Thermal Acoustic Insulation Blankets
Thermal acoustical insulation blankets are widely used in the aviation industry to protect the aircraft interior from temperature variations and noise. The Swissair MD-11 was primarily fitted with metallized PET (M-Pet). M-Pet covered Mylar insulation blankets are used throughout the MD-11 aircraft, including extensive use in the ceiling area forward and aft of the cockpit bulkhead. On October 14th 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.
As discussed in Section 1.1 above, The catastrophic electric failure exacerbated other underlying shortcomings in the construction of the aircraft including the characteristics of the thermal acoustic insulation blankets (M-Pet) that lined the aircraft.
3.1 FAA’s Flammability Test of Mylar Misleading
After extensive flammability testing, the TSB determined that such blankets are ‘susceptible to being ignited by small ignition sources, such as electrical arcing or sparking and will propagate a fire’.
Accordingly, on August 11th 1999, the TSB issued Aviation Safety Recommendation A99-07 noting ‘it is likely that MPET cover material was a significant source of the combustible materials that propagated the fire’ and ‘that the operation of aircraft outfitted with thermal acoustical insulation blankets incorporating MPET cover material constitutes an unnecessary risk.’ The TSB 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 MPET-covered insulation blankets in aircraft.
On the same date the TSB issued Aviation Safety Recommendation A99-08 requesting regulatory authorities to 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 that are representative of actual in-service system performance.
The recommendations are similar to those subsequently issued on August 28th 2001 in respect of aircraft wiring (See Section 2.6 above) in that they refer in this instance to actual in-service performance and in respect of the former to realistic operating conditions. In other words, laboratory testing is acceptable provided it correlates to the real world performance of these materials. A Bunsen burner is not the same as an in-flight fire thirty thousand feet above the Atlantic Ocean.
3.2 Selected Incidents of Mylar Propagating Fire aboard Aircraft
In Appendix A of their final report, the TSB cited five accidents representing ‘selected occurrences in which M-Pet insulation blanket cover material was involved’. It should be noted that these are representative of the problem and represent a sample of the many incidents in which M-Pet insulation blankets have propagated fire.
1 November 24th 1993: 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 M-Pet covered insulation blankets acted as fuel sources that helped to spread the fire. [Aircraft Accident Investigation Board, Denmark]
2 September 6th 1995: 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 M-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]
3 November 26th 1995: 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 M-Pet covered blankets. [Civil Aviation Department, Republic of Italy]
4 November 8th 1998: 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 September 6th 1995 vintage covered insulation blanket. [National Transportation Safety Board, U.S.]
5 March 29th 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 M-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.]
The September 6th 1995 incident is of particular note in determining the adequacy of the FAA’s response to the problem. In a report dated 24 May 1996, which was forwarded to the FAA, the Civil Aviation of China (CAAC) recommended that the manufacturer be advised, "the insulation blanket installed in the Boeing 737-300, [and] MD-11 airplanes is fire flammable. They should make a prompt and positive response."
In a response to the CAAC report dated 24 July 1996, the FAA stated that they intended to investigate the behaviour of insulation blanket materials under larger scale conditions. The FAA also stated that, while the tests conducted by the CAAC on the PET were illustrative, the type of CAAC testing conducted (igniting at the sewn edge of the sample material) was not required for certification.
In the context of the FAA’s October 14th 1998 statement (above) that “the test criteria used to certify the flammability characteristics of thermal acoustical insulation materials were inadequate”, one has to ask precisely what did the FAA do in response to the CAAC correspondence? Many have wondered if the fate of the 229 people aboard Swissair Flight 111 might have been different had the FAA responded more robustly to the CAAC on July 24th 1996.
3.3 IASA’s Response
As with the wiring issue, IASA first gathered together all available data before taking our findings to the FAA. We directed correspondence to the FAA and obtained from them copies of the CAAC correspondence. Whilst we had anticipated that the FAA response would be lethargic at best, we had not anticipated just how lethargic.
As noted in Section 2.4 above, the FAA initially gave airlines four years to remove M-Pet insulation blankets, however, if this was not bad enough this was subsequently extended to five years. However, the M-Pet fiasco is made all the more alarming by virtue of a seemingly innocuous question IASA put to the FAA seeking an update as to how many of the hundreds of affected aircraft had removed the M-Pet insulation blankets. The answer we do not routinely monitor each airline’s compliance with Airworthiness Directive … although we have the ability to do so, we trust that they do. Is this the approach that was adopted with Alaska Airlines? Trust? A Regulator should Regulate and to regulate means to take a proactive lead in ensuring not only that it tackles problems before they turn into disasters but making sure that when it does mandate action it consistently monitors action taken in response to it. Otherwise it makes a mockery of the title of ‘regulator’.
The FAA issued two NPRMs (99-NM-161-AD and 99-NM-162-AD). The NPRMs proposed the removal of MPET-covered insulation blankets from all US-registered aircraft. The final rule regarding these proposals came in May 2000 when the FAA issued two ADs (AD 2000-11-01 and AD 2000-11-02). On July 31st 2003 the FAA issued a revised final rule allowing airlines until September 2nd 2005 to comply. Considering the TSB recommendations were issued on an ‘urgent basis’ it is alarming to us that it will be six years from the date of the recommendation to compliance and that unfortunately is not the end of this issue.
In September 1997, one year prior to the crash of Swissair 111, the US Department of Transportation published a document titled Evaluation of Fire Test Methods for Aircraft Thermal Acoustical Insulation stating:
This report presents the results of laboratory round robin flammability testing performed on thermal acoustical insulation blankets and the films used as insulation coverings. This work was requested by the aircraft industry as a result of actual incidents involving flame propagation on the thermal acoustical blankets. . . . The grade of metallized PET film evaluated in this round robin is flammable and possibly could propagate a fire in a realistic situation.
In other words, when subjected to a test that reflected actual experience, the FAA’s text reflected the reality of flammable events.
The deadline for compliance, June 30th 2005, has been and gone and to this day we are not certain whether all affected aircraft have removed the M-Pet. It is the case that many of the affected aircraft were likely sold to third world and emerging economies, while others were converted into cargo aircraft, however, for the FAA to take close to nine years to address a problem that was brought firmly to its attention by the CAAC is alarming to say the least.
 TSB Final Report. Section 126.96.36.199 Other Thermal Acoustic Insulation Materials at Risk.