Cougar Helicopters Flight 91

The TSB used two remotely operated vehicles (ROVs),[10] operating from Atlantic Osprey, to locate and examine the sunken aircraft, which was found on the bottom largely intact but with significant structural damage, and with the tail boom broken off and lying separately nearby.[15][16] The damage to the airframe was severe enough to prevent immediate recovery of the wreckage as originally planned,[17] and efforts instead focused on recovering the remains of the passengers and crew. Images from the ROVs indicated the presence of between 10 and 13 bodies in the aircraft fuselage. Nine bodies were recovered from the wreckage on 14–15 March and were returned to St. John's in the early hours of 16 March aboard Atlantic Osprey. On 17 March, the TSB announced that all bodies had been recovered, as were the Flight Data Recorder (FDR) and Cockpit Voice Recorder (CVR). The S-92 FDR/CVR is one box called a Multi-Purpose Flight Recorder, commonly referred to as a "combi-unit", manufactured by Penny and Giles in the United Kingdom. The FDR/CVR was transferred to Ottawa, Ontario for analysis by TSB personnel.[8] On 18 March Atlantic Osprey docked in St. John's carrying the main chassis of the helicopter in a basket on deck.[18] The TSB's lead investigator indicated on 19 March that about 80% of the wreckage had already been recovered;[19][20] by 26 March this figure had been increased to 95%.[21]

TSB identified a broken titanium stud as part of the gearbox oil filter assembly. Sikorsky had previously recommended that the titanium stud be replaced with a steel stud within one year or 1,250 flight hours of a 28 January 2009 Alert Service Bulletin, following a total loss of oil and emergency landing in Australia in August 2008.[22] On 21 March Les Dorr, a spokesman for the FAA, indicated that it would release an Airworthiness Directive calling for the replacement of the studs on other S-92A aircraft, most likely on Monday 23 March, but that the directive would apply only to US-registered helicopters.[23] The FAA had issued previous Airworthiness Directives AD 2005-12-03, AD 2006-11-14, and AD 2006-15-19 for problems with the main gearbox of that type. The FAA had also issued Special Airworthiness Information Bulletin (SAIB) number SW-09-19 Sikorsky S92A Main Gearbox Emergency Procedures dated 19 March 2009. The SAIB indicated that a recent procedural change, Sikorsky Safety Advisory (SSA) SSA-S92-08-006, dated 26 September 2008 may not have been appropriate and that it had not been approved.[24] The European Aviation Safety Agency had already acted to highlight this problem.

Late on 23 March, Sikorsky issued a news release indicating that it had furnished replacement studs and tools to all operators and that 50 of 91 aircraft had been reworked already.[25] The FAA later grounded flights until the parts had been replaced,[26] issuing Emergency Airworthiness Directive 2009-07-53 dated 23 March that required the replacement of the studs before further flight.

TSB disclosed at a news conference on 26 March that the flight data record indicated that oil pressure was lost, but that there was no anomaly other than the broken stud to explain that loss. The aircraft descended at 1,000 feet per minute (5.1 m/s). The aircraft lost electrical power, interrupting the data record. Damage analysis indicated that it struck the water belly-down and tail first with an acceleration of 20 g (20 times that of the Earth's gravity).[21][27]

In 2003, the S-92A initially failed a FAR/JAR-29 additional oil system loss of lubrication test (sometimes called the "run dry" test) conducted to determine whether it could sustain 30-minute operation without main gearbox lubrication, failing after 10 minutes.[28] Subsequent design changes implemented an oil cooler bypass valve to eliminate what were seen to be the most likely sources of leakage, the cooler and external lines and fittings. Certification was obtained without meeting the 30-minute test as the chances of oil loss were calculated as being "extremely remote",[29] a statistical chance of failure of approximately one in every 10 million flight hours. This was based on the erroneous assumption that all leaks would occur from the oil cooler, and so did not represent the type of leak that occurred to Flight 91 or to a CHC S-92A in Australia the previous year.[30][31]

All offshore helicopter flights from St Johns were suspended following the accident. Regular passenger flights to the platforms resumed on Monday, 18 May 2009; Cougar Helicopters is limiting the maximum altitude for passenger flights to 2,133.6 metres (7,000 ft) as an additional safety precaution.[32]

On 16 June 2009, the FAA released an additional Airworthiness Directive, AD 2009-13-01, requiring the Rotorcraft Flight Manual for the S-92A helicopter be modified to clarify emergency procedures in the event of a main gearbox failure due to loss of oil pressure, and in particular to identify the urgency of an immediate landing in the event of an oil pressure loss.[33][34]

The TSB issued an update on the investigation on 18 June 2009, indicating that the pilot may have been trying to perform a controlled landing at the time of the accident. The main blades were apparently rotating at the time of impact; however, the tail rotor drive gears were severely damaged, which would result in a loss of thrust. An engine shutdown was initiated at an altitude of 500 feet (150 m), consistent with a tail rotor drive failure. The TSB was continuing to investigate the failure of the flotation system, which reportedly had been activated but did not operate correctly.[35][36]

The Inquiry Commissioner took some interim measures to secure improved emergency response times in the North West Atlantic pending completion of the Commission's Report.[37]

On 23 October 2009, the European Aviation Safety Agency issued an airworthiness directive in response to the discovery of cracks in the mounting bolts of the main gearbox of S-92 helicopters operating in the North Sea.[38]

On 9 February 2011, the Transportation Safety Board released its final report[39] on the accident. It identified 16 separate factors that led to the fatal crash but no single one was to blame.[40]