Questions and answers - A09A0016

What caused this accident?

There was no single cause of this accident. The TSB investigation, which looked at the role of the crew, the manufacturer, the operator, training providers, and the regulators, identified a complex equation of 16 causes and contributing factors. None of these 16 factors, which are listed below, stand out above the others.

Findings as to Causes and Contributing Factors

  1. Galling on a titanium attachment stud holding the filter bowl assembly to the main gearbox (MGB) prevented the correct preload from being applied during installation. This condition was exacerbated by the number of oil filter replacements and the re-use of the original nuts.
  2. Titanium alloy oil filter bowl mounting studs had been used successfully in previous Sikorsky helicopter designs; in the S-92A, however, the number of unexpected oil filter changes resulted in excessive galling.
  3. Reduced preload led to an increase of the cyclic load experienced by one of the titanium MBG oil filter bowl assembly attachment studs during operation of CHI91, and to fatigue cracking of the stud, which then developed in a second stud due to increased loading resulting from the initial stud failure. The two studs broke in cruise flight resulting in a sudden loss of oil in the MBG.
  4. Following the Australian occurrence, Sikorsky and the Federal Aviation Administration (FAA) relied on new maintenance procedures to mitigate the risk of failure of damaged mounting studs on the MBG filter bowl assembly and did not require their immediate replacement.
  5. Cougar Helicopters did not effectively implement the mandatory maintenance procedures in Aircraft Maintenance Manual (AMM) Revision 13 and, therefore, damaged studs on the filter bowl assembly were not detected or replaced.
  6. Ten minutes after the red MBG OIL PRES warning, the loss of lubricant caused a catastrophic failure of the tail take-off pinion, which resulted in the loss of drive to the tail rotor shafts.
  7. The S-92A rotorcraft flight manual (RFM) MBG oil system failure procedure was ambiguous and lacked clearly defined symptoms of either a massive loss of MBG oil or a single MBG oil pump failure. This ambiguity contributed to the flight crew's misdiagnosis that a faulty oil pump or sensor was the source of the problem.
  8. The pilots misdiagnosed the emergency due to a lack of understanding of the MBG oil system and an over-reliance on prevalent expectations that a loss of oil would result in an increase in oil temperature. This led the pilots to incorrectly rely on MBG oil temperature as a secondary indication of an impending MBG failure.
  9. By the time that the crew of CHI91 had established that MBG oil pressure of less than 5 psi warranted a "land immediately" condition, the captain had dismissed ditching in the absence of other compelling indications such as unusual noises or vibrations.
  10. The captain's decision to carry out pilot flying (PF) duties, as well as several pilot not flying (PNF) duties, resulted in excessive workload levels that delayed checklist completion and prevented the captain from recognizing critical cues available to him.
  11. The pilots had been taught during initial and recurrent S-92A simulator training that a gearbox failure would be gradual and always preceded by noise and vibration. This likely contributed to the captain's decision to continue towards CYYT.
  12. Rather than continuing with the descent and ditching as per the RFM, the helicopter was leveled off at 800 feet asl, using a higher power setting and airspeed than required. This likely accelerated the loss of drive to the tail rotor and significantly reduced the probability of a successful, controlled ditching.
  13. The captain's fixation on reaching shore combined with the first officer's non-assertiveness prevented concerns about CHI91's flight profile from being incorporated into the captain's decision-making process. The lack of recent, modern, crew resource management (CRM) training likely contributed to the communication and decision-making breakdowns which led to the selection of an unsafe flight profile.
  14. The throttles were shut off prior to lowering the collective, in response to the loss of tail rotor thrust. This caused significant main rotor rpm droop.
  15. The pilots experienced difficulties controlling the helicopter following the engine shut-down, placing the helicopter in a downwind autorotative descent with main rotor rpm and airspeed well below prescribed RFM limits. This led to an excessive rate of descent from which the pilots could not recover prior to impact.
  16. The severity of the impact likely rendered some passengers unconscious. The other occupants seated in the helicopter likely remained conscious for a short period of time, but became incapacitated due to the impact and cold water shock, and lost their breath hold ability before they could escape the rapidly sinking helicopter.

Is the S-92 helicopter required, by regulation, to have a 30 minute run dry time following a massive loss of oil from the main gearbox?

The S-92 helicopter was certified under FAR Part 29, which requires that a helicopter be capable of continued flight for at least 30 minutes after perception by the flight crew of the loss of lubricant from the main gearbox. However, these regulations include a provision which exempts a manufacturer from demonstrating that the main gearbox is capable of continued operation for 30 minutes following a loss of lubricant, if such a failure was considered "extremely remote."

When the S-92 was undergoing certification, regulators accepted that a massive loss of main gearbox oil from the MBG was an extremely remote possibility.

The S-92 helicopter was certified under FAR Part 29 using the extremely remote provision; as a result, it was not required to demonstrate a true 30 minute run dry capability.

What run dry capability does the S-92 helicopter have?

During the S-92 helicopter's initial certification testing, it ran for approximately 11 minutes following a complete loss of lubricant from the main gearbox. In this occurrence, CHI91 experienced a loss of drive to the tail rotor due to overheating of the main gearbox approximately 11 minutes after losing all main gearbox oil. While this was not a causal factor, the Board found certification standards that do not require a clear capability of continued flight for 30 minutes, present a risk to the system. That is why the Board recommends that all Category A helicopters, including the S-92, and new helicopters certified to these standards in the future should be able to fly for at least 30 minutes following a massive loss of main gearbox oil.

Further the Board found that with distant offshore operations, even 30 minutes may not be enough time to optimize landing opportunities. Given advances in technology, the Board is recommending that the FAA take a look at today's operating environments, and decide whether even 30 minutes is enough time.

These issues are covered in detail in sections 1.18.5, 2.1 and 3.2 and 4 of the TSB's report.

Is the TSB calling for the S-92A helicopter to be grounded?

The decision whether or not to ground an aircraft is up to the regulators. The causes of this exact accident have largely been addressed.

Early in the investigation, the TSB focused on the broken titanium studs. Those studs were quickly replaced with steel ones and the main gearbox oil filter bowl housing has been re-designed.

Pilot training has been improved and the rotorcraft flight manual now provides clearer guidance in the event of a total oil loss.

This operator has added upgraded emergency flotation systems and improved crew resource management training. Also the oil companies now provide all occupants on board these helicopters with emergency underwater breathing apparatus.

All of these measures have lowered the risk. To make the system as safe as possible the TSB made 4 recommendations.

Recommendation A11-01
The Federal Aviation Administration, Transport Canada and the European Aviation Safety Agency remove the "extremely remote" provision from the rule requiring 30 minutes of safe operation following the loss of main gearbox lubricant for all newly constructed Category A transport helicopters and, after a phase-in period, for all existing ones.

Recommendation A11-02
The Federal Aviation Administration assess the adequacy of the 30 minute main gearbox run dry requirement for Category A transport helicopters.

Recommendation A11-03
Transport Canada prohibit commercial operation of Category A transport helicopters over water when the sea state will not permit safe ditching and successful evacuation.

Recommendation A11-04
Transport Canada require that supplemental underwater breathing apparatus be mandatory for all occupants of helicopters involved in overwater flights who are required to wear a PTSS.

What is the difference between a crash and a ditching and is there an emergency flotation system designed to keep helicopters afloat after a crash?

A 1995 United Kingdom Civil Aviation Authority report described a ditching as a controlled descent (with some measure of warning) into a non-hostile sea. They considered a crash to encompass all uncontrolled or inadvertent impacts with the water, controlled descents into a hostile sea, and a helicopter falling off a heli-deck.

The Emergency Flotation System (EFS) in the S-92A, like all other modern helicopters certified under the current rules, is only required to withstand the forces that would be expected for a controlled ditching.

Unlike some other modern helicopters that have EFS designed to withstand being deployed in flight at speeds up to 120 knots and to withstand water landing speeds up to 30 knots, the S-92A's EFS is not approved for deployment in flight and the helicopter must be ditched in the water before the EFS is deployed.

CHI91 's EFS was rendered inoperable by significant impact forces which caused the fuselage to immediately break up, resulting in multiple breaks in the EFS system's associated wires, cables, and lines.

In section 3.2 (#24) the Board concluded that "If offshore helicopter EFS systems are only designed to withstand the force associated with a ditching there is a continued risk that these systems will be disabled in survivable impacts contributing to occupant deaths from drowning."

What does the TSB report conclude about the steps taken to address the problem of titanium studs after the Australian incident?

The Australian registered helicopter involved in the July 2008 incident was owned by CHC Australia and was operating not far from Broome, Australia. The crew of that helicopter experienced a massive loss of main gearbox oil, they had a suitable landing area available and they were able to land the helicopter approximately seven minutes later with no resulting damage to the main gearbox.

In the months after the incident, the studs from this helicopter were examined 4 times and the picture became clearer after each examination. After the first examination in the field it was determined that the studs on the main oil filter bowl assembly had broken possibly due to unapproved maintenance actions. Following the second examination in Australia, it was determined that the studs had failed in overload and that further metallurgical examination was needed. The third examination was carried out by an independent laboratory in Canada. They determined the studs had fractured due to fatigue and identified galling as a possible cause. The final examination was carried out by Sikorsky in September of 2008 and at that time, galling was confirmed and the risks presented by the studs were assessed.

Following this examination and risk assessment, stepped up inspections and maintenance procedures were put in place. This is what is referred to as Aircraft Maintenance Manual Revision 13. In addition, the titanium studs were required to be replaced with steel studs within 1250 flying hours or 1 year. The facts and analysis of these issues are covered in sections 1.18.3 and 2.1 of the TSB report.

In section 3.1, the Board concluded that "Following the Australian occurrence, Sikorsky and the Federal Aviation Administration (FAA) relied on new maintenance procedures to mitigate the risk of failure of damaged mounting studs on the MBG filter bowl assembly and did not require their immediate replacement."

What is the TSB recommending?

Recommendation A11-01
The Federal Aviation Administration, Transport Canada and the European Aviation Safety Agency remove the "extremely remote" provision from the rule requiring 30 minutes of safe operation following the loss of main gearbox lubricant for all newly constructed Category A transport helicopters and, after a phase-in period, for all existing ones.

Recommendation A11-02
The Federal Aviation Administration assess the adequacy of the 30 minute main gearbox run dry requirement for Category A transport helicopters.

Recommendation A11-03
Transport Canada prohibit commercial operation of Category A transport helicopters over water when the sea state will not permit safe ditching and successful evacuation.

Recommendation A11-04
Transport Canada require that supplemental underwater breathing apparatus be mandatory for all occupants of helicopters involved in overwater flights who are required to wear a PTSS.

What probability is there that your recommendations will get implemented? What do you think companies and regulators will do in response?

Historically, of the total number of recommendations made by the TSB since 1990, almost all have been accepted by industry and regulators. We have rated the response to around 70% as Fully Satisfactory. It is difficult to predict if these four recommendations will be fully implemented but the TSB will certainly be following up with Transport Canada, the European Safety Agency and the Federal Aviation Administration in an effort to impress upon them the importance of these recommendations. We will also continue to monitor the official responses, will post these responses on our website along with our assessment of the responses.

If not enough action has been taken, we will meet with regulators again to discuss their position and what more can be done to address these important safety issues. In our public reports the TSB can only report on safety action that has been already taken. However Sikorsky has advised the TSB that they are in the process of redesigning the MBG on the S-92A with the goal of having a 30 minute loss of lubricant capability.

What role will the TSB play in the second phase of the Offshore Helicopter Safety Inquiry?

The Commissioner of the Offshore Helicopter Safety Inquiry, Mr. Justice Robert Wells, will undertake a review of the sections of the TSB report that deal with matters which are specifically within the mandate of the Canada-Newfoundland and Labrador Offshore Petroleum Board and decide whether further recommendations are required. The Commissioner will determine what process will be followed in Phase II. Further information may be found at www.oshsi.nl.ca.