The North Sea fleet of Eurocopter EC225 Super Pumas — grounded for 10 months after two ditchings in 2012 — is headed back to sea after corrective actions that civil aviation authorities say were designed to “restore an acceptable level of safety” to offshore operations.
CHC Helicopters — one of three North Sea operators of Super Pumas — conducted the first flight in early August, carrying energy company managers from Aberdeen, Scotland, to the North Alwyn platform to tell offshore workers about their plans to resume use of EC225s for routine crew-change flights.1
A representative of Bristow Helicopters said the company was “in the process of making the required modifications” to its EC225s in the U.K., Norway and Australia and that it plans to return the helicopters to service after the work is completed, probably between October and December of 2013.
The EC225, an 11-ton twin-engine helicopter heavily used in support of offshore energy development, is the most recent model in Eurocopter’s family of Super Pumas.
An expanded version of the AS332, the EC225 was certificated in 2004. It has Turbomeca Makila 2A1 turboshaft engines rated at 1,776 kW (2,382 shp). Maximum cruise speed at maximum continuous power is 142 kt, maximum range with standard fuel tanks of 2,020 L (534 gal) is 838 km (452 nm), and maximum endurance is 5 hr 38 min.
Maximum flight altitude is 20,000 ft.
The EC225 can seat two pilots and as many as 19 passengers, or, in search-and-rescue configuration, four medical crewmembers and up to 20 passengers or three stretchers.
Sources: Eurocopter, Jane’s All the World’s Aircraft
A Bond Offshore Helicopters spokesman said that Bond “continues to work on training its pilots and engineers on the safe implementation of the approved preventative, monitoring and modification measures.” The company “will only resume flying EC225 aircraft once we are sure it is safe to do so,” he said.
The CHC flight came after the issuance of a series of alert service bulletins by Eurocopter, airworthiness directives by the European Aviation Safety Agency (EASA) and safety directives by the U.K. Civil Aviation Authority (CAA) and the Civil Aviation Authority–Norway, all of which outlined the actions required to return the helicopters to safe operation.
In publishing Airworthiness Directive 2013-0138R1, EASA said that its action “approves and mandates the technical solution proposed by Eurocopter. … The Eurocopter solution comprises a set of modifications and inspections which aim at monitoring and detecting vertical shaft crack conditions and reducing the likelihood of any shaft crack initiation.”
The U.K. CAA added, “As a result of the modifications and inspections required by EASA, the CAA has determined that the restrictions placed on the operation of an affected Super Puma can now be lifted, subject to the requirements of the airworthiness directive being met on that aircraft.”
A previous directive that suspended operations for affected EC225s will remain in effect for aircraft that have not undergone the required modifications, the CAA said.
Norwegian authorities issued a similar safety directive.
The modifications ordered by EASA focus on preventing cracking in the main gearbox (MGB) bevel gear vertical shaft of affected EC225s and AS332s — the model from which the EC225 was derived. Such cracking occurred in both helicopters involved in the 2012 ditchings, and in both cases, EASA said, “a full circumferential crack of the lower vertical shaft of the MGB bevel gear occurred in the area where the two sections of the shaft are welded together. As a result, the vertical shaft ceased to drive the main and backup oil pumps, leading to warning indications of the loss of the MGB main and standby oil lubrication systems.”
Eurocopter President and CEO Guillaume Faury said he was confident that the “designed and approved measures are the right ones, allowing a safe return to service of the entire fleet.”
The U.K. Air Accidents Investigation Branch (AAIB) is continuing its investigations of both accidents, but in a series of special preliminary reports, the agency said that in each case, after the bevel gear vertical shaft fractured, there were warnings of loss of MGB oil pressure. Each crew conducted the “Total Loss of MGB (Main Gear Box) Oil Pressure” checklist, which called for them to activate the MGB emergency lubrication system.2
“In both cases, approximately 30 seconds later, the MGB EMLUB [emergency lubrication] caption illuminated on the CWP [central warning panel], indicating that the emergency lubrication system had failed, resulting in the subsequent ditching of the helicopters,” the AAIB said.
The agency added that the two accidents are “the only two known occasions in which the emergency lubrication system has been activated in operational flight.” Examinations of the MGBs indicated that, in each case, the system had lubricated and cooled the MGB before it failed.
The first ditching, on May 10, 2012, occurred at 1114 local time during a scheduled flight from Aberdeen (Scotland) Airport to the Maersk Resilient platform in the North Sea, about 150 nm (278 km) east of Aberdeen. About 34 nm (63 km) east of the airport, the helicopter was in cruise flight at 3,000 ft when MGB oil pressure warning lights illuminated.
The commander took the controls, reduced airspeed to 80 kt, turned toward the coast, began a descent and activated the emergency lubrication system. During the descent, the MGB EMLUB caption illuminated, the report said, adding, “The associated procedure is to land immediately.
“The commander briefed the passengers and carried out a controlled ditching. The total flight time was 27 minutes.”
After the ditching, the emergency floatation gear helped keep the helicopter upright, and the two pilots and 12 passengers evacuated to one of the helicopter’s life rafts. A search-and-rescue helicopter and a lifeboat rescued all 14. Two passengers received minor injuries.
The second ditching occurred at 1425 on Oct. 22, 2012, about 32 nm (59 km) southwest of Sumburgh, in the Shetland Islands during a planned 226-nm (419-km) flight from Aberdeen north to the West Phoenix drilling rig.
The helicopter was in cruise at 3,000 ft when warning lights illuminated, and the crew performed the checklist actions, including activation of the MGB emergency lubrication system.
“Within a minute, the MGB EMLUB caption illuminated on CWP, indicating that the emergency lubrication system had failed,” the AAIB said.
The crew ditched the helicopter near a ship, and, along with the passengers, boarded two life rafts. They were subsequently rescued and taken to the ship. No injuries were reported.
In the most recent of its preliminary reports, published in March, the AAIB said that, although it had found fatigue cracks in the bevel gear vertical shafts, it was continuing to search for reasons for the cracking, with testing of material properties and a review of fracture mechanics.
The AAIB described the bevel gear vertical shaft (Figure 1) as consisting of a main bevel gear and a vertical shaft, joined by an electron beam weld.
“To ensure the integrity of the weld, the disrupted material at the end of the weld is removed by drilling and reaming a diameter 4 mm [0.2 in] hole,” the AAIB said. “The inner and outer surface of the weld region is then machined to remove the cap and root of the weld. A plug is fitted into the hole to prevent leakage of the lubrication oil.”
In the MGB, the bevel gear vertical shaft is supported by bearings at the top and bottom of the shaft. Gear teeth on the vertical shaft are surface hardened, but the design does not call for similar treatment of the vertical shaft or the part of the bevel gear wheel that is welded to the shaft, the AAIB said.
The reports said the bevel gear vertical shaft has a life of 20,000 flight hours and must be overhauled every 2,000 hours. In the helicopter that was involved in the May ditching, the bevel gear vertical shaft had been in operation for 167 flight hours — approximately 20 million shaft cycles — since new. In the helicopter involved in the second ditching, the bevel gear vertical shaft had been operated about 1,800 flight hours, with an overhaul expected in about 200 hours.
“The failure of the bevel gear vertical shaft on both … helicopters occurred as a result of high cycle fatigue cracking in the area of the weld and is thought to be as a result of the shaft bending (flexing) as it rotates,” the AAIB said.
The Helicopter Safety Steering Group (HSSG) — made up of helicopter operators, the energy industry, labor unions representing pilots and offshore workers, and regulatory authorities — said that Eurocopter had determined that the “fatigue life of the shafts was reduced by manufacturing residual stress and stress concentration in the weld area. Then, due to further weakening caused by corrosion pits, and active corrosion attack from trapped moisture in the weld area, small cracks formed.3
“With the high shaft rotation speed, the shafts rapidly accumulate millions of … fatigue cycles, and the cracks grew until the shafts failed, after about 20 hours of flying since crack initiation.”
In a briefing note on the Super Puma investigation, HSSG said that Eurocopter developed corroded metal test samples in a laboratory and fatigue-tested them until small cracks formed.
In this situation, corrosion occurs if there is stress associated with flight loads, if the temperature is warmer than about 80 degrees C (176 degrees F) and if moisture is trapped against the shaft, HSSG said.
In the accident helicopters, the fatigue cracks began in the high-stress area near the weld but at different points, HSSG said. In the helicopter involved in the May event, the crack initiated in a corroded area at the weld stop hole. Water was found in a space between a plug and countersink; the water was believed to have been there when the gear shaft was manufactured. In the helicopter involved in the October event, the crack began in a corrosion pit in the weld area where moisture had been trapped in layers of a pasty substance that consisted of a combination of fine metallic power from the gear splines and oil from the lubrication system, HSSG said.
Water in the Gear Shaft
Water typically accumulates in the gear shaft as condensation on the hollow metal interior.
“The shafts are … ventilated by air laden with water vapour,” HSSG said. “The MGB is hot with the rotor turning but cools when the engines are shut down. The water vapour condenses on the metal surface as droplets. …
“At full speed, the shaft rotates at 2,400 rpm, causing high centrifugal force. Oil sprayed into the lower shaft by jets is carried up to the weld area and splines by centrifugal force. Water is denser than oil, so the force on the water is higher than on the oil, throwing the water out against the inside of the shaft under the paste.”
Corrective actions focus on removing moisture and thereby stopping corrosion, as well as imposing a schedule of frequent gear shaft inspections and monitoring in-flight vibration to ensure early detection of small cracks. If cracks are found, EASA said, the vertical shaft should be replaced before the next flight.
Summarizing the corrective steps, HSSG noted that maintenance will be performed to clean the affected bevel gear vertical shaft components and replace an MGB oil jet to improve lubrication, regular periodic inspections will be conducted for early detection of any future cracking and in-flight monitoring will allow pilots to land immediately if they note a loss of drive to the oil pumps.
HSSG said that, as a long-term solution, Eurocopter plans a new shaft design to “minimise stress factors and hot spots and prevent active corrosion.”
The AAIB’s preliminary accident reports also examined the EC225 emergency lubrication system, noting that the Joint Aviation Requirements that governed helicopter certification called for a helicopter to be capable of continuing in safe flight for at least 30 minutes after failure of the MGB lubrication system.
In EC225 LPs, the requirement has been met “with an emergency lubrication system that uses a mixture of glycol and water, called Hydrosafe 620, which cools and lubricates the MGB,” the AAIB said. During a certification test conducted on the ground, an EC225 engine was drained of oil and then ran for more than 30 minutes while pressurized air, intended to simulate engine bleed-air, and Hydrosafe 620 were sprayed into the gearbox. The MGB experienced no significant damage, the AAIB said in a 2013 report on the ditchings.
The report added, “Although the emergency lubrication sub-systems were tested individually, no test was carried out on the complete system during certification, either on a test rig or installed on the helicopter.”
The report also noted that the MGB EMLUB caption was designed to illuminate for any one of three reasons: high oil pressure, low oil pressure or an erroneous signal. However, the report added, “An error in the specification issued to the pressure switch manufacturer resulted in all EC225 LPs, with MOD 0752520 [a 2010 modification] having a pressure switch configuration that results in illumination of the MGB EMLUB caption once the system is activated and after the 30-second delay. This was the most likely cause of the MGB EMLUB caption during the accident flights. … With the erroneous pressure switches, the MGB EMLUB caption will always illuminate after activation of the emergency lubrication system, requiring the crew to land immediately.”
Eurocopter issued an alert service bulletin (ASB) in February to require wiring modifications; a subsequent EASA airworthiness directive made compliance with the ASB mandatory.
The AAIB added that Eurocopter plans to begin using replacement pressure switches “with lower thresholds and tighter tolerances, as well as improved maintenance procedures that will provide the crew with an accurate indication of the status over the entire operating envelope of the helicopter.”
- CHC’s flight was the first involving a helicopter from the North Sea’s grounded EC225 fleet. Operators in other offshore areas also had suspended EC225 operations, and Eurocopter said that the first to return were two helicopters operated by SonAir in Angola. Two SonAir EC225s made a total of three flights in late July between Luanda and two offshore platforms.
- AAIB. Special Bulletins S2/2012, S3/2012, S5/2012, S6/2012, S7/2012 and S2/2013.
- HSSG. Briefing Note.