The EASA approved the interim solution–a combination of vibration monitoring, inspections and part modification–that Eurocopter devised for the EC225 to address main gearbox (MGB) lubrication issues after the type experienced two controlled ditchings last year. The manufacturer’s fix is intended to reduce the likelihood of an undetected crack, leading to in-flight rupture and ditching, to one per billion flight hours. At issue is the bevel gear vertical shaft, a part that is critical to main gearbox lubrication. The first of the 80 helicopters essentially grounded by a series of ADs was expected to return to service last month.
After conducting trials on test benches on the ground and a flight demonstration of the failure sequence, up to rupture, Eurocopter claims now to understand the root cause of the problem. The EASA describes it as “a combination of several factors, including stress hot-spots induced by the shaft geometry, residual stresses in the shaft weld material resulting from the manufacturing process and corrosion pitting inside the shaft on areas where gear spline wear particles had accumulated.”
To prevent crack initiation, a new MGB oil jet is to be installed. In addition, repetitive cleaning is required, at intervals ranging from 150 to 400 flight hours, to eliminate the residue from spline wear, which traps moisture.
To detect crack initiation on the ground, the eddy current method is to be replaced by an ultrasonic one. In flight, an upgrade to vibration health monitoring (VHM, also known as M’Arms) is intended to warn the crew early, thus allowing a safe return to base. The key upgrade for M’Arms is designated Mod45. Inspections and Mod45 data analysis are to be conducted at intervals of between three and 12 flight hours.
As a longer-term solution, Eurocopter plans to offer a redesigned shaft in next year’s second half. The thicker shaft will feature better surface finish and lubrication and eliminate hot spots. In addition, the geometry will be improved to reduce sharp angles to avoid corrosion accretion.
All Super Puma series helicopters in the world have been subject to restrictions and strict monitoring. The most affected area has been the North Sea, where British, Danish and Norwegian authorities prohibited maritime overflights. This effectively grounded those EC225s serving the offshore oil-and-gas industry, at great inconvenience for operators and people working on the rigs. AS332s (in the Super Puma series, the EC225 is a derivative of the AS332) could be retrofitted with an older shaft design.
The British CAA announced its plans to lift its restrictions.
In its approval, the EASA does not mention the emergency lubrication system. Since May, an AD requires crews to land or ditch immediately if the MGB’s backup lubrication system activates, thus making the backup useless for continued operations. Eurocopter determined that the system’s performance, in one area of the flight envelope, is different from that assumed during certification. Eurocopter says improvements are imminent to restore the 30-minute backup in case of total loss of oil.
The Helicopter Safety Steering Group, a committee of helicopter users in the North Sea, said it welcomes the decisions by the EASA and the CAA. “The safety barriers proposed will give several layers of assurance that this helicopter is safe to fly,” the group emphasized. However, it also stated it “recognizes the nervousness of the workforce about the return to service and remains committed to ensuring sufficient information is provided.”