Patricia Cahill

This report discusses the flammability tests conducted on aviation and nonaviation electrical wiring that were performed to re-evaluate the effectiveness of the current Federal Aviation Administration (FAA)-mandated 60° Bunsen burner flammability test requirement for aircraft wiring. The evaluation included a 60° flammability test, an intermediate-scale vertical flammability test, and an intermediate-scale cabin attic flammability test. Test results showed that the 60° single wire Bunsen burner flammability test may not be adequate to qualify wire when bundled and subjected to a severe ignition source.

Huiqing Zhang

The intrinsic relationships between polymer structure, composition, and fire behavior have been explored to develop new fire-safe polymeric materials.

Three milligram-scale methods (pyrolysis-combustion flow calorimetry (PCFC), simultaneous thermal analysis, and pyrolysis gas chromatography/mass spectrometry (GC/MS)) have been combined to fully characterize the thermal decomposition and flammability of polymers and polymer composites. Thermal stability, mass loss rate, char yield, and properties of decomposition volatiles were found to be the most important parameters in determining polymer flammability. Most polymers decompose by either an unzipping or a random chain scission mechanism with an endothermic decomposition of 100-900 J/g. Aromatic or heteroaromatic rings, conjugated double or triple bonds, and heteroatoms such as halogens, N, O, S, P, and Si, are the basic structural units for fire-resistant polymers. The flammability of polymers can also be successfully estimated by combining the pyrolysis GC/MS results or chemical structures with the thermogravimetric analysis.

The thermal decomposition and flammability of two groups of inherently fire-resistant polymers—poly(hydroxyamide) (PHA) and its derivatives and bisphenol C (BPC II) polyarylates—have been systematically studied. PHA and most of its derivatives have extremely low heat release rates and very high char yields upon combustion. PHA and its halogen derivatives can completely cyclize into quasi-polybenzoxazole structures at low temperatures. However, the methoxy and phosphate derivatives show a very different behavior during decomposition and combustion. Molecular modeling shows that the formation of an enol intermediate is the rate-determining step in the thermal cyclization of PHA. BPC II-polyarylate is another extremely flame-resistant polymer. It can be used as an efficient flame-retardant agent in copolymers and blends. From the PCFC results, the total heat of combustion of these copolymers or blends changes linearly with composition, but the change of maximum heat release rates also depends on the chemical structure of the components.

The flammability of various polymers and polymer composites measured by PCFC; cone calorimeter, ASTM E1354; and the Ohio State University (OSU) calorimeter, ASTM E906, were also compared. For pure polymers, there was a relatively good correlation between different methods. However, for polymer composites with inert fillers or flame-retardant additives, the OSU and cone calorimetries are more suitable evaluation methods.

Steven M. Summer

This report discusses experiments to determine the reduction in oxygen concentration required to prevent a fuel tank explosion. A simulated aircraft fuel tank containing JP-8 fuel of an amount equivalent to a mass loading of approximately 4.5 kg/m3 was used to determine the limiting oxygen concentration (LOC) at pressures corresponding to altitudes ranging from 0 to 38,000 ft. In addition, the peak pressure rise was measured at various altitudes (pressures) due to ignition occurring at O2 levels approximately 1% to 1.5% above the LOC.

From these tests, it was determined that the LOC at sea level through 10,000 ft is approximately 12% O2, with a linear increase from 12% at 10,000 ft to approximately 14.5% at 40,000 ft. Tests with various sparks/arcs as ignition sources at sea level showed little variation in results, with the LOC ranging from 11.9% to 12.8%. The single ignition event falling below 12% O2 is attributed to inherent error in the oxygen measurement system, whose sensitivity is stated to be ±1% of the full-scale value (25% O2). In addition, a heated surface capable of igniting a flammable fuel air mixture proved insufficient for ignition in a tank inerted to just 14%. Peak pressures resulting from ignition at oxygen concentrations 1% to 1.5% above LOC values decreased as the altitude was increased to 30,000 ft, while the duration to reach the peak pressure increased.

Harry Webster

This report documents the findings of a series of tests conducted to determine the flammability characteristics of primary lithium batteries and the dangers associated with shipping them in bulk form on commercial transport category aircraft.

Michael Burns, William M. Cavage, Richard Hill, & Robert Morrison

Extensive development and analysis has illustrated that fuel tank inerting could potentially be cost-effective if air separation modules (ASM), based on hollow-fiber membrane technology, could be used in an efficient way. To illustrate this, the Federal Aviation Administration has developed an onboard inert gas generation system that uses aircraft bleed air to generate nitrogen-enriched air (NEA) at varying flow and purity (oxygen concentration) during a commercial airplane flight cycle. A series of ground and flight tests were performed, in conjunction with Airbus, designed to prove the simplified inerting concept. The system was mounted in the cargo bay of an A320 operated by Airbus for the purposes of research and development and used to inert the aircraft center wing fuel tank during testing. The system and center wing fuel tank were instrumented to allow for the analysis of the system performance and inerting capability.

The results of the tests indicated that the concept of the simplified inerting system is valid and that the air separation module dynamic characteristics were as expected. ASM pressure had the expected effect on flow rate and purity; however, bleed air consumption was greater than expected during cruise. The fuel tank inerting results illustrated that no stratification or heterogeneous oxygen concentrations occurred in the tank. The measured average tank ullage oxygen concentration data agreed well with a simple analytical model applied to the flight test data. The measured effect of the high-flow mode was significant, allowing the single-membrane configuration to maintain an inert ullage during the entire flight cycle, even with the very high rate of descent employed for the flight tests. Fuel had virtually no effect on the resulting oxygen concentrations observed in all the tests.