Aviation Investigation Report A98H0003

1.14.1  Aircraft Certification Standards

  1. - Development of Material Flammability Standards
  2. - Material Flammability Standards – Testing Procedures  Development of Material Flammability Standards (STI1-90)

Among the CAAs, the FAA has traditionally taken a lead role in research and development to improve fire safety in aviation. In 1988, the United States Aviation Safety Research Act mandated the FAA to conduct fundamental research related to aircraft fire safety. The FARs are used internationally as the primary source for aircraft certification requirements, including material flammability standards. Current FAA regulations reflect a philosophy adopted following a study in 1975 to 1976 to determine the feasibility of, and the trade-offs between, two basic approaches to providing fire safety improvements to a modern, wide-bodied transport aircraft fuselage. The purpose of the study was to examine the impact of in-flight, post-crash, and ramp fires on fuselage compartments, and assess the fire protection requirements.

The first approach looked at the potential of applying the latest available technologies in early-warning fire-detection and fire-extinguishing systems. This approach would involve what was described as a "fire management system"; that is, one that would incorporate fire detection, monitoring, and suppression throughout the aircraft.

The second approach looked at the potential for improving the flammability standards of materials to be used in cabin interiors so that they would have high fire-retardant qualities, and low emissions of smoke and toxic gas.

The study concluded that there were merits and limitations to each approach, and that an approach combining a fire management system with selective material improvements may offer the most potential for providing timely fire protection in all cases.

Subsequently, as recommended in the FAA's SAFER[62] Advisory Committee report, the FAA's main research and development efforts were directed toward what was determined to be the greatest threat: a post-crash fire. The post-crash fire scenario that was envisioned was an intact fuselage adjacent to a fire being sustained by uncontained aviation fuel. It was determined that the most significant threat to surviving passengers in such a scenario would be from burning cabin interior materials. FAA research concluded that in such a scenario, surviving passengers could become incapacitated owing to toxic gases generated by a phenomenon known as "flashover."[63] Therefore, to increase survivability, the FAA concentrated its efforts on improving the flammability standards for cabin interior materials to delay the onset of flashover.

In-flight fires were considered to be rare, and the FAA concluded that the best defence against them would be through the use of cabin materials that had high fire-containment and ignition-resistance properties, and through the use of fire detection and suppression devices in "potential fire zones."

Research and development related to in-flight fires has led to increased fire protection in areas such as cargo compartments and lavatories.  Material Flammability Standards – Testing Procedures (STI1-91)

As part of the FAA aircraft certification process, materials to be used in the construction of aircraft are required to meet specified performance (test) criteria or standards when exposed to heat or flame. These flammability test criteria are designed, in principle, to expose a given material to a representative in-service fire environment. When deciding on the type and amount of testing for a particular material, assessments are made of the composition of the material, the quantity to be used, and its location within the aircraft. The testing is designed to measure the tendency of each material to ignite and propagate a flame.

For the majority of materials used in the pressure vessel, the flammability tests in place at the time the MD-11 was certified consisted primarily of a variety of Bunsen burner tests. A single Bunsen burner was used as the ignition source. Each test could be varied in several ways. For example, the orientation of the material to the flame could be varied from the horizontal through to the vertical. The orientation was specific to the test objectives, which were based on the perceived threat. The vertical burn test would normally be the most severe. Also, the length of time that the material was exposed to the flame could be varied. A longer exposure time would normally equate to a more severe test.

For each of the various Bunsen burner tests, requirements were established to differentiate between a pass or a fail for the material being tested. The following is a list of criteria that could be used to measure a material's flammability characteristics:

  • Ignition time (how long it takes the material to ignite when exposed to the Bunsen burner flame; the tests typically use either 12, 15, 30, or 60 seconds of flame exposure);
  • Glow time (the average time the material continues to glow after the ignition source is removed);
  • Flame time (the average time the material continues to produce a flame after the ignition source is removed);
  • Drip flame time (the average time that any dripped material continues to produce a flame);
  • Burn length (average value for burn length measured to the nearest 0.3 cm (0.1 inches)); and
  • Rate of burn (measured in inches per minute).

In accordance with individual Bunsen burner test requirements, the performance of the material was averaged over a minimum of three test specimens.

Except for selected materials in Class C cargo compartments, the most stringent material flammability standards were applied to those materials that were to be used in the occupied areas of the aircraft. Of particular interest were large surface panels, such as side walls, ceilings, stowage bins, and partitions. Not only were the materials used in the panels subjected to the most aggressive test procedures, the materials also had to be self-extinguishing; that is, they would not propagate flame beyond a certain distance, typically less than 20 cm (8 inches). Cabin materials were also subjected to tests for heat release and for smoke. No testing was required for toxicity. (STI1-92)

As a consequence of the testing requirements, less stringent material flammability standards were applied to those materials that were intended for use within the pressure vessel but that were outside the occupied areas. Certain materials only required the horizontal Bunsen burner test. To pass, the material could not exceed a certain rate of burn. Depending on the intended use of the material, the rate of burn could not exceed either 6 or 10 cm (2.4 or 4 inches) per minute. No requirement existed for these materials to be self-extinguishing.

In effect, the different flammability testing requirements, as described above, resulted in the following material flammability hierarchy:

  • Materials that would self-extinguish within an acceptable flame time and burn length;
  • Selected cabin materials that would self-extinguish and release no more than a predetermined amount of heat and smoke; and
  • Flammable materials with an acceptable rate of burn.

Therefore, many aircraft materials were certified even though they were either flammable or would burn within established performance criteria.

Many materials are installed in aircraft as part of a system, even though they are normally tested individually for flammability. For example, thermal acoustic insulation materials are typically installed as a system that includes cover material, insulation, and related components, such as splicing tape, fasteners, and breathers. However, by regulation, the testing of the "finished product" only consists of insulation and cover material together. Consequently, the "as-installed" thermal acoustic insulation materials may pose a different propensity to ignite and propagate fire than its testing would reveal. (STI1-93 (video clip))

[62]    Special Aviation Fire and Explosion Reduction dated 1980. FAA-ASF-80-4. Office of Aviation Safety. Washington, D.C. 20591.

[63]    The definition for flashover is not universally accepted, but can be defined in general as a sudden and rapid spread of fire within an enclosure.

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