Backgrounders

Swissair 111 Investigation Report - Executive Summary

The following is an executive summary of the report by the Transportation Safety Board of Canada (TSB) into the accident involving Swissair Flight 111 near Peggy's Cove, Nova Scotia, on the night of 2 September 1998. The aircraft, a McDonnell Douglas MD-11 with 215 passengers and 14 crew members on board, was on a scheduled flight from New York, United States of America, to Geneva, Switzerland.

Summary of Occurrence

About 53 minutes after departure, while flying at Flight Level 330 (about 33,000 feet), the flight crew smelled an abnormal odour in the cockpit. A small amount of smoke became visible in the cockpit; then, it is likely that the smoke stopped entering the cockpit for an undetermined length of time. The flight crew assessed that there was an anomaly associated with the air conditioning system.

After conversing with air traffic services, the flight crew decided to divert to the Halifax International Airport in Nova Scotia, Canada. While the flight crew was preparing for the landing in Halifax, they were unaware that a fire was spreading above the ceiling in the front area of the aircraft.

About 13 minutes after the abnormal odour was first detected, the aircraft's flight data recorder began to record a rapid succession of aircraft systems-related failures. The flight crew declared an emergency and indicated a need to land immediately. About one minute later, radio communications and secondary radar contact with the aircraft were lost, and the flight recorders stopped functioning. About five and one-half minutes later, at 10:31 p.m. Atlantic daylight saving time (ADT), the aircraft crashed into the ocean about five nautical miles southwest of Peggy's Cove, Nova Scotia, Canada. The aircraft was destroyed and there were no survivors.

Summary of Findings and Board Recommendations

Approximately 98 per cent of the aircraft, measured by weight, was recovered. From the examination of the pieces of wreckage, it was determined that a fire had occurred in the forward, overhead area of the aircraft. Portions of the front section of the aircraft were reconstructed to allow a thorough analysis of potential ignition sources, and to assess how the fire propagated.

It was determined that the fire most likely started from an electrical arcing event that occurred above the ceiling on the right side of the cockpit near the cockpit rear wall. The arcing event ignited the flammable cover material on nearby metallized polyethylene terephthalate (MPET) covering on the thermal acoustic insulation blankets. As the fire spread across the surface of the insulation blankets, other flammable materials became involved, including silicone elastomeric end caps, hook-and-loop fasteners, foams, adhesives, thermal acoustic insulation splicing tapes, and metallized polyvinyl fluoride (MPVF) insulation blanket cover material. The fire progression was rapid, and involved a combination of these materials that together sustained and propagated the fire.

 

Material Flammability
The Board concluded that aircraft certification standards for material flammability were inadequate, in that they allowed the use of materials that could be ignited and sustain or propagate fire. In a recommendation released during the investigation, the Board called for regulatory authorities to take action, on an urgent basis, to reduce or eliminate the risk associated with the use of MPET-covered insulation blankets in aircraft. The Board also recommended that regulatory authorities test all thermal acoustic insulation materials against more rigorous test criteria, and that flammability standards for material used in the manufacture of any aeronautical product be revised, based on realistic ignition scenarios, to prevent the use of any material in the construction of aircraft that sustains or propagates fire.

Initial Wire Arcing
Reconstruction of the wreckage indicated that a segment of arced electrical cable associated with the in-flight entertainment network (IFEN) had been located in the area where the fire most likely originated. The Board concluded that the arc on this electrical cable was likely associated with the fire initiation event. The Board also concluded that it is likely that one or more additional wires were involved in the lead arcing event, and that the additional wire or wires could have been either IFEN or aircraft wires. Therefore, it could not be concluded that the known arcing event on the IFEN cable located in the area where the fire most likely originated was by itself the lead event.

During the course of the investigation, in efforts to find and eliminate potential ignition sources in MD-11 aircraft, more than 70 Airworthiness Directives have been promulgated by the Federal Aviation Administration (FAA). These airworthiness directives required various measures to be taken, such as one-time inspections of wires and electrical components, primarily in the front ceiling area of the aircraft.

During the lead arcing event, the associated circuit breaker or breakers did not trip. The Board concluded that, although the type of circuit breakers used in the aircraft, including those used for the IFEN, were similar to those in general aircraft use, the circuit breakers were not capable of protecting against all types of wire arcing events. The Board recommended that a certification test regime be mandated that evaluates aircraft electrical wire failure characteristics under realistic operating conditions, and against specified performance criteria, with the goal of mitigating against the risk of igniting nearby flammable material.

There was no information available to indicate that the SR 111 crew reset circuit breakers. However, during the course of the investigation, it became evident that the circuit breaker resetting philosophy and procedures varied considerably among manufacturers, operators, flight crews, and maintenance personnel throughout the industry. While considerable standardization has taken place during the course of the investigation, the Board is calling for clearer guidance from regulators.

 

Fire Detection and Suppression
Airflow patterns in the aircraft and fire propagation scenarios were analyzed to assess cues that may have been available to the pilots during the early stages of the fire. It was determined that the small amount of odour and smoke first noticed by the pilots originated from a small creeping fire propagating aft from the area of the initial ignition, toward the area above the forward passenger cabin ceiling.

As the fire propagated aft, it is likely that the associated smoke temporarily stopped migrating forward into the cockpit. As the aircraft was not required to be equipped with built-in fire detection in the hidden area where the fire was located, the pilots were not alerted to the presence of the fire. The Board concluded that the actions by the flight crew in preparing the aircraft for landing, including their decisions to have the passenger cabin readied for landing and to dump fuel, were consistent with them being unaware that an on-board fire was under way. A theoretical descent profile calculation, conducted by the TSB during the investigation, confirmed that, because of the rapid progression of the fire and its adverse effects on various aircraft systems and the cockpit environment, they would not have been able to complete a safe landing in Halifax, even if they had undertaken to do so at the time of the PAN PAN urgency radio communication at 10:14 p.m. ADT.

The Board issued several recommendations to mitigate against potential fires in hidden areas of aircraft, including a recommendation that appropriate regulatory authorities, together with the aviation community, review the methodology for establishing designated fire zones within the pressurized portion of the aircraft, with a view to providing improved smoke and fire detection and suppression capability.

Adequacy of In-Flight Firefighting
The available information indicates that, by the time the aircraft crew became aware that there was an in-flight fire, the fire had developed to a condition where it is unlikely that available firefighting equipment and methods would have been effective. The Board concluded that industry-wide changes are necessary to provide aircraft crews with effective means to detect and suppress fires in hidden areas, including the provision for ready access to hidden areas for the purpose of firefighting.

There was no integrated in-flight firefighting plan in place for the accident aircraft, nor was such a plan required by regulation. Therefore, the aircraft crew did not have procedures or training directing them to aggressively attempt to locate and eliminate the source of the smoke, and to expedite their preparations for a possible emergency landing. In the absence of such a firefighting plan, the aircraft crew concentrated on preparing the aircraft for the diversion and landing. The Board recommended that appropriate regulatory authorities, in conjunction with the aviation community, review the adequacy of in-flight firefighting as a whole, to ensure that aircraft crews are provided with a system whose elements are complementary and optimized to provide the maximum probability of detecting and suppressing any in-flight fire.

 

Systems Failure Leading to Increased Fire Propagation
During the fire, the failure of silicone elastomeric end caps installed on air conditioning ducts resulted in the addition of a continuous supply of conditioned air that contributed to the propagation and intensity of the fire.

The aluminum cap assembly used on the stainless steel oxygen line above the cockpit ceiling was susceptible to leaking or fracturing when exposed to the temperatures that were likely experienced during the last few minutes of the flight. Such failures would exacerbate the fire and potentially affect the crew oxygen supply. It could not be determined whether this occurred on the flight.

There is no requirement that fire-induced failures be considered when completing the system safety analysis required for aircraft certification. The Board recommended that, as a prerequisite to certification, all aircraft systems in the pressurized portion of an aircraft, including their subsystems, components, and connections, be evaluated to ensure that those systems whose failure could exacerbate a fire in progress are designed to mitigate the risk of fire-induced failures.

Standby Instruments – Positioning and Power Supply
For at least some portion of the last minutes of the flight, primary flight displays ceased operating. A lack of outside visual references forced the pilots to rely on the standby instruments.

In the deteriorating cockpit environment, the positioning and small size of the standby instruments would have made it difficult for the pilots to transition to their use, and to continue to maintain the proper spatial orientation of the aircraft.

The Board called upon Transport Canada to work with the FAA and Joint Aviation Authorities to address identified safety concerns, including the lack of a power supply for the standby instruments that is independent of the aircraft electrical system and battery.

The Board believes that standby instruments should be in a standard grouping layout similar to the primary flight instruments, and that they should be positioned in the normal line of vision of the flight crew. The Board also believes that with current technology, providing independent standby instrumentation for secondary navigation and communication is feasible.

 

Additional Safety Risks Identified
During the course of this investigation, some additional risks that have the potential to degrade aviation safety were identified. Although these factors could not be shown to have played a direct role in this occurrence, the associated deficiencies could potentially lead to other accidents if the deficiencies are not rectified.

Areas of Concern

- checklists that do not adequately deal with smoke conditions;

- aircraft designs that do not facilitate the rapid de-powering of electrical systems;

- MD-11 map light design and installation;

- lack of clarity in guidance material and regulations regarding wire separation in confined areas; and

- inadequacy of Supplemental Type Certificate standards to ensure that add-on equipment is compatible with the aircraft's type certificate

In addition to the 14 safety recommendations that the Board has issued during the course of the investigation, nine recommendations are presented in the final report:

  • Two recommendations that deal with testing and flammability standards of in-service thermal acoustic insulation materials.
  • One recommendation that deals with the application of existing standards for the certification of other materials.
  • Two recommendations that focus on aircraft electrical systems, including additional measures for certifying supplementary add-on systems and industry standards for circuit breaker resetting.
  • Four recommendations that propose improvements to the capture and storage of flight data as it relates to cockpit voice recorders, flight data recorders, and cockpit image recording systems.

The final report also identifies some safety concerns that require additional follow-up. The TSB will continue to work with regulatory authorities and the aviation industry to help ensure that the recommended safety improvements are carried out as effectively as possible. For a complete list of the Board's recommendations, see Part 4, "Safety Action," of the Swissair 111 Investigation Report, available at: www.tsb.gc.ca.

 

Safety Action Taken

As a result of the TSB's findings and recommendations during the course of this investigation, considerable safety action has been taken by various regulatory authorities, airlines, and manufacturers to address the recommendations, advisories, and observations made by the Board. Such action taken has significantly improved aviation safety worldwide.

Safety action taken to date includes the following:

  • MPET-covered thermal acoustic insulation blankets have been ordered removed from aircraft;
  • new flammability testing criteria have been developed;
  • flight crew reading lights have been re-designed;
  • additional guidance material for dealing with smoke situations has been promulgated to flight crews;
  • aircraft checklists have been modified;
  • numerous inspections have been completed on wiring and components to look for and eliminate potential ignition sources;
  • the IFEN system was removed voluntarily from Swissair aircraft; subsequently the design was de-certified; and
  • new FAA policies are in place for the certification of in-flight entertainment systems.

Other safety measures stemming from the TSB's recommendations are also being implemented. See Part 4, "Safety Action," of the Swissair 111 Investigation Report for a complete list.