MGB component failure eyed in fatal Cougar crash

 - May 1, 2009, 11:15 AM

As Sikorsky prepares to deliver its 100th medium-lift, twin-engine S-92A, the role main rotor vibration and other past problems with the helicopter’s main rotor gearbox (MGB) played in the fatal crash of a Cougar Helicopters S-92 off Newfoundland on March 12 remains under study by Canada’s Transportation Safety Board (TSB).

However, over the last several years some flight crews have been complaining privately about high blade loading when the four-blade S-92 is flown at high gross weight. They have also expressed concerns about component durability when the helicopters are repeatedly subjected to these loads. At least one of those components on the crashed Cougar helicopter, the main gearbox oil bowl mounting studs, failed.

Sikorsky does not formally acknowledge that the S-92 has a vibration problem and will not comment on any particulars of the Cougar crash. However, the company has made several moves that could be construed as a tacit addressing of some issues. They include advising operators that the company is working on an improved main rotor anti-vibration system, unveiling a new S-92 “cockpit hush kit” at this year’s Heli-Expo in Anaheim and developing a Canadian military variant, the CH-148 Cyclone, with fly-by-wire (FBW) controls.

Perhaps most telling, last year, when it re-bid the Air Force’s Combat Search and Rescue (CSAR-X) program, Sikorsky offered the Pentagon the HH-92, a proposed S-92 variant with 20 percent more powerful engines, FBW and a five-blade main rotor system. (The Obama Administration deleted CSAR-X from the FY2010 defense budget. For the time being, the Air Force’s fleet of 25-year-old HH-60 Pave Hawks will continue in that role.)   

More immediately, examination of the Cougar wreckage prompted Sikorsky to recommend the grounding of all S-92s unless their titanium main gearbox oil bowl mounting studs had already been replaced with steel ones. Investigators for the TSB found a broken stud on the Cougar S-92 that crashed into the Atlantic off Newfoundland minutes after the pilot reported “zero” MGB oil pressure. The two pilots and 15 of the 16 oil rig workers aboard were killed. TSB investigators suspect the broken stud was a “contributing factor” in the crash and recommended that the FAA issue an emergency AD grounding all S-92s that had not had the part replaced. The FAA did so March 23 and within a week Sikor-sky reported that 50 of the 91 S-92s in service already had been fitted with the new studs. Work was completed on the remainder of the fleet by early last month.

Before the crash, Sikorsky had issued an alert Service Bulletin recommending that the studs be replaced within one year or 1,250 hours, which- ever came first. That requirement became mandatory and immediate after the Cougar crash.

A TSB spokesman declined to speculate if the studs failed due to excessive vibration, design deficiency or improper installation. He also would not comment on reports that the downed helicopter’s flight data recorder showed falling MGB oil pressure just minutes after liftoff, perhaps at a point when the helicopter could have safely returned to land. Also unknown at this point is to what extent Cougar’s maintenance personnel worked on the helicopter’s MGB before the crash. On February 23, Sikorsky announced an agreement with Cougar’s parent company, VIH Aviation Group, that authorized Cougar to repair and overhaul S-92 gearboxes.  

The MGBs themselves have been the subject of some controversy. The two pumps that lubricate the system have been revised four times to address various design and component defects. While the transmissions have a recommended TBO of 6,000 hours, the gearbox housings must be replaced every 2,700 hours due to fatigue life and some past cracks that have produced MGB oil “seepage,” according to Brian Young, Sikorsky’s commercial programs director.

MGB certification standards for the S-92 appear to have been relaxed, with Sikorsky obtaining a JAA exemption from a requirement that the gearbox run dry for 30 minutes. Sikorsky is designing a back-up MGB lubrication system for the 28 Canadian CH-148s on order, first deliveries of which now have been pushed back to 2012, as a condition of that contract. However, it is not known if the system will be offered to commercial customers.  

The S-92’s four-blade main rotor system and associated components, including the MGB, might be subjected to additional stress as Sikorsky pushes to expand the mtow envelope from the current 26,500 pounds to near 27,000 pounds. That increase, along with Category A certification for confined areas, is coming soon, said Young.

The commercial programs director defends the S-92’s four-blade design as building on the successful heritage of the time-tested S-70 Black Hawk series and in terms of meeting customers’ cost and maintenance requirements. Two-thirds of the S-92s delivered to date have been to the cost-competitive offshore oil market. “An extra blade adds 25 percent to the cost of the rotor system,” said Young. “We believe that [with] Sikorsky’s proprietary technology, to be able to aerodynamically design the blades, to have the vibration control systems, that four [blades do] a very good job. As we [raise] the gross weight beyond where it is today, one of the things we will obviously look at is the number of rotor blades. But right now we have no plans to do anything beyond the four.”

Currently, the S-92 employs two types of vibration control: a traditional bifilar pendular absorber affixed to the main rotor above the rotor head, and an active vibration control system on the fuselage that uses computer-controlled force generators to produce out-of-phase vibration-canceling signals. Three force generators are standard and an additional three are optional. Young said the extra generators provide “a more uniform cabin vibration environment from seat to seat and from cockpit to cabin.”

Sikorsky has made numerous improvements and added significant capabilities to the S-92 since its first delivery in 2004, including the development of SAR, multi-mission and VIP variants; increasing maximum gross weight from 26,150 to 26,500 pounds; adding an auxiliary fuel system; and giving the avionics management system more capabilities. To date, the fleet has amassed 140,000 hours and achieved an overall dispatch rate of 90 percent. Young said the company hoped to boost that “into the high 90s” with help from Sikorsky’s S-92 flight management operations center (FMOC). The FMOC constantly monitors all S-92s in service and analyzes operator health and usage monitoring system data with an eye to extending part life and reliability and ensuring an adequate supply of field spares.

Young said that the FBW system in the CH-148 could find its way into civil S-92s “at some point in the future.”