The Transportation Safety Board (TSB) says passenger helicopters should be able to fly for at least 30 minutes without oil in their main gearbox — and perhaps longer in harsh environments, such as the Newfoundland and Labrador offshore.
On Wednesday, the TSB outlined a “complex web” of factors that led to the deaths of 17 people in the crash of a Sikorsky S-92 flown by Cougar Helicopters.
Some of those factors occurred long before the S-92’s titanium studs failed the morning of March 12, 2009, allowing oil to leak out of the main gearbox that powers the helicopter.
Eleven minutes after the flight crew noticed the drop in oil pressure and headed back to land, the helicopter plunged into the North Atlantic.
“This tragedy was about more than failed titanium studs, and had our investigation stopped there that would have been too simplistic,” said Wendy Tadros, TSB chairwoman.
“In the months that followed, we dug deeper and we found many underlying problems.”
One of them was the aircraft certification standard: how long a helicopter can continue to fly after losing all oil from the main gearbox.
All but one of the 18 people aboard Cougar Flight 491 drowned as the helicopter quickly sank once it hit the water.
“Due to the crashworthy features of the S-92, all 18 people survived the initial impact with the water,” said Mike Cunningham, TSB investigator in charge.
“However, only two of the helicopter’s occupants escaped the wreckage and, as we know, only one of those folks survived.”
The sole survivor, Robert Decker, spent 76 minutes in the frigid water before he was rescued.
The four TSB recommendations are outlined below.
The TSB also called on the U.S. Federal Aviation Administration (FAA) to reassess the way helicopters are certified.
The Canadian agency has no power to compel the FAA to follow its recommendations.
“We have a very high rate of acceptance of our recommendations and we’ll be having discussions with the FAA next month to talk specifically about these issues,” said Tadros.
“But it would be my hope that they would look at everything in this report very carefully and be quite mindful of its recommendations.”
The FAA is the certifying authority for Sikorsky, the U.S.-based helicopter manufacturer. In turn, Transport Canada certifies the aircraft for use in this country.
FAA rules require helicopters have a 30-minute run-dry capability — unless failure of the helicopter’s oil lubricating system is demonstrated to be “extremely remote.”
This was the case for the S-92, which failed the run-dry test and operated 11 minutes once the main gearbox lost oil, according to the TSB.
Although the TSB said the certification of the S-92’s run-dry capability wasn’t a cause of the crash, it does remain a risk.
“The 30-minute requirement is negated by the extremely remote provision,” said Tadros. “Therefore, it needs to go.”
The TSB said the Sikorsky S-92 is the only helicopter certified using this provision.
Delayed crew response
Mike Cunningham, TSB investigator in charge, said many factors that led to the crash were beyond the control of the flight crew.
“The written emergency procedures began with a non-critical gradual loss of oil, eventually concluding with a critical loss of oil,” he said. “This delayed the crew’s response to this emergency.”
Cunningham said the loss of oil procedures in both the flight crew’s manual and training emphasized that increased oil temperature would be indicated.
“Instead, the crew saw a normal temperature indication.”
That temperature indicator contributed to the crew’s belief that the low oil pressure was the result of a sensor or pump problem — not a total loss of oil from the main gearbox.
He said the crew was “seeing something for the first time.”
“Based on their understanding of the situation and the belief that there was still oil in the main gearbox, they decided that continuing flight toward St. John’s was less risky than ditching.”
Once the tail rotor began to fail, Cunningham said the crew wasn’t able to safely ditch the helicopter.
“A tail rotor failure is one of the most difficult emergencies to control, and this was the first documented case of one in an S-92.
“They did their best faced with an extremely challenging situation.”
A July 2008 incident in Australia suggested the possibility of complete loss of main gearbox oil wasn’t as remote as suggested.
An S-92 operated by CHC Helicopters lost oil pressure when two titanium studs broke in flight.
They were over water at the time, but crew was able to land safely on shore about eight minutes after noticing the pressure drop.
Mark Clitsome, TSB director of air investigations, said an independent laboratory analysis of those studs confirmed the bolts broke because of metal fatigue.
“One possibility for this fatigue was galling,” he said. “Galling happens when nuts wear down the threads as they are removed and reinstalled to change the oil filter.”
Clitsome said titanium doesn’t react well to this type of wear and tear.
On Jan. 28, 2009, Sikorsky Aircraft Corp. issued an alert service bulletin advising S-92 operators to replace titanium mounting studs on the main gearbox oil filter assembly with stronger, steel mounting studs.
It said the studs should be replaced within 1,250 flight hours.
Cougar Helicopters ordered the new parts Feb. 19.
“They had not, however, effectively performed enhanced inspection and maintenance procedures,” said Clitsome.
“This meant damaged studs were neither detected nor replaced.”
Cougar updates procedures
Hank Williams, Cougar’s general manager, issued a release Wednesday saying the company has stepped up its helicopter maintenance reviews and quality assurance procedures.
The company said it has installed a redesigned main gearbox, attachment studs and oil filter on its fleet of S-92s.
“Cougar is confident in the improved design and increased robustness of the S-92 main gearbox and the airworthiness of the S-92 helicopter,” said the release.
The company also said it has updated and improved its emergency check list for pilots, and integrated those changes into its pilot training program.
Paul Jackson, spokesman for Sikorsky Aircraft Corp., declined comment on the TSB report until it finishes analyzing it.
“We recognize the great effort that went into preparation of this report, and we will need time to study it fully,” he said in an email.