Aircraft hangars are primarily used for either aircraft assembly or maintenance. In this article, we will just focus on maintenance (or servicing) hangars.
The hazards of maintenance hangars can be fairly obvious. Aircraft are assumed to be fueled (some larger aircraft can hold thousands of gallons of fuel) and aircraft interiors are almost entirely combustible, using plastic materials that generate high heat and dense smoke. Add in other hazards such as hot work, painting, fuel transfer, and repair of onboard hydraulic systems and a significant fire risk can be present. This risk can be compounded by the value of the aircraft, which is usually higher than that of the hangar.
NFPA 409, Standard on Aircraft Hangars, offers extensive guidance for the protection of these facilities. Let’s review some of the major elements for fire protection:
The key to understanding the fire protection requirements of an aircraft hangar is to first determine which grouping applies. Maintenance hangars are classified by their size and door height as follows:
- Group I hangers have an access door height exceeding 28 ft., or have a single fire area in excess of 40,000 sq. ft., or have provision for housing an aircraft with a tail height exceeding 28 ft.
- Group II hangars have an access door height of less than 28 ft., and a single fire area of 5,001 to 40,000 sq. ft., depending on the type of building construction.
- Group III hangars have an access door height of less than 28 ft. and a single fire area of 5,000 to 30,000 sq. ft., depending on the type of building construction.
- Group IV hangar is any structure constructed of a membrane-covered rigid-steel frame.
Construction materials and arrangement must be chosen as applicable.
- Group I or II hangars are to be either Type I or II construction. Other groups may be ordinary construction.
- Demising walls are needed when other occupancies are in the hangar such as shops, offices, storage, or separate servicing areas are present. These walls are either 2 or 1 hour rated depending on the type of occupancy.
- Aprons adjacent to access doors need to be sloped away from the aircraft hangar, so any spilled fuel does not enter the hangar.
- Due to the possibility of a fuel spill, floor drainage is needed for all hangar groups. The capacity should be large enough to prevent buildup of flammable liquids and water over the drain inlet when all fire protection systems and hose streams are discharging at the design rate. Typically, a trench drain is placed parallel to the access doors to capture spilled fuel before it flows onto the apron and reaches a parked aircraft. Additional trench drains are placed on both sides of an aircraft bay to capture spilled fuel or other flammable liquids from either reaching the aircraft or spreading the spill to other parts of the hangar.
Fire suppression systems are a critical component in the overall protection scheme and should meet the following.
For Group I Hangars:
- A foam-water deluge system, installed at the roof, along with supplementary protection systems (oscillating monitor nozzles) if a single aircraft has wing areas greater than 3,000 sq. ft., or,
- Combination of automatic sprinklers at the roof and an automatic low-level, low-expansion foam system, or,
- Combination of automatic sprinklers at the roof and an automatic low-level high-expansion foam system.
- A minimum of two fire pumps are required to supply these systems.
For Group II Hangars:
- Foam-water deluge systems utilizing air-aspirating discharge devices, or
- A combination of automatic sprinklers at the roof and an automatic, low-level, low-expansion foam system, or
- A combination of automatic sprinklers and an automatic, high-expansion foam system, or
- A closed-head foam-water sprinkler system.
For Group III Hangars:
- Where hazardous operations, including fuel transfer, welding, torch cutting, torch soldering, doping, and spray painting, are performed, fire protection as specified for Group II hangars is needed.
For Group IV Hangars:
- When the total area exceeds 12,000 sq. ft., a low-expansion foam or high-expansion foam system is needed.
Hangars housing unfueled aircraft follow a different set of criteria and NFPA 409 should be consulted for those requirements.
It is important to acknowledge the scope of NFPA 409 includes guidelines for construction and protection of the aircraft hangar, but does not include protection goals for the aircraft contained within the hangar. In the event of a fuel fire, it is likely that the aircraft of fire origin will be subjected to severe thermal damage, regardless of the protection features in place. However, with proper design of passive and active fire protection features, including closely spaced emergency drainage, it may be possible to not only protect the hangar, but also limit direct thermal damage to adjacent aircraft.
Risk Logic can recommend fire protection design criteria, help develop preventive maintenance and property loss control programs at your facility. Please contact us to schedule a property survey at your facility by one of our engineering specialists.