The purpose of this document is to describe the testing and results of the evaluation of pentafluoroethane (HFC 125) and sulfur hexafluoride (SF₆) as simulants for Bromotrifluoromethane (Halon 1301), the agent of choice in aircraft total flooding fire extinguishing systems. The idea behind the use of a simulant is to reduce the amount of halon, an ozone-depleting agent, discharged in the atmosphere during the certification testing of aircraft Halon 1301 total-flooding fire suppression systems.

On April 15, 1997, the International Halon Replacement Working Group (IHRWG) organized a new task group to evaluate candidate Halon 1301 simulants. The thrust behind the creation of this new task group was the fact that the Federal Aviation Administration (FAA) was planning to release Notice of Proposed Rule Making (NPRM) 97-10 to change existing regulations regarding aircraft cargo compartments. The NPRM upgraded the fire safety standards for cargo compartments in aircraft by eliminating the Class D cargo compartment configuration in existing and future aircraft fleets; i.e., large cargo compartments without active fire detection and suppression systems. By the year 2001, all aircraft operating under Federal Aviation Regulation (FAR) Parts 121 and 135 are required to have a fire protection system installed in their Class D cargo compartment (FAA, 1997), thus converting them to a Class C compartment. FAR 25.855 requires certification tests of fire suppression systems to ensure their proper operation and that adequate concentration of extinguishing agent is maintained. This means that each Class D cargo bay configuration and each unique fire protection system will have to be certified by the FAA. Halon 1301 continues to be the agent of choice in the aviation community. Even though the production of Halon 1301 ceased in the industrialized world in 1994, the U.S. Environmental Protection Agency has not yet restricted its usage in civil or military aviation operations (EPA, 1995). It is expected that large quantities of Halon 1301 will be discharged into the atmosphere during certification testing in order to meet the new requirement before the year 2001. The goal of the IHRWG task group is to minimize or eliminate the usage of the Halon 1301 ozone-depleting gas during the certification testing of aircraft cargo compartment fire protection systems.

Currently there is no advisory circular to direct an acceptable means to conduct these certification tests to comply with FAR 25.855. Even though there is no written guidance, a common procedure, based on years of testing and experience, has been used by aircraft certification engineers to approve certificates. Briefly, the applicant needs to demonstrate to the FAA that the cargo fire extinguishing system works as designed by performing a combination of flight testing and analysis. The aircraft flight test conditions are at a steady-state cruise profile with a maximum cabin-to-ambient pressure differential. The cargo compartment is empty with the ventilation system set as indicated in the airplane flight manual for a fire situation. If the system is designed for Halon 1301, a 5% by volume minimum concentration is required for initial knockdown of the fire and a sustained concentration of 3% by volume is required for the duration of the flight, typically 60 minutes for normal non-Extended Twin-Engine Operations (ETOP) aircraft. The certification engineers examine the system interface such as the connection between the fire suppression system and the smoke detection system. They also review the flight crew interface and system feedback, such as crew cockpit view of the fire alarm and usability of the control panel. Other certification considerations include system redundancy, fire squib debris blockage, system freezing effects, and squib power source (Lam, 1998).

Published reports from the U.S. Navy and other entities identified two agents (SF₆ and HFC 125) that are excellent simulants for Halon 1301 fire protection systems used in shipboard machinery spaces (DiNenno et al., 1990) and aircraft engine nacelles (Grosshandler et al., 1995).

Sulfur hexafluoride (SF₆) is a stable, non-toxic, nonflammable, colorless, odorless, non-corrosive gaseous chemical and is not suspected of contributing to ozone depletion. This gas is used as a dielectric or insulating medium in electrical equipment due to its desirable electrical, physical, and thermal properties. The major industrial use of SF₆ is in the electric utility industry and in areas such as high-voltage circuit breakers, low-voltage switchgear, gas-insulated power stations and transmission lines, radar equipment, and Van de Graff and linear particle accelerators. It has also been used in non-electrical applications such as the blanketing of molten magnesium, leak detection, and dry processing in the semiconductor industry (Air Products, 1983). It has a relative vapor density of approximately 5.1, which is equal to that of Halon 1301. The drawback of this gaseous agent is that it has an atmospheric lifetime of 3,200 years with a 100-year, 500-year, and 1,000-year global warming potential (GWP) of 16100, 26110, and 32803, respectively (EPA, 1995). The U.S. Navy identified SF₆ as an excellent simulant for shipboard total-flooding Halon 1301 systems based on its physical properties, discharge rate, distribution through a piping network, leakage from a compartment (Carthart et al., 1989), air mixture (Carthart et al., 1991), and full-scale testing (DiNenno et al., 1990). In 1995, the EPA in its final Protection of Stratospheric Ozone ruling stated that "SF₆ is acceptable for use as a discharge test agent in military uses and civilian aircraft uses only."

HFC 125 is an acceptable total-flooding fire and explosion protection agent under the EPA’s Significant New Alternatives Policy (SNAP) Program, but due to its extinguishing design concentration, 14.2%, it is limited to unoccupied areas; it exceeds the EPA’s 7.5% No Adverse Observable Effect Limit. HFC 125 is used in applications such as military aircraft engine nacelles, dry bays, electronic and electrical cabinets, under floor spaces, turbine or engine compartments, as a Halon 1301 simulant for aircraft engine nacelle extinguishing systems, and as an explosion suppression agent in grain elevators (Dupont, 1995). Its relative vapor density is approximately 4.2. Grosshandler and Womeldorf (1995) concluded that HFC 125, filled in storage containers to 77% of the mass of Halon 1301, provides an excellent simulant for the nacelle's fire protection system. The Navy has promoted its usage in qualification testing on board the F/A-18E/F and V-22 (Department of Navy, 1998).