EASA has certified a redesigned vertical bevel gear shaft for the Airbus Helicopters EC225, which was grounded for nine months in 2012 and 2013 after a series of in-flight failures. Manufacture of the redesigned gear shaft is under way for production aircraft and retrofits, with first installations (for both applications) planned for this year’s second half.
The new design provides corrosion resistance, compensates for residual stress and eliminates stress hot spots, eliminating all three factors that, combined, caused two unexpected vertical shaft failures.
To combat corrosion, a smoother shape inside the shaft prevents “mud” retention areas. Moreover, new oil jets continually flush the welding area while the volume of oil injected inside the shaft for lubrication has been doubled thanks to the addition of two lubrication holes. This minimizes wear on the splines and, as a consequence, keeps paste from forming, according to Airbus.
To compensate for residual stress designers reinforced the welding area, allowing the shaft to withstand three times more stress in the critical areas. Finally, an improved internal geometry removes angles, thus eliminating hot spots, as a small radius is a stress concentration factor.
The new design has been tested extensively for the entire flight envelope. Fatigue tests substantiated a 20,000 flight-hour service life limit. Damage tolerance tests–when artificial damage was created in the welding area–demonstrated high safety margins, according to the company. Under loads around three times higher than maximum flight loads, no crack initiation was found. In addition, the new bevel gear shaft was tested for 200 test hours in a gearbox on a power bench. This reflected the most stringent flight profiles, according to Airbus.
The module-level exchange procedure is “not very complicated,” said Airbus Helicopters CTO Jean-Brice Dumont. The operator’s mechanics will remove the main gearbox module and replace it with one that contains a new shaft. Airbus Helicopters or a certified depot-level maintenance center (such as Heli-One) will replace the shaft inside the module. “We will rotate the main modules; such an industrial organization will allow us to act swiftly,” Dumont said. Delivering the new shafts quickly has been a major concern, lest a customer get the impression that a competitor is served first, he explained.
After installation of the upgraded module, the operator’s mechanics will test the main gearbox on the ground. Alternatively, Airbus Helicopters specialists can perform the trial, using a test bench. Finally, according to the procedure Airbus has devised, the operator will test the main gearbox in flight. It will be the operator’s responsibility to release the aircraft to service. Downtime will be approximately six days.
Airbus plans to begin retrofits in the second half and complete them within two years. “Every operator will be contacted individually, with a retrofit package, dates and all other necessary information,” an Airbus spokesperson said.
The interim measures approved last summer, which involved a smaller hardware modification, thorough monitoring and stricter maintenance, will thus gradually become obsolete.