EASA has cleared the Airbus Helicopters EC225 LP and AS332 L2 for return to service. However, aviation authorities in both the UK and Norway have decided to keep their flight bans on the helicopters in place at this time. Most of the worldwide fleet of these helicopters was grounded June 2 following the fatal crash of a CHC EC225 LP on April 29 in Norway, during which the entire main rotor hub detached from the helicopter in flight, killing all 13 aboard. While the exact cause of that accident remains undetermined, investigators did discover abnormal wear on a main gearbox planet gear.
According to EASA, “Airbus Helicopters (AH) has investigated possible accident contributory factors and determined that the likely cause relates to the rupture of the second stage planet gear, which was found with fatigue and surface degradation. Although the root cause of this failure is still not fully understood, it involved cracking of the planet gear bearing outer race, some spalling and propagation of a crack into the rim of the gear, finally resulting in its rupture.”
Return of grounded helicopters to service is subject to the following conditions outlined this morning by EASA: replacement of the accident type main planet gear with one that has demonstrated better reliability and service life, and reducing the service life of the replacement gear by half; new daily/every 10-flight-hour inspections of chip detectors; new 10-hour inspections of main gear box oil filters; and withdraw all main gear boxes from service that have experienced “unusual events,” even if they exhibit no visual damage.
Airbus Helicopters has issued AS332 Emergency Alert Service Bulletin (ASB) 63.00.83 and EC225 ASB 63A030 (single document at Revision 1), and AS332 Emergency ASB 05.01.07 and EC225 ASB 05A049 (single document at Revision 2), to introduce the necessary instructions to allow return to service.
“There are two configurations of planet gear within the current type design. In-depth review of the design and service data showed that one configuration has higher operating stress levels that result in more frequent events of spalling, associated with rolling contact fatigue, while the other exhibits better reliability behavior," EASA noted. "By limiting the type design to the gear configuration with lower stress levels and better reliability and specifying a reduced life limit, combined with more effective oil debris monitoring procedures and other operational controls, an acceptable level of safety can be restored…This AD is considered to be an interim action and further AD action may follow.”
Airbus Helicopters said the company “takes note of EASA’s decision to lift the temporary suspension it had put in place on June 2, 2016 for the H225 and AS332 L2 fleet. We are providing assistance to our customers and working with related stakeholders to help them return their aircraft to service at the appropriate time. Meanwhile, we maintain our full support to the Accident Investigation Bureau of Norway (AIBN) in the frame of the ongoing investigation.”