Aircraft hangar protection is driven by a fuel spill scenario rather than by an ordinary structure fire. An aircraft on the hangar floor may hold thousands of gallons of jet fuel in wing tanks a few feet above a flat, unobstructed concrete surface. A tank breach or a fuel system failure during maintenance releases that fuel as a rapidly spreading unconfined pool, and a pool fire of that area develops heat release rates that conventional sprinkler protection cannot control.
NFPA 409 sorts hangars into Groups I through IV based on size, door height, and construction, and the group determines the protection required. The larger groups generally require foam-water deluge across the hangar floor, frequently with supplementary protection under wings and over specific areas, because foam is what secures a burning liquid surface while water alone would spread it. Hangar height also defeats conventional detection, which is why optical flame detection is common in this application.
The current complication in this category is foam chemistry. Aqueous film-forming foam has been the standard agent for decades and contains PFAS compounds now under significant regulatory and environmental pressure. Facilities are transitioning to fluorine-free alternatives, which is not a drop-in substitution: proportioning rates, discharge device compatibility, and system performance all need verification. At the same time, the environmental liability attached to any discharge, including an accidental one, has risen sharply, which makes false discharge prevention a much bigger operational concern than it once was.