News releases

TSB # A 16/99

TSB ISSUES AVIATION SAFETY RECOMMENDATIONS FOR THERMAL ACOUSTICAL INSULATION MATERIALS AND FLAMMABILITY TEST CRITERIA

(Hull, Quebec, 11 August 1999) - As a result of its ongoing investigation into the fatal Swissair Flight 111 accident, the Transportation Safety Board of Canada (TSB) has identified safety deficiencies associated with the thermal acoustical insulation materials and has issued two interim aviation safety recommendations.

On September 2, 1998 Swissair Flight 111 (SWR 111) departed John F. Kennedy airport in New York, en route to Geneva, Switzerland. On board were 215 passengers and 14 crew members. Approximately 53 minutes after take-off, the crew noticed an unusual smell in the cockpit. Within about three and a half minutes, the flight crew noted visible smoke and declared the international urgency signal "Pan Pan Pan" to Moncton Area Control Centre, advising the Air Traffic Services (ATS) controller of smoke in the cockpit. SWR 111 was cleared to proceed direct to the Halifax airport, Nova Scotia.

While the aircraft was being manoeuvred in preparation for landing, the crew advised ATS that they had to land immediately and that they were declaring an emergency. Approximately 20 minutes after the crew first noticed the unusual smell, and about seven minutes after the crew's "emergency" declaration, the aircraft struck the water near Peggy's Cove, Nova Scotia, fatally injuring all 229 occupants on board. Because the aircraft crashed into the ocean, there was no post-crash fire.

To date, the investigation has revealed fire damage in the ceiling area forward of and several metres aft of the cockpit bulkhead. While the source of ignition has yet to be determined, there are indications that a significant source of the combustible materials that sustained the fire was thermal acoustical insulation blanket material. Burnt remnants of this material, quenched by the sea water, were found in the wreckage. Shortcomings related to the in-service fire resistance of some thermal acoustical insulation materials, and shortcomings in the test criteria used to certify those materials, have been identified during this and other recent aircraft occurrence investigations.


Thermal Acoustical Insulation Blanket

Thermal acoustical insulation blankets are widely used in the aviation industry to protect the aircraft interior from temperature variations, noise and moisture. Widely used cover materials in the aviation industry are metallized polyvinyl fluoride (PVF), known under the trademark of Tedlar, and metallized and non-metallized polyethylene terephthalate (PET), known under the trademark of Mylar. The MD-11 involved in this accident was primarily fitted with metallized PET.

Metallized PET-covered insulation blankets are used throughout the MD-11 aircraft, including extensive use in the ceiling area forward and aft of the cockpit bulkhead where fire damage has been discovered in the accident aircraft. The investigation has found samples of metallized PET which had been burning.

With the in-service history, the demonstrated flammability of the metallized PET cover material, and the discovery, in the Swissair Flight 111 wreckage, of remnants of insulating blankets with cover material burnt, it is likely that this material was a significant source of the combustible materials that propagated the firre.

It is the Board's view that the operation of aircraft outfitted with thermal acoustical insulation blankets incorporating metallized PET cover material constitutes an unnecessary risk. Therefore, the Board has recommended that:

Regulatory authorities confirm that sufficient action is being taken, on an urgent basis, to reduce or eliminate the risk associated with the use of metallized PET-covered insulation blankets in aircraft. [A99-07]


Flammability Test Criteria

The flammability test, for thermal acoustical insulation, insulation covering, and insulation blankets necessitates a vertical flammability test of samples using an approved burner. The type of cover material on the insulation blankets installed on the Swissair aircraft had been subjected to this test and met the applicable flammability test criteria for Federal Aviation Administration (FAA) certification.

In-service fires of the metallized PET cover material, and inconsistent results from the vertical burn test method, prompted manufacturers to seek additional flammability test criteria. Subsequently, aircraft manufacturers developed a "cotton swab" test which yielded more consistent results when testing the flammability characteristics of the various cover materials. This additional testing was adopted by several major aircraft manufacturers who subsequently modified their internal material specifications.

On 14 October 1998, the FAA stated that the test criteria used to certify the flammability characteristics of thermal acoustical insulation materials were inadequate, and committed itself to conducting the research necessary to establish a more comprehensive test standard. At the same time, the FAA indicated that because materials containing polyimide film have performed well in preliminary flammability tests, these materials would be considered compliant under the new regulation. Until adequate flammability test criteria are available, it is not possible to determine whether polyimide film, or other materials, provide adequate protection against fire propagation. Thermal acoustical insulation materials are installed in aircraft as a system, including such related components as tape, fasteners, and breathers. The Board believes that thermal acoustical insulation materials for use in aircraft must be judged against more valid flammability test criteria, not as individual components, but as a system. Therefore, the Board has recommended that:

On an urgent basis, regulatory authorities validate all thermal acoustical insulation materials in use, or intended for use, in applicable aircraft, against test criteria that are more rigorous than those in Appendix F of FAR 25.853, and similar regulations, and that are representative of actual in-service system performance. [A99-08]

As the investigation proceeds, should the Board identify additional safety deficiencies in need of urgent attention, it will make further aviation safety recommendations.

The Transportation Safety Board of Canada is an independent agency operating under its own Act of Parliament. 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.

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INTERIM AIR SAFETY RECOMMENDATIONS

The Honourable David Michael Collenette, P.C., M.P.

Minister of Transport

Mr. K. Koplin, Secretary General

Joint Aviation Authorities, The Netherlands

The Honourable Jane Garvey, Administrator

Federal Aviation Administration, United States

SUBJECT: Thermal Acoustical Insulation Materials

Background

On 02 September 1998 at 2118 Atlantic daylight saving time, Swissair Flight 111 (SWR 111), a McDonnell Douglas MD-11 aircraft, HB-IWF, departed John F. Kennedy airport in New York, en route to Geneva, Switzerland. On board were 215 passengers and 14 crew members. Approximately 53 minutes after take-off, as the aircraft was cruising at Flight Level 330, the crew noticed an unusual smell in the cockpit. Within about three and a half minutes, the flight crew noted visible smoke and declared the international urgency signal "Pan Pan Pan" to Moncton Area Control Centre, advising the Air Traffic Services (ATS) controller of smoke in the cockpit. SWR 111 was cleared to proceed direct to the Halifax airport from its position 58 nautical miles southwest of Halifax, Nova Scotia. While the aircraft was manoeuvring in preparation for landing, the crew advised ATS that they had to land immediately and that they were declaring an emergency. Approximately 20 minutes after the crew first noticed the unusual smell, and about seven minutes after the crew's "emergency" declaration, the aircraft struck the water near Peggy's Cove, Nova Scotia, fatally injuring all 229 occupants on board. Because the aircraft crashed into the ocean, there was no post-crash fire.

To date, the investigation (A98H0003) has revealed fire damage in the ceiling area forward of and several metres aft of the cockpit bulkhead. While the source of ignition has yet to be determined, there are clear indications that a significant source of the combustible materials that sustained the fire was thermal acoustical insulation blanket materials. Burnt remnants of this material, quenched by the sea water, were found in the wreckage.

Shortcomings related to the in-service fire resistance of some thermal acoustical insulation materials, and shortcomings in the test criteria used to certify those materials, have been identified during this and other recent aircraft occurrence investigations.


Thermal Acoustical Insulation Blanket

Thermal acoustical insulation blankets are widely used in the aviation industry to protect the aircraft interior from temperature variations, noise and moisture. Typically, blanket construction consists of a batt of insulating material encapsulated by a cover or film. Depending on the blanket size required, tape may be used to seal several blankets into a single unit. Selection of cover material is based on factors such as durability, fire resistance, weight, impermeability, and installation considerations. The most widely used cover materials in the aviation industry are metallized polyvinyl fluoride (PVF )(1) and metallized and non-metallized polyethylene terephthalate (PET).(2) The MD-11 involved in this accident was fitted with metallized PET.

The Douglas Aircraft Company first introduced reinforced plastic film insulation coverings during the development of the DC-10. By 1987, the manufacturer (then McDonnell Douglas Corporation) began installing insulation blankets having metallized PET cover material on production aircraft. Further research and development resulted in the use of lighter, non-metallized PET thermal acoustical insulation blankets, which were introduced in 1994 and which superseded metallized PET cover material on new production aircraft. The McDonnell Douglas aircraft produced with metallized PET-covered insulation blankets include the following models: DC-10, MD-80, and MD-11. The total number of aircraft worldwide that have used this material for replacement or repair has not been ascertained. However, it is clear that a large number of aircraft are using, in whole or in part, thermal acoustical insulation blankets incorporating metallized PET cover material.

Metallized PET-covered insulation blankets are used throughout the MD-11 aircraft, including extensive use in the ceiling area forward and aft of the cockpit bulkhead where fire damage has been discovered in the accident aircraft. The investigation has found samples of metallized PET that had been burning. Appendix A provides an overview of some other notable aircraft fires in which metallized PET insulation blanket covering was considered to have aggravated the damage in the occurrence.

In September 1996, prompted by several MD-80 and MD-11 ground fire incidents involving insulation blankets with metallized PET cover material, McDonnell Douglas advised operators to discontinue the use of this material. Additionally, the company stated that it was currently installing non-metallized PET cover material in production aircraft. By 1997 metallized PVF was being used in production aircraft. By October of that year McDonnell Douglas had issued a Service Bulletin (MD-11-25-200) that encouraged MD-11 operators to replace insulation blankets covered with metallized PET material with blankets covered with metallized PVF material. The Service Bulletin also stated that the non-metallized PET cover material that had been used in production aircraft since September 1996 was discontinued, as it did not consistently pass a particular McDonnell Douglas flammability test. That Bulletin also stated that McDonnell Douglas was now using metallized PVF in new production aircraft. Similar Service Bulletins, regarding the use of metallized PET cover material, were issued to DC-8, DC-9, DC-10, MD-80, and MD-90 operators.

Manufacturer's Service Bulletins are advisory in nature unless mandated by the issuance of an Airworthiness Directive by the appropriate regulatory authority.

Thermal acoustical insulation, insulation covering, and insulation blankets must comply with the flammability requirements as described in U.S. Federal Aviation Regulation (FAR) 25.853, Appendix F. In 1997, concerned with the number of incidents involving flame propagation on thermal acoustical insulation blankets, the Federal Aviation Administration (FAA) Research and Development Division conducted a study to evaluate flammability test conditions beyond those called for in Appendix F of FAR 25.853. The study involved testing a variety of insulation blanket cover materials, including metallized PET. The metallized PET samples failed the expanded set of test conditions, prompting the study to conclude that the particular grade of metallized PET cover material used in the evaluation was flammable and possibly could propagate a fire under certain conditions. In March and May 1999 the FAA conducted burn tests as part of its continuing efforts to improve the test criteria required under FAR 25.853 Appendix F. A mock-up was fitted with insulation blanket material to simulate the top part of the fuselage of a commercial aircraft. The preliminary results demonstrated that metallized PET materials could be ignited and that the resulting fires could spread, generating large amounts of smoke under certain conditions and thus exacerbating the emergency associated with an in-flight fire. These results are consistent with observations from the previously referenced in-service fires and other FAA testing results.

With the in-service history, the demonstrated flammability of the metallized PET cover material, and the discovery, in the Swissair Flight 111 wreckage, of remnants of insulating blankets with cover material burnt, it is likely that this material was a significant source of the combustible materials that propagated the fire. It is the Board's view that the operation of aircraft outfitted with thermal acoustical insulation blankets incorporating metallized PET cover material constitutes an unnecessary risk. Therefore, the Board recommends that:

Regulatory authorities confirm that sufficient action is being taken, on an urgent basis, to reduce or eliminate the risk associated with the use of metallized PET-covered insulation blankets in aircraft.

A99-07


Flammability Test Criteria

The flammability test for thermal acoustical insulation, insulation covering, and insulation blankets, as stated in Appendix F of FAR 25.853, necessitates a vertical flammability test of samples using an approved burner. The type of cover material on the insulation blankets installed on the Swissair aircraft had been subjected to this test and met the applicable flammability test criteria for FAA certification.

In-service fires of the metallized PET cover material, and inconsistent results from the vertical burn test method specified by FAR 25.853, prompted manufacturers to seek additional flammability test criteria. Subsequently, aircraft manufacturers developed a "cotton swab" test, which yielded more consistent results when testing the flammability characteristics of the various cover materials. This additional testing was adopted by several major aircraft manufacturers who subsequently modified their internal material specifications. In 1996, based on results of the "cotton swab" test, McDonnell Douglas advised its customers not to use metallized PET, and discontinued its use in production aircraft. In 1997 an FAA sponsored study confirmed that the "cotton swab" test was a more reliable and reproducible test method to assess the flammability characteristics of metallized PET cover material; however, the FAA did not amend FAR 25.853, Appendix F to improve test standard requirements.

As the incidents listed in Appendix B attest, the limitations of the FAR 25.853, Appendix F, test criteria may not be confined to its inability to accurately and reliably identify the flammability characteristics of metallized PET cover material.

On 14 October 1998 the FAA stated that the test criteria used to certify the flammability characteristics of thermal acoustical insulation materials were inadequate, and committed itself to conducting the research necessary to establish a more comprehensive test standard. At the same time, the FAA indicated that because materials containing polyimide film have performed well in preliminary flammability tests, these materials would be considered compliant under the new regulation. Until adequate flammability test criteria are available, it is not possible to determine whether polyimide film, or other materials, provide adequate protection against fire propagation. Thermal acoustical insulation materials are installed in aircraft as a system, including such related components as tape, fasteners, and breathers. The Board believes that thermal acoustical insulation materials for use in aircraft must be judged against more valid flammability test criteria, not as individual components, but as a system. Therefore, the Board recommends that:

On an urgent basis, regulatory authorities validate all thermal acoustical insulation materials in use, or intended for use, in applicable aircraft, against test criteria that are more rigorous than those in Appendix F of FAR 25.853, and similar regulations, and that are representative of actual in-service system performance.

A99-08

As the investigation proceeds, should the Board identify additional safety deficiencies in need of urgent attention, it will make further aviation safety recommendations.

Benoît Bouchard

Chairperson

On behalf of the Board


Appendix A

The following accident synopses represent selected occurrences in which metallized PET insulation blanket cover material was involved.

  • 24 November1993: a McDonnell Douglas MD-87 experienced a fire while taxiing. Initially, the smoke emerged from the aft right side of the cabin. After the passengers and crew had disembarked, the fire intensified dramatically and spread quickly. Investigators determined that the metallized PET-covered insulation blankets acted as fuel sources that helped to spread the fire. [Aircraft Accident Investigation Board, Denmark]
  • 06 September1995: a McDonnell Douglas MD-11 experienced a fire in the Electronics and Engineering bay. Investigators found that molten metal from arcing wires had fallen on metallized PET-covered insulation blankets adjacent to the fuselage skin causing extensive flame propagation and widespread fire damage. [Minister of General Administration of Civil Aviation of China, People's Republic of China]
  • 26 November1995: a McDonnell Douglas MD-82 experienced a cabin fire prior to take-off. A ruptured light ballast case ignited a fire, which spread rapidly with extensive flame propagation on the metallized PET-covered blankets. [Civil Aviation Department, Republic of Italy]
  • 08 November1998: a fire broke out during loading operations of a McDonnell Douglas MD-11. Indications are that a cargo pallet was inadvertently pulled over an electrical cable that supplied power to one of the cable deck floor rollers. A box containing electronic circuitry sparked, which ignited a nearby metallized PET-covered insulation blanket. [National Transportation Safety Board, U.S.]
  • 29 March 1999: a McDonnell Douglas MD-11 freighter undergoing maintenance was discovered to have insulation blanket material displaying evidence of fire damage. Preliminary investigation results reveal that chafed wires, located under the floorboards of the aft cargo compartment, had arced, causing nearby metallized PET-covered insulation blanket to ignite. The fire propagated to cover an area of insulation blanket of approximately 60 inches by 26 inches. [National Transportation Safety Board, U.S.]


Appendix B

The following accident synopses represent selected occurrences in which metallized PVF insulation blanket cover material was involved.

  • 10 October 1994: after landing, ground crew detected a burning smell on a Boeing 737-100. Upon investigation, it was discovered that an improperly installed wire clamp had caused a short circuit. The subsequent arcing ignited nearby insulation blanket that used metallized PVF cover material. [Minister of General Administration of Civil Aviation of China, People's Republic of China]
  • 13 November 1995: during a maintenance inspection on a Boeing 737-300 a nut bolt had to be removed using an air drill. This action produced hot metal chips that ignited the metallized PVF insulation blanket cover material under the floor. Flames propagated to consume an area of 18 inches by 40 inches. [Minister of General Administration of Civil Aviation of China, People's Republic of China]

1. Insulation blanket cover material commonly known by the trademark "Tedlar"

2. Insulation blanket cover material commonly known by the trademark "Mylar".



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.

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