage as possible so that the UK’s Air Accidents Investigation Branch (AAIB) could do its work. According to an AAIB special bulletin released on May 13, the event began, for the crew, when the main-gearbox oil-pressure gauge indicated zero, along with other warnings. The gearbox oil temperature started to increase.

The commander reduced speed to 80 knots, turned back toward the coast and initiated a descent. The crew activated the emergency lubrication system, which is supposed to allow 30 minutes of flight. During the descent, the warning light for this system illuminated. The associated procedure is to land immediately, the AAIB said. The commander briefed the passengers and carried out a controlled ditching.

The AAIB also made public on May 13 its discovery of a “360-degree circumferential crack in the bevel gear vertical shaft in the main gearbox.” The crack appeared in the vicinity of a manufacturing weld, and the shaft failure disabled the drive to both mechanical oil pumps. After the AAIB released its bulletin, Bond extended its flight suspension to the Eurocopter AS332L2 Super Puma, a helicopter in the same series as the EC225.

This lasted until May 15, when Bond announced it would resume all flights within 24 hours. The move followed what Bond termed “a rigorous engineering analysis and safety risk assessment” that included inspection of the entire fleet. Bond engineers conducted “borescope tests” on the bevel gear shaft of each of its EC225s to establish the absence of a crack.

Bond will continue to use the health usage and monitoring system (Hums) on its EC225s and AS332L2s to monitor vibration in the main gearbox. The Hums had recorded increasing vibration a few flight hours before the failure, the AAIB said.

Bond also said it received enough “information and assurances” from Eurocopter to resume flights. On May 14 Eurocopter released a service information notice that was agreed with the EASA, UK CAA, BEA (France’s investigation bureau) and AAIB. Notably, it contained reminders about pre-flight checks and flight manuals. “The EC225 and AS332L/L2 fleets can be safely operated within the standard operational limits,” Eurocopter stated.

In an emergency AD issued on May 18 for some EC225s, the EASA required operators to keep a close eye on the vibration health monitoring system. For those not equipped with the system, the AD restricts overwater flights to day VFR.

Gearbox History

The helicopter manufacturer and Bond have somewhat different views about the gearbox, however. A spokesman for Eurocopter said this is the first time such an accident has happened. He referred to some 4.5 million hours of experience with this design, notably on the AS332. But Bond maintains that the failed gearbox was fitted with a bevel shaft specific to the EC225 and certified in 2004, which implies less experience than the millions of hours Eurocopter cites. In any case, the emergency lubrication system has logged less flight time than the original gearbox design.

In a statement released on May 15, Bond said it continues to work with Eurocopter and the regulatory authorities “to find a permanent solution to the [EC225] issues that have challenged the industry.” The operator added it is “confident there is no link between the gearbox-related accidents that Bond Helicopters has experienced, the causes of which have been proved to be beyond [Bond’s] control.”

The Eurocopter spokesman pointed out that neither the AAIB nor the EASA has required any aircraft grounding. As part of the investigation, Eurocopter is studying the crew’s decision to ditch the EC225 immediately.

The manufacturer is also scrutinizing the maintenance history of the accident gearbox. Investigation of the 2009 fatal crash of a Bond-operated Eurocopter AS332L2 in the North Sea revealed that a particle had been found on the epicyclic module magnetic chip detector 36 flight hours before the accident. The operator’s maintenance technicians did take action, but they did not recognize the presence of the particle as indicating degradation of the planet gear. One of the final investigation report’s highlights was its assertion of imperfections in maintenance programs.

The Helicopter Safety Steering Group (HSSG) met on May 14 “to share preliminary information” about the incident. After the meeting, the HSSG said, “We understand that detailed discussions involving … Eurocopter, the AAIB and CAA have concluded that the particular models in question are fully airworthy. We have had assurances by the helicopter operators that their flight operations remain safe.” Three helicopter operators in the North Sea–Bond, CHC and Bristow–have EC225s in their fleet. The HSSG consists of helicopter operators, oil and gas operators and contractors, offshore trade unions, Balpa, regulators HSE and CAA, and trade association Oil & Gas UK.