Helicopter’s redesigned main gearbox still prone to cracks
A Cougar Sikorsky S-92 helicopter taxis on the runway with offshore workers onboard after landing at the company’s headquarters at St. John’s International Airport in this file photo. — Photo by Joe Gibbons/The Telegram
Improvements to the main gearbox of the Sikorsky S-92 helicopter haven’t solved the problem of cracking foot mounts.
Earlier this month, the U.S. Federal Aviation Authority (FAA) expanded its previous airworthiness directive (AD) to include regular inspections of the new main gearbox assembly “that is prone to the same cracks and corrosion” as the old assembly.
It directs S-92 operators to continue inspecting the main gearbox mounting foot pad and rib for cracks and corrosion every 10 flight hours.
The S-92 is the same model helicopter used to fly workers to the offshore oilfields off Newfoundland and Labrador.
It’s also the same model that crashed 55 kilometres east of St. John’s last year, killing 17 of the 18 people on board.
Since the crash, Sikorsky Aircraft Corp. has redesigned the
S-92’s main gearbox lubrication system.
Among the changes, the company replaced the three-stud oil filter attachment with a six-stud filter attachment.
“Since we issued the first (airworthiness directive), we found out that the main gearbox assembly and housing that has the six-stud attachment is prone to the same cracks as the one listed in the other AD,” said Les Dorr, FAA spokesman in Washington, D.C.
“Sikorsky is still looking into the root cause, but we needed to take action to expand the AD to those main gearbox assemblies as well.”
The pilots of Flight 491 reported a loss of oil pressure in the main gearbox as they were transporting offshore workers to the White Rose and Hibernia oilfields March 12, 2009.
Minutes after the pilots headed back to land, the helicopter crashed.
Weeks later, the Transportation Safety Board of Canada (TSB) said two of three mounting studs broke in flight resulting in a loss of oil from the oil lubrication system.
Those studs attach the oil filter assembly to the main gearbox.
Without them, oil can leak out of the main gearbox, which powers the helicopter’s rotor drive.
Sikorsky is still investigating the root cause of cracks in the main gearbox mounting feet.
“Contributing factors may include corrosion and the bushing press fit in the mounting foot bolt hole,” said the FAA directive issued Nov. 19.
“The actions specified by this AD are intended to prevent the loss of the main gearbox and subsequent loss of control of the helicopter.”
If S-92 operators discover a crack, the directive instructs them to replace the main gearbox before the next flight.
If corrosion, bubbled paint or paint discolouration are detected, the main gearbox must be repaired before the next flight.
Dorr said the FAA will be notified once the root cause is determined.
“As always, you have to make sure you understand the root cause of the problem,” he said.
“Second, you have to make sure that any fix you develop does not cause unintended consequences.”
The FAA will then issue another airworthiness directive.
Since the FAA is the certifying authority for U.S.-based Sikorsky, it issues those directives.
Both Transport Canada and the European Aviation Safety Agency have followed suit and notified S-92 operators of the new inspection requirements.
Eleven S-92 helicopters are registered in Canada — and all but one is owned by Cougar Helicopters, according to the Transport Canada registry.
Cougar, which transports workers offshore, has previously said it checks the main gearbox for cracks after every flight.
The company last reported a foot mounting crack in May.