Main Gearbox Remains Helicopters’ Achilles Heel
In Greek mythology, when Achilles was a baby, it was foretold that he would die young. To improve his chances of immortality, his mother, Thetis, took Achilles to the River Styx, which was supposed to offer powers of invulnerability, and dipped his body into the water. But Thetis held Achilles by the heel… The rest of the myth is well known. If the mother of the helicopter concept ever dipped it into the River Styx, she probably held it by the main gearbox.
As recent accidents (as investigators call them) or incidents (the term preferred by the manufacturer and the operator) have shown, the main gearbox has consistently been the Achilles’ heel of helicopters, despite the efforts of design engineers to refine main-gearbox architectures for decades. Civil aviation authorities have regularly published rules and airworthiness directives. At some point, the industry even thought some helicopters (the Eurocopter EC225) had a reliable backup system in case the primary system for gearbox lubrication failed.
Apparently this was not enough. Two workhorses of the offshore transport industry, the EC225 and the Sikorsky S-92, have had repeated, serious problems. Despite industry efforts, one fundamental problem remains: a gearbox cannot be made redundant.
However, some parts are continuously improved. As part of an effort to enhance vertical-shaft durability and hardness, Eurocopter switched from 16NCD13 steel alloy to 32CDV13 steel alloy. The switch to this “second generation” shaft was for the EC225. The second-generation shaft is also known as the “nitrided” shaft, but the subsequent problems appeared in an area unaffected by nitriding, a surface-hardening process.
The EC225 experienced two incidents last year, one on May 10 (on a Bond-operated EC225) and the other on October 22 (with a CHC-operated aircraft). In both cases, cracks developed in the welding area between the shaft itself and the bevel gear, ultimately causing a loss of lubrication since the bevel gear drives the oil pump. Part of the cause might be the gearbox housing’s reaction to vibration, a French industry source told AIN.
Gradual Return to Service
In the wake of the second failure, helicopters produced or retrofitted with the new shaft were grounded, affecting a significant number of AS332 Super Pumas, causing widespread inconvenience for the operators and the offshore workers they serve.
The 19 EC225s flying in the North Sea could not be retrofitted with the “first generation” shaft. Eurocopter thought intermediate measures, based on more restrictive maintenance, could ensure continued operation of the EC225. Intervals for downloading vibration-monitoring data were shortened to three flight hours. Flight over water was prohibited for those helicopters without a vibration health-monitoring system. But the UK CAA and its Norwegian counterpart took actions that effectively grounded the EC225, except for search-and-rescue flying.
The 20 or so AS332s could be retrofitted with the first-generation shaft, and by late November Eurocopter and local partner Heli One had virtually completed the retrofit of the North Sea fleet. Heli-One also participated in the effort for the CHC fleet. Other retrofits have already been performed in Angola and China. “Fortunately, we have not been limited by the availability of first-generation shafts,” Derek Sharples, Eurocopter executive v-p for support and services, told AIN.
Sharples hopes his company will be able to recommend a return to flight in February, by which time it expects to understand fully the root cause and have a final fix available. Both ground and flight tests will have been conducted. The emergency lubrication system has been tested during a ground run on one of the first serial-production aircraft. Extensive flight-testing, focused on the shaft and the emergency lubrication system, is planned, too.
The emergency system, a much touted back-up Eurocopter has installed on its EC225 to keep it flying for 30 minutes after a loss of lubrication, has become a headache for its designers. In both 2012 controlled ditchings, the pilots received indications of a failure of the main gearbox lubrication system and, subsequently, of the emergency lubrication system as well. In the May 10 event the emergency lubrication system gave the crew a false alarm. It is thought the October 22 event also involved a false alarm, but investigators have yet to confirm this.
In the first incident, the UK’s Air Accidents Investigation Branch (AAIB) found no leak in the back-up system. Moreover, the back-up pump was operating normally from the time the system was activated until the aircraft ditched, the investigators determined. They suggest the bleed-air pressure sensor triggered the warning. According to the AAIB, during the development of the helicopter no test was carried out on the complete back-up system, either on a test rig or installed on the helicopter itself. Sharples maintained that it was “fully tested, during a ground run on one of the first serial-production aircraft.”
Eurocopter is not the only helicopter manufacturer having problems with the main gearbox of its heaviest civil model. Sikorsky has been struggling with the S-92’s gearbox since the 2009 fatal crash of a Cougar-operated aircraft off Newfoundland.
Last summer, the FAA issued a further AD after a crack was found in the gearbox housing of one S-92. The crack, discovered in “the cored passage area adjacent to the scissor bracket mount,” allowed oil to leak.
In the inspections the FAA required, no additional cracks were found, and the problem is now solved, according to Sikorsky. “The cause has been identified in the manufacturing process and corrective measures have been put in place,” a spokesman told AIN.
In 2011, after a series of upgrades, Sikorsky announced it was developing a “next-generation” main gearbox. It was said to feature an automatic oil-cooler bypass switch, unipleat oil filter and improved durability with redundant scavenge. Also, an emergency lubrication system was to be added.
“Development timeline is for testing to commence in 2013 with implementation into the production line following a successful testing and initial fielding effort,” a spokesman said last month. The new design is sized to accommodate planned weight and power growth. It will also fit existing aircraft.
Manufacturers Eye New Approaches
AgustaWestland’s approach to main-gearbox lubrication is to design integral, independent and redundant systems within the main gearbox itself to mitigate the chances of failure, according to Giuseppe Gasparini, head of transmission systems design and development for the company.
“Several decades of development and service experience have demonstrated that there are some weak points on the main-gearbox lubrication system that can cause rapid and complete loss of oil: they are mainly the oil pipes and fittings connecting coolers, filters and other components of the lubrication system itself,” Gasparini said. “It is for this reason that for many years AgustaWestland’s main gearboxes have been designed to exclude the use of any external pipes and fittings. This is not easy because the filter, cooler, fan and so on have to be fully integrated in the main-gearbox castings, and all the pipes are replaced by cored passages in the castings, but it represents a dramatic improvement over conventional designs, in reducing the probability of actual gross oil leakage.”
Gasparini said AgustaWestland takes other measures to ensure the robustness of its main gearboxes, including securing any cover subjected to oil pressure, such as the filter head, with multiple fasteners and operational testing for the loss of at least one fastener. He also noted that AgustaWestland’s main gearboxes have dual lubrication pumps working in parallel. “If one fails or jams for any reason, it is excluded automatically and the remaining pump provides the needed oil. While this failure condition will generate a main-gearbox oil-pressure warning to the pilot, he can immediately distinguish it from a total loss of main-gearbox oil and should be able to complete the mission,” Gasparini said. He noted that AgustaWestland main gearboxes do not have a standby/emergency lubrication pump “because of the added weight, complexity and the risk of dormant failure.”
Eurocopter, too, is adopting a new lubrication system on the in-flight-test EC175 rather than use the same one as on the EC225. The EC175’s main gearbox has two lubrication pumps–a main one and an emergency one–each driven by rotating parts located in different areas of the gearbox. This architecture ensures that one single failure cannot affect both pumps, the company claims.
The three big certification authorities are considering making the rules more stringent, but it is a lengthy process. Following the Cougar accident, the EASA, FAA and Transport Canada created a joint team to “review helicopter main gearbox CS29/Part 29 certification requirements that affect loss of gearbox lubrication.” According to an EASA spokesman, the team report considers that changes to certification rules “could have a significant influence on the design and cost of future helicopter types.” Therefore, the team recommends that representatives from the helicopter industry should be involved in such a rulemaking task. In other words, passengers and crews will have to wait a long time until new rules improve their safety.
Sidebar: “Thirty-minute run-dry?”
Current certification requirements of the FAA, EASA and Transport Canada require that the aircraft must be able to continue safe flight for at least 30 minutes after the crew has detected lubrication system failure or loss of lubrication. This is sometimes referred to as “the 30-minute run-dry requirement.”
There is a provision within that regulation for all three agencies that adds the caveat “unless such failures are [determined to be] extremely remote.” This was the provision under which the Sikorsky S-92 was certified, and it drew the glare of safety advocates such as Canada’s Transportation Safety Board.
Bell Helicopter is confident that the proprietary main-gearbox technology in both its 429 light twin and under-development 525 medium heavy twin can meet the 30-minute “run-dry” (without lubricant) requirement under FAR Part 29.
“We have a robust run-dry capability and we are going to test it [on the 525] all the way until it fails,” said Larry Thimmesch, Bell’s vice president of commercial programs. “It will easily exceed the requirements and we ran it for many hours on the Bell 429. It is an internal design and there are a lot of different aspects to it, including super-finishing the gears and a lot of details within the design that give it that capability. There are a lot of details within the design that keep the main gearbox from leaking oil, but if it does leak oil it will be able to continue to fly safely to the nearest destination,” Thimmesch said.
Thimmesch said that the run-dry tests in the 429 “definitely exceeded way beyond 30” minutes of run-dry without gearbox failure. “It’s one of our competitive advantages, one of our strengths. We brought a lot of industry best practices into designing and manufacturing the main gearbox. It is not really a concern for us,” he said.
Eurocopter says that in the event of an oil leak the EC175 can fly for 30 minutes without any main-gearbox lubricant, thanks to the design of the main gearbox. The manufacturer said it developed this confidence as the result of system tests conducted on the ground.
Operators are not exactly talkative in the aftermath of recent failures. CHC is “encouraged” by Eurocopter’s efforts. Bristow “supports Eurocopter in its ongoing efforts to resolve the matter.” A Bond spokesman told AIN that “Eurocopter remains a firm part of Bond’s strategic plans for the future; we are confident that Eurocopter is making every effort.” Grounded EC225s represent 18 percent of Bond’s fleet.
Jake Molloy, regional organizer of the RMT union of transport workers, was more willing to talk. He said that confidence “has been severely damaged” by the two helicopters that ditched in 2012. Initially, Molloy was unhappy with Eurocopter’s response. But in November “their efforts improved significantly, and today I am pleased with the process they are going through,” he said later.