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Aircraft Fire Extinguishing Systems
Halon 1301 fire extinguishing systems in aircraft are employed in ways related to the size and mission of the aircraft and the number of engines. Application areas include engine nacelles or engine bays of twin-engine tactical aircraft; dry bays—those void spaces adjacent to, or beneath, fuel tanks and fuel lines; portable, hand-held extinguishers in cockpits and cabin spaces where aircrew personnel are located; and miscellaneous uses such as inerting a fuel tank's ullage (space above fuel in a less-than-full tank) to prevent incendiary bullet-initiated explosions, and to extinguish fires in auxiliary power units.
Fires Aboard Aircraft
Aircraft applications involve suppression of four distinct types of fires. These include hydrocarbon/air diffusion flames characteristic of engine nacelle and bay fires; premixed fuel vapor/aerosol/air deflagrations in dry bay applications; explosions of premixed fuel/air mixtures in fuel cells; and solid combustible diffusion flames involving cable and wire insulation or other combustibles that are typically suppressed using hand-held portable extinguishers.
The most critical fires are those in engine bays and explosion/deflagration events in dry bays. Each of these situations is addressed briefly below.
Halon 1301 is now used extensively to protect engine nacelles/bays. Here, the agent is discharged at a high rate through a series of nozzles to mix with the air stream through the engine and form a transient extinguishing concentration as it passes through the nacelle/bay and extinguishes the in-flight fire. The agent is discharged on command of the pilot after a positive indication of an engine fire, usually from a combination of thermal fire detection activation and/or anomalies in engine operating parameters. Thus, while the agent is discharged in only a few seconds, tens of seconds may elapse between the first indication of trouble and discharge of the suppression agent.
The basic mechanism of extinguishment of engine bay fires is identical to suppressing any hydrocarbon/air diffusion flame, such as a fuel oil fire in a ship machinery space. The primary technical challenge in aircraft is to disperse a sufficiently high concentration of agent into the engine bay such that an extinguishing concentration is maintained within the very high air flow environment of the bay. Generally, this extinguishing concentration is maintained for only a few seconds. Given the high air velocities present in engine bays, the flame strain rate is much higher than what occurs in quiescent, buoyant diffusion flames. As a result, these flames can be extinguished at lower agent concentrations. The reduced partial pressure of oxygen at flight altitude also simplifies the suppression process. A complicating factor in system design is the highly obstructed nature of the engine bay, which impacts nozzle design, flow rate, and agent mixing behavior.
The suppression of explosions (inertion) in dry bay applications requires sensing the presence of an explosion kernel before the flame front has expanded to a damaging size, and then rapidly applying a suppression agent in the vicinity of the ignition to quench the deflagration wave. This sequence must occur within a time scale of tens of milliseconds in order to effectively limit explosion damage, hence the need for automatic system actuation.
The primary scenario for initiation of a dry bay explosion is ordnance or shrapnel penetrating a fuel cell adjacent to the bay and subsequent ignition of the resulting fuel/air mixture. Dry bay explosion suppression systems are designed primarily to ensure the survivability of aircraft in combat. An alternative, albeit much less efficient approach, is to inert bays and voids prior to combat damage. This approach has been tried with some aircraft but has not been pursued in more recent aircraft designs.