Aircraft Hangar Fire Suppression Systems: Foam, Dry Chemical and NFPA 409

An aircraft hangar is protected against one scenario above all others: a fuel spill under a parked aircraft that ignites and spreads across the floor faster than anyone can reach a hose. NFPA 409, the Standard on Aircraft Hangars, sorts hangars into groups by size and door height and sets the suppression for each, which for the larger groups has historically meant foam-water deluge sprinklers, low-level high expansion foam generators, or both, on top of a floor designed to drain burning fuel away from the aircraft. Outside the doors, on the apron and at the fueling positions, the hazard is the same fuel without the building around it, and the agent there is dry chemical on ARFF vehicles, wheeled units and skids.

How NFPA 409 groups hangars and what each group needs

NFPA 409 divides hangars into four groups. Group I sits at the large end: an aircraft access door taller than 28 ft, or a single fire area above 40,000 sq ft, or provision for an aircraft with a tail height over 28 ft. Group II keeps the door at 28 ft or less with a fire area up to 40,000 sq ft, the exact ceiling set by construction type. Group III is smaller again, commonly capped near 12,000 sq ft and lower for lighter construction, which is where most general aviation and corporate hangars land. Group IV covers membrane-covered rigid steel frame structures.

The group sets the protection. A Group I hangar gets a foam-water deluge system at roof level, or automatic sprinklers combined with a low-level foam system, low or high expansion, backed by a supplementary hand hose system and a floor drained to carry burning fuel away from the aircraft. Group II has the same choices plus closed-head foam-water sprinklers. Group III requires no fixed system by default, though the authority having jurisdiction can call for one, and hazardous operations such as fuel transfer, welding or spray painting push a Group III hangar up to Group II protection.

The drainage is as much a part of the system as the foam. A hangar floor is sloped to trench drains that carry spilled fuel and foam solution to a separator outside the building, so the spill does not pool under a wing and a fire that starts is pulled away from the aircraft. A hangar with a flat floor and no drains is a different fire problem regardless of what hangs from the ceiling.

Foam-water deluge and low-level high expansion foam

A foam-water deluge system is a deluge sprinkler system with a foam proportioner on the supply, discharging solution through open heads across the whole bay on detection. It lays a foam blanket on the floor from above at the application rate set by NFPA 409 and NFPA 11. The weakness is the aircraft itself: the wings shadow the floor beneath them, which is exactly where the fuel collects. Low-level high expansion foam solves that problem from the other direction. Generators at floor level along the walls produce foam at expansion ratios above 200 to 1 and fill the bay to depth within minutes, flowing under the wings to smother the fire and suppress the vapor.

Both depend on the concentrate, and the concentrate is the problem. Hangar systems were built around AFFF, usually at 3 percent, for its speed on a jet fuel spill, and AFFF is going away in the US because its fluorinated surfactants are PFAS. The Department of Defense, which owns more large hangars than anyone, is converting to fluorine-free foam under MIL-PRF-32725, and civil operators are following as state restrictions and disposal costs land on them. Fluorine-free foam is more viscous, often needs different proportioning equipment, and is not a direct swap for AFFF on a thin spill under a deluge pattern, so a conversion means re-proportioning and a test discharge. A false discharge fills the hangar with foam solution, and the damage to avionics and engines has made accidental activations expensive enough that owners now weigh the system against the fire.

The case for water only, and its limits

Those accidental discharges, combined with the PFAS problem, moved the standard. The 2022 edition opened routes away from a mandatory foam system: a performance-based design path, a risk assessment that lets an owner propose an alternative scheme with or without foam, and acceptance of an ignitable liquid floor drainage assembly in place of foam. It also exempted Group II hangars that prohibit hazardous activities from the foam requirement. The argument is that a hangar with real drainage and no fueling inside presents a spill scenario that sprinklers and the floor can handle. The counter-argument is that a wing tank punctured during maintenance puts hundreds of gallons on the floor, and water on a jet fuel spill spreads the fire before the drains take it. The decision sits with the owner, the insurer and the AHJ.

The water-only path does not change the need for an agent that kills a fuel fire at the point of ignition. Whatever hangs from the ceiling, the hand hose system and the portable and wheeled units on the floor are expected to handle an incipient fuel fire before the ceiling system trips, and those units are dry chemical. Purple K, potassium bicarbonate, is the aviation standard: roughly twice the extinguishing rate per pound of sodium bicarbonate on a jet fuel fire, and supplied in grades compatible with most foam concentrates, so a dry chemical knockdown followed by a foam blanket does not tear the blanket apart. Wheeled Purple K units, commonly 150 lb, stationed along the hangar walls within reach of every aircraft position, are the usual answer.

Dry chemical on the apron and at the fueling positions

Outside the hangar doors the foam system stops and the fuel does not. The apron, fuel farm, hydrant pits and fueling positions carry the same spill hazard with no building and no drains designed around it, and the first line there is dry chemical. NFPA 407, the standard for aircraft fuel servicing, sets the floor: a listed dry chemical extinguisher of at least 20 lb at each fueling position and loading rack, and two of them, one per side, on every fuel servicing tank vehicle. That is a minimum, not a design, and the same standard bars ABC multipurpose (ammonium phosphate) extinguishers off fueling vehicles, ramps and aprons. Operators who take the hazard seriously add a wheeled Purple K unit of 150 lb at each position.

ARFF vehicles carry dry chemical as the complementary agent alongside water and foam, with 14 CFR 139.317 setting the smallest airport index at 500 lb of sodium based dry chemical or 450 lb of potassium based with water and AFFF. The twin-agent attack, dry chemical for knockdown and foam to hold the surface, is the standard technique for a running fuel fire on the apron.

Skids fill the gap between the wheeled units and the ARFF truck. A fuel farm, hydrant pump house or remote apron minutes from the fire station needs more agent than a wheeled unit carries and cannot wait for the truck. A nitrogen-expelled skid with 350 to 1,000 lb of Purple K, a hose reel, a stream that carries 25 to 40 ft and no dependency on water or power covers that hazard from a fixed position. Salus Global builds UL listed skids in 350, 1,000 and 2,000 lb capacities with the agent tank fabricated to ASME Section VIII and nitrogen held in separate charged cylinders at 150 to 200 bar, so the tank sits unpressurized for years on the apron and the agent stays fluidized on discharge. The skid range can be supplied with Purple K for aviation fuel hazards, and for a fuel farm with fixed targets such as a loading rack or pump skid, a fixed dry chemical system with piped nozzles gives automatic protection. The twin-agent skid puts dry chemical and foam on one frame for a position that needs both.

Dry chemical's limits on an airfield are the familiar ones. It does not cool, so a fuel fire that has heated a wheel assembly or an engine nacelle can reignite after knockdown, which is why the foam follows. Purple K residue on an aircraft is a cleaning job rather than the corrosion problem monoammonium phosphate leaves behind. Wind across an open apron scatters the cloud, so the operator approaches from upwind and the nozzle reach matters more than it does inside the hangar.

Specifying hangar and apron protection together

The design review for a new or converted hangar runs from the group classification and the foam versus water decision through detection (optical flame detection is the norm), proportioning and concentrate, drainage, the hand hose system, and the wheeled and portable units by position. The apron is often handled separately by the airport or the fuel operator, which is how a hangar with a good foam system ends up with an expired 20 lb extinguisher at the fueling position outside its door. One plan covering hangar, apron and fuel supply closes that. Concentrate selection belongs there too: a fluorine-free foam listed for the fuel, proportioned at its listed rate.

Frequently asked questions

Do all aircraft hangars need a foam system?

No. Group I hangars need a foam-water deluge system or sprinklers with low-level foam, and Group III needs no fixed system unless the authority having jurisdiction or a hazardous operation calls for one. The 2022 edition exempted Group II hangars that prohibit hazardous activities from the foam requirement and added risk assessment and performance-based design paths, so a hangar can be protected on an alternative basis where the AHJ agrees.

What dry chemical is used for aircraft fires?

Potassium bicarbonate, Purple K. It extinguishes jet fuel fires faster per pound than sodium bicarbonate, it is compatible with foam so a twin-agent attack works, and its residue does not corrode aircraft structures the way monoammonium phosphate does. ARFF vehicles, wheeled units at fueling positions and apron skids all carry it.

Is AFFF still allowed in hangar foam systems?

AFFF is being phased out in the US because of PFAS, with federal and state restrictions on purchase, use and disposal, and fluorine-free foams are replacing it. An existing system can often be converted, but the proportioner, application rate and discharge duration have to be re-evaluated for the new concentrate, and the system should be test discharged to confirm performance.

Salus Global dry chemical skids are UL listed and assembled, filled and inspected at our facility in Texas. Call +1 360 901 9828 or email info@salusglobal.co.