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TSB issues update on Sikorsky S-92A accident

Main gear box after removal from wreckage (Photo: TSB of Canada)

June 18, 2009, Gatineau, Que. – The Transportation Safety Board of Canada (TSB) is well into a comprehensive investigation of the accident of a Sikorsky S-92A helicopter, Cougar Helicopters Flight 491, which occurred 30 nautical miles east of St. John’s, Newfoundland and Labrador, on March 12, 2009.

A thorough, unbiased investigation is necessary to understand as completely as possible all the contributing factors involved in this accident.

To this end, a dedicated team of TSB investigators and several TSB Engineering Laboratory specialists is working towards completing the investigation. A number of other specialists and observers from Cougar Helicopters, Transport Canada, Sikorsky, and the Federal Aviation Administration (FAA), along with the accredited representative of the United States National Transportation Safety Board (NTSB), continue to provide valuable contributions to the TSB investigation.
Work Completed to Date

A significant amount of work has been completed so far, but much remains to be done. Dozens of interviews have been conducted with individuals from various organizations. The latest interviews were conducted in May, including a follow-up interview with the sole survivor. More interviews remain to be conducted in the coming weeks. Hundreds of technical and operational documents, weather reports, air traffic control communications, incident reports, studies, and research papers have been gathered, and the review of this material continues. The TSB has examined the main gearbox (MGB), the tail rotor drive shaft, the flight data recorder (FDR), and cockpit voice recorder (CVR). As reported previously, the FDR stopped recording at approximately 1225:17, (1) while the helicopter was about 800 feet above sea level (asl).

The reason the recorder stopped remains under investigation. The TSB Engineering Laboratory, in concert with manufacturer’s specialists, has been able to successfully retrieve additional data from the aircraft’s Health and Usage Monitoring System (HUMS) and flight control computer (FCC) to be able to piece together most of Cougar Helicopters Flight 491’s flight profile below 800 feet. While this portion of the flight profile is still in a preliminary stage, and further analysis is required, the following additional factual information can be released at this time.

Factual Information

Examination of the MGB indicates that there was no loss of main rotor drive and that the main rotor blades were rotating at the time of the impact.

The examination of the MGB also revealed that the tail rotor drive gears had been severely damaged, resulting in a loss of drive, causing it to stop producing thrust. Further examination is being carried out by the TSB Engineering Laboratory to determine the cause and sequence of this loss of tail rotor drive.

The metallurgical examination of the titanium oil filter attachment studs revealed fatigue cracking in the studs as well as evidence of thread damage. A detailed metallurgical examination of the studs, nuts, and filter bowl is under way to identify the origin of the fatigue cracks and to determine the fracture mechanism.
Just before the recorder stopped, engine power was reduced, a descent from 800 feet was initiated, and the speed of Flight 491 began to decrease from 133 knots. The helicopter continued to descend and to slow down in a controlled manner, until about 1225:44, at which time driving power to the tail rotor was lost. At this time, Flight 491 was heading 290 degrees magnetic (M) at 85 knots and was descending through approximately 500 feet. At 1225:47, a shut-down of both engines was initiated, which is consistent with a tail rotor drive failure emergency.

Subsequently, the aircraft experienced a number of large and rapid attitude changes. At 1225:54, Flight 491’s pitch attitude increased from approximately 10 degrees nose down to about 16 degrees nose up, which is consistent with a flare for an engines-off landing. The helicopter struck the water at approximately 1226 in a slight right-banked, nose-high attitude at an approximate location of 47 degrees 26’03” N, 051 degrees 56’34.8” W, with moderate speed and a high rate of descent. The wreckage was found at a depth of 165 metres on a bearing of 283 degrees from the surface position.

The Sikorsky S-92A flotation system activation switch was found in the armed position after recovery. The helicopter experienced significant forces during the impact with the water, and examination of the inflation bottles indicates that they had not released their compressed gas to inflate the flotation collars. The reason the collars failed to inflate is still under investigation.

Investigation Activities in Progress

Continued investigation activities being finalized include evaluations of the Flight 491 flight profile in an S-92A simulator. Investigators from the TSB and other agency specialists will recreate as closely as possible the accident flight profile to add to the understanding of the challenges encountered by the pilots of Flight 491. In addition, pilot training, human performance aspects, crew resource management, and cockpit ergonomics will be evaluated.

Oil filter bowl studs on all Sikorsky S-92A aircraft have been replaced with new steel studs in accordance with a Sikorsky Aircraft Alert Service Bulletin (ASB). Compliance with the Sikorsky Aircraft ASB was subsequently mandated by an FAA Airworthiness Directive (AD).

The investigation has revealed that, even though the Sikorsky S-92A MGB was certificated to meet requirements of Part 29 of the Federal Aviation Regulations (FAR 29) of the United States FAA, there is a perception in some areas of the aviation community that the MGB can be run in a dry state-that is, without lubricating oil-for 30 minutes. FAR 29 does not require run-dry operation of a gearbox to meet the 30-minute “continued safe operation.”

Based on the applicable guidance material at the time of certification, the lubrication failure modes of interest were limited to the failure of external lines, fittings, valves, and coolers. This practice was consistent with industry experience, which had found that loss of lubrication tended to be associated with external devices. Therefore, the possibility of a failure at the oil filter was considered to be extremely remote. As a result of the fracture of the filter bowl mounting studs, resulting in the loss of a large quantity of oil, the certification guidance material is being reviewed.

Additionally, the FAA and Sikorsky Aircraft are working to identify all the modes of failure that might lead to Sikorsky S-92A MGB oil loss, determining their probability of occurrence, and developing appropriate mitigation strategies.

The Sikorsky S-92A Rotorcraft Flight Manual (RFM) has been reviewed regarding MGB oil pressure loss below 5 pounds per square inch (psi) and the need for pilots to land immediately. An RFM revision has been approved by the FAA and Transport Canada.

A number of issues regarding survivability such as passenger immersion suit and crew flight suit effectiveness, use of underwater breathing devices, adequacy of survival training, adequacy of general ditching procedures, personal locator beacons, weather/sea state flight limitations, and Sikorsky S-92A flotation system are currently under investigation.

Safety Action

As the TSB investigation progresses, the team continues to work closely with the other agencies involved. Safety concerns have been communicated directly to these change agents and have resulted in safety action taken by Cougar Helicopters, Sikorsky Aircraft, and the various regulatory bodies.
Cooperation with Other Agencies

The TSB is working in cooperation with other external agencies such as the Canada-Newfoundland and Labrador Offshore Petroleum Board (C-NLOPB) to ensure that the upcoming public inquiry by retired Justice Robert Wells does not impinge upon the work being done by the TSB under our mandate.

To this end, several exchanges of information have occurred and meetings have been held to coordinate activities. The general concerns of offshore workers have been passed to the TSB and those concerns appropriate to the TSB’s mandate have been checked against the investigation issues already under consideration by the investigation team. The other concerns of offshore workers have been addressed by an external Helicopter Operations Task Force, a working group composed of representatives from the helicopter operator and the oil companies.

As always, the thoughts of the TSB investigation team go out first to the families who lost loved ones on board Flight 491. The investigation work of the TSB team on this accident will help all understand what happened in this accident and hopefully prevent any similar accidents from happening again in the future.

The TSB is an independent agency that investigates marine, pipeline, railway and aviation transportation occurrences. Its sole aim is the advancement of transportation safety. It is not the function of the Board to assign fault or determine civil or criminal liability.