Flight and Emergency
On 12 March 2009, at 0917 Newfoundland and Labrador daylight time, Cougar Helicopters Flight 91 (CHI91), a Sikorsky S-92A registered as C-GZCH (serial number 920048), departed St. John's International Airport for the Hibernia oil production platform. The helicopter carried 16 passengers and 2 flight crew. The captain was the pilot flying, occupying the right seat; the first officer was the pilot not flying.
At approximately 0945, after leveling at 9000 feet above sea level for about 13 minutes, the main gearbox (MGB) oil pressure warning light illuminated. The flight crew declared an emergency and initiated a descent toward St. John's. By 0945:25, MGB oil pressure had dropped below 5 psi, constituting a "land immediately" condition per the Rotorcraft Flight Manual. The crew discussed ditching and prepared their immersion suits.
Ditching and Rescue
At 0955, about 35 nautical miles from St. John's, the crew reported they were ditching. Less than one minute later, the helicopter struck the water in a slight right-bank, nose-high attitude with low speed and high rate of descent. The fuselage was severely compromised and sank quickly in 169 meters of water. One passenger survived with serious injuries and was rescued approximately 1 hour and 20 minutes after the accident. The other 17 occupants died of drowning. No signals were detected from the emergency locator transmitter or personal locator beacons.
Investigation Findings
The Transportation Safety Board of Canada (TSB) investigation focused on the loss of MGB oil pressure. During the transit toward shore, loss of MGB oil led to failure of the tail rotor take-off pinion, causing loss of tail rotor drive and forcing an autorotative descent.
Certification history: The S-92A was certified by the FAA under a rule that allowed exemptions for "extremely remote" failure modes. Neither Sikorsky nor the FAA considered failure of the MGB oil filter bowl attachment system as possible. The helicopter had failed initial loss-of-lubricant testing, but was certified based on the "extremely remote" rationale, which was accepted by the Joint Aviation Authorities and Transport Canada.
Following a 2008 Australian occurrence involving another S-92A with similar MGB oil filter bowl stud galling, Sikorsky issued an Alert Service Bulletin (SSA-S-92A-08-007) and an Airplane Maintenance Manual revision (Revision 13, 05 November 2008) mandating enhanced inspections. The TSB's examination of new studs showed galling damage developed during initial installation and worsened with reuse. An Airworthiness Directive (AD 2009-07-53) was issued on 23 March 2009, requiring replacement of damaged studs.