More than six months after the end of a nine-month halt to flights over hazardous environments by the Eurocopter (now Airbus Helicopters) EC225, all operators of the heavy helicopters, the majority of which are used to support oil and gas platforms, have returned it to the air having implemented a series of interim fixes certified by the European Aviation Safety Agency (EASA) last July.

To finally eliminate the preventive and precautionary measures put in place in order to keep the fleet flying, Airbus engineers are now looking toward redesigning the bevel gear vertical shaft, the component that failed in two Aberdeen, Scotland-based EC225s in 2012, leading to controlled ditchings into the North Sea.

The redesigned shaft should be available for retrofit onto customer helicopters in the third quarter of 2014. It is planned to feature improvements in surface, finish, lubrication and geometry to avoid what the company calls “corrosion accretion”—a build-up of spline powder and oil that forms a paste inside the humid gearbox. The revised shaft has yet to fly, but it has been “tortured and cracked” in the company's test facilities, says Jean-Brice Dumont, head of engineering at Airbus Helicopters. The new shaft is slightly heavier than the previous one.

For now, however, operators must continue to implement the EASA-mandated procedures to operate the aircraft, including the use of ultrasonic, non-destructive inspection of the component, a new shaft-cleaning procedure and the Mod 45 system, a modification of the EC225's Modular Aircraft Recording Monitoring System (M'ARMS) onboard health and usage monitoring system (HUMS).

The system represents the company's first real-time HUMS use to warn the pilots of potential issues. A sensor in the gearbox enables the Mod 45 to monitor vibration levels on the bevel gear vertical shaft every 5 min. In the event of a warning, signaled by lights on the cockpit instrument panel, the helicopter will be able to continue safely operating for 2 hr. and must return to a base where depot-level maintenance can be performed. Safety documentation states that once a crack has initiated, the main gearbox must be replaced.

Normally, operators download HUMS data at the end of a working day or after each flight, then engineers analyze it and conduct maintenance as necessary. Operators see a move toward real-time HUMS data collection as a step closer to the use of such data in fixed-wing flight operations.

“It has been a very fruitful experience,” Dumont says. “We are starting to think about how we can next use the technology.”

Adding further HUMS detail to the cockpit could increase the complexity of what is already a high-workload environment, Dumont says. And more work would be needed on recording data so that false warnings do not affect operations. “We have to look at whether we make this information available to crews or, by real-time transmission, automatically to the engineers at the home base,” he says.

During the short period of operations with Mod 45, some operators have experienced what has been described as a spurious “Mod 45 fail,” a warning that caused some flights to and from offshore platforms to be aborted, prompting concern among oil workers.

Such a warning occurs when Mod 45 acquisition data is not collected twice consecutively. Engineers found that the data collected for Mod 45 was occasionally being erased on the HUMS by a sensor collecting data for rotor track and balance.

“The fault did not appear during our testing of the system but, after the issues with the EC225, this was very sensitive . . . and we worked very quickly to understand the situation,” Dumont says. “We have now modified the software and this has been certified, and five aircraft are currently flight testing the software. This will shortly increase to 10 and it will be installed into the HUMS of all operational aircraft shortly after.”

SonAir of Angola was the first operator to return the EC225 to flight operations in July, although eight helicopters continued operations without interruption in Vietnam and China during the suspension.

Brazilian operators BHS Helicopter, OMNI and Aeroleo Taxi Aereo were the last to return the EC225 to flight in December due to administrative issues, although not all of their helicopters are back in revenue service. Bristow, which uses the rotorcraft mainly for operations in the North Sea, is in the process of returning the rest of its EC225s to revenue operations. The company says it was cautious about the interim fixes, ensuring they could be put into practice without affecting daily operations. As part of its work, Bristow added the Mod 45 to its flight simulator to familiarize crews with its use.