The aircraft industry in partnership with the Federal Aviation Administration (FAA) formed a task group in 2013 to consider using the American Society for Testing and Materials (ASTM) D7309 “Standard Test Method for Determining Flammability Characteristics of Plastics and Other Combustible Solid Materials Using Microscale Combustion Calorimetry” (MCC) as an alternate means of complying with 14 CFR 25 flammability regulations when a combustible constituent of a certified cabin construction is changed due to availability, economics, performance, or environmental concerns. A combustible constituent may be an adhesive, potting compound, film, fiber, resin, coating, binder, paint, etc., formulated with a new flame retardant, pigment, etc., that is used in the construction of a cabin material and can be tested in the MCC at the milligram scale. The use of ASTM D7309 for high precision measurements of aircraft cabin materials for regulatory purposes required a level of accuracy and reproducibility that was beyond the capability of the 2013 version of the ASTM D7309 standard when the FAA-Industry task group was formed. At the time, the calculation of the flammability characteristics did not include a correction for baseline driftwhich can be a significant source of error for low flammability aircraft cabin materials. The calculation of the calorimeter signal was revised in 2019 to include the effect of combustion gases, which improved the accuracy of the flammability parameters, and was codified as ASTM D7309-19 and later versions. Correction for baseline drift was complicated by random fluctuations of the MCC signal that precluded the subtraction of a pre-recorded background signal, as is routine in thermal analysis. This report describes an analytic approach to baseline correction that is specific to the MCC and can be used to correct the calorimeter signal for temperature-dependent drift during the test to improve the accuracy and reproducibility of MCC flammability parameters of combustible materials.

A method and criterion are described to assess the no-effect level of a constituent change on the fire performance of aircraft cabin material or construction. A constituent may be a thermosetting resin, coating, composite, adhesive, potting compound, film, fabric, elastomer, rubber, or thermoplastic. This can be used in the construction of a cabin material whose heat of combustion can be reliably measured using a 1-10 milligram sample in American Society for Testing and Materials (ASTM) D7309-21, Standard Test Method for Determining Flammability Characteristics of Plastics and Other Solid Materials Using an MCC. Bench-scale fire testing as per 14 Code of Federal Regulations (CFR) § 25.853 (Compartment Interiors, 2020) was conducted on several dozen cabin materials with changed constituents. Results showed that the relative difference in the microscale Fire Growth Capacity (FGC) of the certified and changed constituent in ASTM D7309-21 must be less than 0.3 (30%) to have no significant effect on the fire performance of the cabin material.

In February 2022, a package containing 140 lithium-ion pouch cells caught fire on a conveyor belt in a sort facility of an all-cargo airline. One of the packages in the shipment was sent to the Federal Aviation Administration’s (FAA) William J. Hughes Technical Center for hazard evaluation. Specialized cell analysis equipment was used to determine the state of charge (SOC) of the cells in the package. The measurements revealed that the cells were approximately at a 70% SOC, exceeding the maximum allowable shipping limit of a 30% SOC for the transport of cells and batteries classified as “UN3480, Lithium-ion batteries (including lithium-ion polymer batteries)”.

It was hypothesized that the fire in this incident started when the terminals of two cells packaged together made contact, causing the cells to short circuit, overheat, and enter thermal runaway. Testing was conducted to validate this hypothesis and to evaluate the fire risk of these cells at various SOCs. Key findings include:

• The cells tested at a higher SOC present a much higher fire risk compared to cells tested at or below a 30% SOC. This was consistent with previous FAA studies.
• The as-delivered 70% SOC cells went into thermal runaway when short-circuiting the cell terminals. The resulting temperature was high enough to propagate to nearby cells despite the surrounding packaging. This suggests that the original cell fire may have started when cell terminals made contact in the original package.
• A spark igniter ignited the gases released from the tested cells during thermal runaway. This suggests that a spark created during thermal runaway could have helped ignite the original package fire, as the ignition of cell gases would have produced a significant flame

There are many types of commercially available fire extinguishing agents used for a wide range of applications. The specific extinguishing agent used for a given application depends on the fire threat and design criteria. For class-C cargo compartments on aircraft, a gaseous flooding agent is used. Halon 1301 is currently the sole extinguishing agent being used in class-C aircraft cargo compartments. It requires a replacement due to its harm to the environment.

The fire threat within cargo compartments is changing compared to the threat that existed when aircraft class-C cargo compartment requirements were first established. The quantity of lithium batteries being shipped in cargo compartments is increasing each year. Lithium batteries can spontaneously catch fire or undergo thermal runaway where they release a significant quantity of flammable gas composed of hydrogen, carbon monoxide and hydrocarbons.

The objective of this study was to evaluate the effectiveness of Halon 1301 and some of its potential replacements against several flammable gases including lithium battery thermal runaway gases.

The United Nations (UN) Subcommittee of Experts on the Transport of Dangerous Goods (SCOE TDG) approves proper shipping names (PSN) and assigns each PSN with a four-digit UN number. Currently, two UN numbers have been assigned to lithium batteries as either UN 3480, Lithium ion batteries (including lithium ion polymer batteries) or UN 3090, Lithium metal batteries (including lithium alloy batteries). This research seeks to support the UN SCOE TDG establish a more performancebased approach to classifying the various types of lithium-ion batteries for transportation. Performance based classifications will support a better understanding of the risks of transporting lithium batteries and promote the industry in further developing safer batteries. A proposed standardized test method was used to assess the combustion hazard from a lithium-ion battery that has undergone thermal runaway. Lithium cobalt oxide pouch cells (3.7 V, 4.8 Ah) and cylindrical cells (3.7 V, 2.6 Ah) were tested at various states of charge (SOCs) and heating rates. The cells were individually heated to induce thermal runaway inside of a pressure vessel, resulting in a venting of gases. The vent gases were analyzed for their constituent’s volume, overall gas volume, and combustion energy. Key findings are as follows:

• The vent gas volume and combustion energy increased linearly with SOC for the pouch and cylindrical cells.
• The top three constituents as a percentage of the vent gas by volume across all SOCs and heating rates for the pouch and cylindrical cells were carbon dioxide, hydrogen, and carbon monoxide
• The vent gas volume and combustion energy increased with the heating rate for the pouch and cylindrical cells.
• When comparing cells heated at 20°C/min to cells heated at 5°C/min, across the tested SOCs, the pouch cells released an average of 14.1% more vent gas and had an average of 34.4% greater combustion energy, the cylindrical released an average of 7.2% more vent gas and had an average of 13.0% greater combustion energy.