Helicopter's redesigned main gearbox still prone to cracks
Improvements to the main gearbox of the Sikorsky S-92 helicopter — the same model that crashed off the coast of Newfoundland and Labrador last year, killing 17 people — haven't solved the problem of cracking foot mounts.
Last month, the U.S. Federal Aviation Authority expanded its previous airworthiness directive 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.
An S-92 helicopter crashed 55 kilometres east of St. John's in March 2009 while ferrying workers to Newfoundland's oilfields. Only one of the 18 people on board survived.
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 (directive)," said Les Dorr, FAA spokesman in Washington.
"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 the ill-fated 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 turned back to St. John's, the helicopter plunged into the ocean and sank.
Weeks later, the Transportation Safety Board of Canada said two of three mounting studs broke in flight resulting in a loss of oil from the 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 directive 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 discoloration 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.
Any one got any more on the 1985 S-61?