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EASA certifies redesigned EC225 gear shaft

By Vertical Mag

By Robert Erdos | April 23, 2014

Published on: April 23, 2014
Estimated reading time 7 minutes, 58 seconds.

The redesigned vertical bevel gear shaft for Airbus Helicopters’ EC225 has been certified, enabling its production and retrofit on the aircraft’s global fleet, as well as its installation on all future EC225s.
The redesigned vertical bevel gear shaft for Airbus Helicopters’ EC225 has been certified, enabling its production and retrofit on the aircraft’s global fleet, as well as its installation on all future EC225s.
The new design, certified by the European Aviation Safety Agency (EASA), eliminates all three factors that, in combination, had caused two vertical shaft ruptures in 2012, resulting in two high-profile controlled ditchings in the North Sea. Airbus Helicopters said the shaft provides corrosion resistance, compensates for residual stress and eliminates stress hot spots.
Manufacturing of the redesigned gear shaft is already underway, and Airbus Helicopters said worldwide retrofit and installation would begin in the second half of 2014.
“Airbus Helicopters has put a very high priority on developing this new shaft for the EC225,” said Jean-Brice Dumont, Airbus Helicopters’ executive vice president of engineering. “EASA’s certification is a major milestone towards getting our customers flying with the final solution.”
Root of the problem
To fully understand what Airbus Helicopters has done to fix the problem (and why) —the mechanical failure mode common to both 2012 ditchings must be explored.
The main gearbox of the EC225 normally contains 22 liters of oil circulated by a pressure lubrication system. The system incorporates a main and a standby oil pump, both of which are mechanically driven by a bevel gear mounted to a vertical shaft at the bottom of the gearbox. This shaft, a two-part welded assembly, is installed in all EC225s and some AS332 L1 and L2 helicopters. 
The certification criteria demand that a failure of both oil pumps should not be catastrophic. EASA regulations under which the EC225 was certified require that helicopters be capable of continued safe flight under prescribed conditions for at least 30 minutes following a total failure of the gearbox lubrication system; a feature typically called a “run dry” capability. In lieu of actually running dry, the EC225 further incorporates a separate emergency lubrication system, consisting of an 11 litre tank of a glycol-water mixture, called Hydrosafe 620. When activated, bleed air pressure from the left engine delivers a spray of Hydrosafe 620 from nozzles around the interior of the gearbox, providing sufficient fluid for at least 30 minutes of gearbox cooling and lubrication. The system is controlled by a dedicated circuit board which monitors inputs from pressure transducers on the engine bleed-air line and the coolant feed line. The onboard vehicle monitoring system (VMS) will warn the crew of emergency lubrication system malfunction if pressure in either line drops below a prescribed threshold. 
Initially, in the case of both accidents, the crew was confronted with indications of loss of gearbox oil pressure, which required activation of the emergency lubrication system. And, in each case, the “MGB EMLUB” caption subsequently illuminated, indicating failure of the back-up lubrication system. In accordance with the checklist procedure to “land immediately”, both crews proceeded to perform a controlled ditching at sea. Fortunately all passengers and crew were rescued.
So, what happened? The accidents were caused by two separate failure modes. Initially a circumferential failure of the weld on the vertical shaft precipitated the loss of gearbox oil pressure. The subsequent failure of the emergency lubrication system can be chalked up to a minor boo-boo with major ramifications. Owing to an error in the specification to a switch supplier, the emergency lubrication system indicated that it had failed when in fact it was operating normally. The faulty specification created a dormant failure of the system; the system working exactly as it was wired, but certainly not as designed. Post-accident investigation on both helicopters confirmed that Hydrosafe 620 had been introduced into the gearbox, and no evidence of heat damage to the gearboxes was found. The crew had no way of knowing that the emergency lubrication system was operative, so under the circumstances ditching the helicopters was the appropriate course of action. 
Finding a Solution
When Vertical visited Airbus Helicopters’ headquarters in Marignane, France, late last year, the company was very forthright about the outcome of the two gearbox-related accidents. I sat down to chat with Andrew Warner, chief test pilot, and Patrick Bremond, head of the Test Center. Their personnel were extensively involved in EC225 investigation for the better part of a year. They said that while inspections found no cracks in other in-service helicopters, the failure mode of the weld could be replicated on a test rig. The vertical shaft was redesigned and tested. 
Interestingly, the original certification requirements did not demand that the emergency lubrication system be tested in an actual helicopter. Certification tests were performed on a ground rig, using pressurized air to simulate engine bleed air to spray Hydrosafe 620 into the gearbox. Subsequent to the accidents, Airbus Helicopters went well beyond those tests. In a remarkable demonstration, Airbus Helicopters Test Pilot Hervé Jammayrac was tasked to fly an EC225 with a deliberately cracked vertical shaft until it failed. The test proved both the integrity of the gearbox following failure and the in-situ performance of the emergency lubrication system. 
One of the outcomes of the investigation was that the onboard health and usage monitoring system (HUMS) was able to register a change in gearbox vibration in the hours before the accidents that proved indicative of the eventual vertical shaft failure. Two data signals which indicate the meshing frequencies of the bevel gear and the oil pump drive pinions were discovered in retrospect to have recorded exceedances. As an outcome of the accidents, an interim measure was taken to install a software update and a HUMS caution light on the instrument panel to facilitate monitoring. In the event that the light illuminated, the crew was to limit flight duration to two hours; sufficient endurance for a return to base. As an ironic milestone from the accidents, Janick Blanc, vice president for EC225 and Super Puma programs, told me that “the EC225 has become the first helicopter to use HUMS for flight limit monitoring.” This experience certainly highlights the value of HUMS as a means of ongoing diagnostic monitoring.
It has been a long road for Airbus Helicopters, and no doubt for EC225 operators, but the manufacturer said it has employed an unprecedented engineering effort to develop the permanent solution. Now, with EASA certification secured, it said validation of the redesigned gear shaft will soon follow from other international airworthiness authorities.
For additional information, visit the EC225 Knowledge Center: http://www.EC225news.com

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