Introduction

By their nature, aircraft hangars pose unique challenges for the fire protection engineer. There are large, open floor areas with tall roof decks to house high-value aircraft contents. Large quantities of liquid jet fuel are present, and aircraft maintenance activities offer a variety of potential ignition sources.

Another characteristic that differentiates hangars from most other occupancies are the large aircraft wings and fuselages that create obstructions to both fire detection and fire suppression. Sometimes, there are large scaffolds, which create further obstructions.

Naturally, as hangars come in all shapes and sizes, some of these features are not always present. A 6,000 ft2 (560 m2) shelter for small aircraft poses different challenges than a 150,000 ft2 (14,000 m2) maintenance complex for overhauling commercial jets.

The main fire threat is posed by a fuel spill finding an ignition source, leading to a challenging fire. A 50 foot (15 m) diameter pool of burning Jet-A fuel can produce a heat release rate on the order of 300 megawatts. A few hundred gallons (liters) of ignited fuel is enough to destroy just about any facility that is not properly protected.

Large hangars call for suppression systems on a scale with which some engineers may be unfamiliar. Fire detection systems must function over unusual heights and distances. Sensitivity is needed for fast response, but this factor must be balanced against protection from nuisance alarms. There are a number of fire suppression options, most of which involve fire pumps, foam systems and sprinkler systems with large design areas. Zoning and distances from equipment rooms to discharge points can also create design complications.

No less challenging for the fire protection engineer is the task of guiding a client or employer, whether that be building owner, construction contractor, code official, A/E firm, etc., through the often confusing array of design options and code requirements. The code requirements are far reaching and have big cost implications. The costs can be hard to reconcile against the loss history data. Hangar fires are low-frequency, but high-consequence events, and the codes require a large amount of protection and redundancy.

NFPA 409, Standard on Aircraft Hangars,1 is the primary document where adopted by the local jurisdiction. Like all NFPA codes and standards, NFPA 409 becomes a legal requirement when referenced in an adopting ordinance by a local governing body. Sometimes, these ordinances include amendments to certain provisions of the document. Even when not specifically adopted, there is often a desire to conform to internationally recognized minimum standards. Where insurance requirements govern, compliance with FM Global standards may be important. For U.S. military projects, matters depend greatly on which branch of service is involved, as there are differences between criteria from the Air Force, Navy, Army and National Guard. This article focuses on NFPA 409.

Historical Perspective

In the 1950s, NFPA began producing what became NFPA 409, taking the place of the earlier pamphlet from the National Board of Fire Underwriters (NBFU). Early NFPA hangar fire protection systems for larger hangars were based on sprinklers. The requirement became deluge-type sprinkler systems (with open sprinklers), and allowed a choice of plain water or foam. If foam was chosen, lesser sprinkler densities were allowed. Protein-based foams and fluoroprotein foams were used, as well as synthetic foams, which became the AFFFs (aqueous film forming foam) of today.

Draft stops (curtains) and floor drainage were important parts of the protection scheme, so the deluge flows could wash the burning fuel safely off the floor and down the drains. In the 1950s and 1960s, large deluge sprinkler systems were the norm. Rows of original deluge risers are often found in hangars constructed during this era. They employed pilot sprinklers or dropweight mechanisms to open the valve clappers. The weights were released by low pressure pneumatic heat detection systems connected to a network of heat-actuated devices (known as HADs) installed beneath the roof deck.

The old deluge systems covered sprinkler zones of up to 15,000 ft 2 (1,400 m2) that were separated by draft curtains. They had design densities of 0.16 to 0.25 gpm/ft2 (6.5 - 10 mm/min). The number of simultaneously flowing zones to be hydraulically calculated was determined by what was known as the "radius rule." The higher the roof deck, the larger the radius of an imaginary circle drawn in the plan view. Any zone touched by the circle had to be included. Hangars with 4, 5 or 6 zones calculated flowing were common, leading to huge sprinkler design areas of 90,000 ft2 (8,000 m2) or more.

With the advent of wide-body aircraft with expansive wing areas, such as the Boeing 747, the NFPA 409 committee became concerned that the aircraft would shelter a fire from the sprinkler discharge, and the water or foam would be too slow to reach the fire. They saw the need for foam to be discharged directly beneath the aircraft. With the 1970 edition, NFPA 409 began requiring “supplemental” foam systems in addition to the deluge sprinklers where there were individual aircraft with shadow areas greater than 3,000 ft2 (280 m2). Supplemental systems almost always employed oscillating monitor nozzles. (High expansion foam is also an option.) Though these nozzles need to only cover the area beneath the aircraft, as a practical matter they must cover a considerably larger area in order to reach all parts of the irregular shape of the aircraft shadow.

In the 1970s and early 1980s, Factory Mutual conducted research that led to the conclusion that sprinklers discharging plain water would fail to control a pool of burning jet fuel on a hangar floor.2 Increasing sprinkler densities was not the answer, since fuel rises above water and can continue to burn or vaporize and reignite.

Because of their physical properties, foams stay above and cling to the surface of burning fuels with a smothering action that provides cooling, cuts off oxygen and suppresses fuel vaporization. Therefore, star ting with the 1985 edition, NFPA 409 eliminated the option of plain water deluge sprinklers for Group I hangars, allowing only foam-water deluge sprinklers.

In the late 1990s, the NFPA 409 committee was presented with research conducted by the U.S. Navy.3 This led the 2001 edition to incorporate the most significant changes to Group I hangars since the foam requirement. The traditional foam-water deluge sprinkler scheme was retained, but as just one of three possible options. The old radius rule governing these deluge designs was revised to be independent of roof height.

The two new options were variants of the Group II protection requirements. In these new options, closed-head sprinklers are used at the roof level, and foam systems, either low-expansion or high-expansion types, are employed to cover the entire hangar floor area. These are termed “low-level” foam systems. Thus, the general historical trend has been to reduce the role of sprinklers from the primary fire suppression system, to a system to cool the steel while a foam system blankets the floor.

Understanding and Applying NFPA 409

The first step in applying NFPA 409 is to address the basics: will the aircraft in the hangar always be unfueled? What “group” should this hangar be classified as?

Allowing only unfueled aircraft in the hangar reduces protection requirements to a simple hazard sprinkler system. Most owners find this unacceptable for their operations. Fueled aircraft are the norm. Regarding hangar groups, rules-of-thumb (full details are in the standard) are as follows:

  • If the aircraft bay is greater than 40,000 ft2 (3,700 m2) and/or if the hangar door is taller than 28 feet (8.5 m), it’s a Group I (the most severe case).
  • If neither condition is true, it’s a Group II (only somewhat less severe, still lots of requirements, including foam and sprinklers).
  • Unless it’s a lot smaller (12,000 ft2 [1,100 m2] or less for common construction types), in which case it’s a Group III. (Few requirements: no sprinklers or foam, no fixed fire suppression systems at all, as long as there are no hazardous activities such as welding, painting, etc.)
  • Finally if the hangar is a “membrane covered rigid steel frame”1 and larger than a Group III, then it’s a Group IV. (A foam system or closed-head sprinkler system is required.) This relatively new type of hangar construction is becoming more popular.

An owner may be interested in considering construction options that allow the facility to be classified at a lower protection level. In some cases, there may be compromises that afford substantial cost savings. Therefore, it’s useful to know where the lines are drawn.

General requirements for construction and passive fire protection for both Group I and Group II hangars are found in Chapter 5. An abbreviated summary of the main requirements are:

  • Construction must be non-combustible.
  • Egress must meet NFPA 101, The Life Safety Code.
  • For hangar fire areas to be considered (calculated) separately, 3-hour walls are needed between aircraft bays. Otherwise, multiple bays are considered as one area with larger water and foam demands.
  • Shops and office areas must be separated from the aircraft bay by 1-hour rated walls.
  • Building columns in aircraft bays must have 2-hour protection or the columns must be sprinklered.
  • Hangar door power must be connected ahead of the building disconnect and must be operable in an emergency.
  • Trench drains are required and must have the capacity to carry away the full design fire flow rate of the fire suppression systems.
  • Any pits or tunnels in the hangar floors must be mechanically ventilated, drained, and treated as Class 1, Division 1 hazardous areas per the National Electrical Code.
  • For Group I hangars only: draft curtains must be provided, enclosing projected floor areas of 7,500 ft2 (700 m2) or less. These draft curtains do not define sprinkler zones and are not needed in Group II hangars.
  • For Group I hangars, fire suppression requirements are found in Chapter 6 and in Chapter 7 for Group II hangars. The main differences between Group I and Group II hangars are:
  1. When closed-head sprinkler systems are chosen, the Group I design criteria is 0.17 gpm/ft2 (7 mm/min) over 15,000 ft2 (1,400 m2), while Group II systems use the same density but with only a 5,000 ft2 (460 m2) design area.
  2. Water flow duration times are approximately 50% longer for Group I hangar systems than for those of Group II.
  3. Draft curtains are not required in Group II hangars.

Group I Choices

Since the options with closed-head sprinklers plus low-level foam systems became available, foam water deluge sprinklers are seen less often. This is particularly true when there are large aircraft with wing areas of more than 3,000 ft2 (280 m2), which invokes the need to add supplemental foam systems. It’s usually more economical to provide a larger low-level foam system instead of a supplemental foam system because the deluge system can be eliminated in favor of closed head sprinklers with a design area of 15,000 ft2 (1,400 m2). Going through the exercise of estimating these demands bears this out.

High-expansion foam usually leads to lower water demands than with other options, sometimes making it an attractive choice. If high-expansion foam is selected as a low-level system, NFPA 409 calls for the foam generators to utilize outside air. This means that the foam generators need to be ducted through the roof to intake hoods. Louvers and dampers will also be needed for relief air. U.S. Air Force and Army design criteria allow the use of inside air, which simplifies matters considerably. Some AHJs may be willing to consider the military approach of using inside air.

Common Sources of Confusion

Low-level vs. supplemental systems is perhaps the single greatest area of confusion in the standard. Supplemental systems are only provided in conjunction with foam water deluge sprinkler systems. Supplemental systems need to cover only the area beneath the aircraft, while low-level systems must be calculated for the entire hangar floor area. The design, objectives and testing requirements for each are different.

High-expansion foam is often used as a low-level system, although the foam generators are usually installed up high, not close to the floor. Low level systems are so named because their purpose is to cover the floor.

NFPA 409 provides a method for calculating the application rate of high-expansion foam. This method does not call for maintaining a submergence volume, because this is a local application approach, rather than a total flooding approach. The high-expansion foam is intended to act in a 2-dimensional manner. Therefore, it does not matter if the hangar doors are open or closed during foam discharge.

While the capacities of the water supply and foam system must be designed for complete coverage of the hangar floor, that does not mean they must all be activated simultaneously. The systems can and should be zoned in order to be discharged in response to heat detection on a zone-by-zone basis. Should the maximum number of zones be needed, the capacity must be available.

Redundancy is specified in different ways for water storage, for fire pumps and for foam supply.

  • Water Storage: Should stored water be necessary, a divided supply is required. The idea is for half the water to be available if one tank is down for repair. It does not mean the water storage quantity is to be doubled. If 200,000 gallons (900 m3) are needed to provide 45 minutes of f low duration, a pair of 100,000 gallon (450 m3) tanks should be provided. They should be piped to be used at the same time.
  • Fire Pumps: Should fire pumps be necessary (as they frequently are), the number and size of the fire pumps needed should be determined, and one additional pump of the same capacity should be provided. The requirement is for full pumping capacity with one pump out of service. Redundant jockey pumps are not required. Suction pipe sizing need not consider the redundant fire pump.
  • Foam Concentrate Pumps: If foam pumps are being used, they are treated in the same manner as fire pumps as far as redundancy is concerned. The schematic piping diagrams in the annex of NFPA 11 do not show all required components, and they do not show how multiple pumps are to be connected.
  • Foam Tanks: The requirement is for a “connected reserve” tank. The primary tank is to have full capacity for the 10-minute duration in the case of low expansion foam, or 12 minutes in the case of high-expansion foam. The connected reserve tank is to be of the same size as the primary tank. Its purpose is to be readily available after an event so the system may be put back into service quickly. As such, it should be connected so that manual operation of valves is needed to utilize its contents. This is true for both pressurized diaphragm tanks and atmospheric pressure tanks.

NFPA 409 is a prescriptive standard. If one wants to vary from its methods, NFPA 409, like most standards, allows for equivalencies or new technologies as long as the level of protection is not lowered. The authorities having jurisdiction have considerable discretion in this area if they choose to exercise it. Alternatives may be considered if one can provide analysis with enough rigor to satisfy the AHJ that a proposed alternative provides an equivalent level of protection.

Michael Aaron is with Rolf Jensen & Associates.

References:

  1. NFPA 409, Standard on Aircraft Hangars, National Fire Protection Association, Quincy, MA, 2011.
  2. “Fire Protection of Large Air Force Hangars,” Krasner, L., Chicarello, P., Fitzgerald, P. & Breen, D. Factory Mutual Research Corp., Norwood, MA, 1974.
  3. “Analysis of High Bay Hangar Facilities for Fire Detector Sensitivity and Placement,” Gott, J., Lowe, D., Notarianni, K. & Davis, W. National Institute of Standards and Technology, Gaithersburg, MD, 1997.