Fire Protection Systems

Status Lights

Status lights are normally green, blue, or white. These indicate operation of systems that should be monitored or recognized by the pilots but that in themselves do not represent abnormal situations. Ice vane deployment on turboprops and bleed-powered ice protection systems on jets fall into this category. Fuel crossfeed is another such condition.

Airplane systems vary so much by aircraft type that what justifies a status light on one airplane may dictate a caution light on another and may be unindicated on a third. Status lights are sometimes installed on the same panel with caution and warning annunciators or may otherwise be scattered around the cockpit, depending on aircraft type.

Audio Advisory and Warning Annunciation

Every turbine aircraft you fly will likely have some sort of an audio advisory and warning system used in conjunction with annunciator and warning systems. Like annunciator lights, these audio systems are designed to advise the flight crew of the status or operation of certain aircraft systems and to warn the flight crew in the event of an impending emergency situation. A variety of audio signals may be found on typical turbine aircraft: bells, horns, chimes, clackers, verbal speech, and C-chord signals, to name a few. Some turbine aircraft may have dozens of different aural signals, and you will be required to know them all.

One popular turboprop associates all warning and caution lights with specific audio chimes. If a red warning light illuminates, it is accompanied by a triple chime, while an amber caution light is accompanied by a single chime. The audio warning chime is repeated every five seconds until the flight crew cancels it by pressing the appropriate master warning or master caution light switch.

Takeoff Configuration Warning System

One specific type of warning system often found in turbine aircraft is the takeoff configuration warning system (TOCWS). This system is designed to warn pilots when an aircraft is not properly configured for takeoff. The TOCWS monitors flap and spoiler positions, trim settings, parking brake status, and a number of other parameters, depending on the airplane (for example, gust locks or condition lever settings in turboprops). If any monitored controls or flying surfaces are in positions not conducive to safe takeoff, a horn sounds and a caution light illuminates upon application of takeoff power by the pilots.

The takeoff configuration warning system can be reset by retarding throttles to idle and reconfiguring the aircraft properly for takeoff.

Fire Protection Systems

A fire is absolutely one of the gravest emergencies that can occur in an airplane. Therefore, numerous systems are installed to combat them. Preflight and before-start checklists always call for fire system tests, as do APU start procedures. Some evidence suggests that most aircraft fire events are over in less than 20 minutes—successfully or unsuccessfully. Clearly, any in-flight fire dictates immediate emergency action, followed by landing as soon as possible.

As you might expect, most larger aircraft have built-in fire detection and/or extinguishing systems. These systems vary tremendously in detail, based on aircraft type, but there are a number of commonalities.

Fire Detection and Extinguishing Systems

Turbine engines are virtually always monitored for fire. Obviously, the combined presence of heat, fuel, and existing combustion makes engine nacelles potentially dangerous.

Fire detection in engine compartments may be accomplished in several manners. The most common is via a system of fire loops, installed around the engines so as to pass through the most likely areas where an uncontained fire could develop. Fire loops operate on the principle that the electrical resistance of a material changes with temperature. Excessive heating of any area on a fire loop signals a nearby detector that electrical resistance has changed and, therefore, that a fire may exist.

Another type of fire detection system operates optically. Infrared light (heat) that exceeds a certain threshold triggers a fire warning. (Optical fire detection systems are subject to false alarms in some aircraft, where sunlight at certain angles can trigger the detectors.)

To fight fires, remotely operated fire extinguisher bottles may be mounted in or near the engine compartment, or they may be remotely located in the fuselage. In some aircraft, multiple extinguisher bottles are cross-plumbed, so pilots can elect to fire multiple bottles sequentially into one engine to snuff a persistent fire (see Figure 6.9).

Aside from the engines, fire detectors may be located in any number of other vulnerable locations. Any plenum through which hot, high-pressure bleed air lines pass is a good candidate for monitoring, as are locations of major electrical components. Wheel wells are often monitored, due to the possibility of brake fires. (Tires on large aircraft are often filled with nitrogen in order to reduce the oxygen available to support brake fires.)

Remote extinguishers may or may not be installed in every one of these various locations. In the case of a fire alarm in a bleed line plenum, the checklist may simply call for shutting off the appropriate bleed source for that area and then proceeding to land.

Pilot Actions and Cockpit Controls

Pilots must learn specific emergency procedures for fires, based upon aircraft type. These sequences usually include verifying the emergency and its location, shutting down the affected engine, pulling the “fire handle,” and then selecting and firing built-in extinguishers, if installed.

Fire handles (or T-handles) are prominently located in the cockpit and often have fire warning lights installed directly in them to eliminate any confusion about which engine is burning. They are designed to completely isolate the engine with a single pilot action. Pulling a fire handle normally shuts off fuel plus hydraulic, electrical, bleed air, and sometimes oil connections to the affected engine. This action also arms the engine fire extinguisher, if installed, for pilot activation (Figure 6.10).

fire handle Pull fire handle to shut off engine fuel valves. Rotate handle to discharge fire bottle.

1 2 bottle

1

Engine

PULL

Fire bottles are mounted away from the engines, usually in the fuselage around the main landing gear bay.

fire bottle fire bottle

1 1 2 2 fire handle

1 2 bottle

2

Engine

PULL

Engine #1 Engine #2

Mounted below the fire bottles are explosive devices known as squibs. When signaled (by rotating the fire handle), the squib fires and breaks a disk on the bottle, releasing extinguishing agent into the selected engine.

Electrical Considerations

If any aspects of fire handle or extinguisher operation are electrically powered, they’re normally supplied off the “hot battery bus” so as to be operational even under limited electrical power.

Any critical electrical fire protection items on other electrical buses are normally powered by the generator of a different engine. For example, on a twin-engined aircraft, emergency electrical items for the left engine are normally powered off the right engine’s generator bus, and vice versa. Obviously, if a left engine fire occurred, it would likely disable the left generator in short order. Therefore, it’s best that left engine emergency electrical needs be filled from the right side (unaffected) generator.

Cabin and Cockpit Protection

Cabins of larger aircraft are normally equipped with smoke detectors, as well as with overheat detectors in environmental air ducts. Cabin and cockpit fires are normally addressed using the hand-held fire extinguishers required by the FARs. Crew members are required to be trained in the use of these extinguishers, and equipment maintenance must be monitored and logged. In addition, pilots are trained to respond appropriately to various types of cabin and cockpit smoke. Procedures for dealing with electrical fires, for example, are different from those addressing environmental system smoke. Training includes proper in-flight donning and operation of personal breathing equipment (PBE, see below) for use in these situations.

fire bell or fire horn

fire warning lights

in annunciator panel or fire handles

fire system test panel master warning light

extinguisher selector

fire handle (or “T-handle”) disconnects fuel, hydraulics, electrical, and bleed air from affected engine (via firewall shutoff valves); arms fire extinguisher

APU fire test panel

FIGURE 6.10 | Fire protection system: cockpit components.

Lavatory Smoke and Fire Protection

Despite the highly publicized fines imposed on offenders, a small minority of passengers continues to smoke in aircraft lavatories. These passengers create a fire risk by disposing their cigarette butts into the lavatory waste bin. For this reason, most aircraft lavatories are equipped with lavatory automatic fire detection and suppression units mounted in the waste container cabinet. These units are automatically activated if the associated heat detector senses excessive heat. When the unit is activated, a small fire bottle mounted on top of the waste container discharges a spray of extinguishing agent into the container to douse the fire.

Lavatory smoke detectors are installed in all commercial aircraft lavatories (Figure 6.11). These smoke detectors are similar to home smoke detectors in that they are usually only activated by a significant reduction in visibility due to thick smoke. Typical lavatory smoke detectors sound an aural alarm continuously until the detector is reset or the battery is removed. Most are safety-wired shut to discourage passengers from tampering with them. Advanced smoke detector systems may also signal the lavatory call light to flash continuously at the flight attendant station to alert the cabin crew. (Note that lavatory smoke detectors do not trigger waste-bin fire extinguishers as lavatory automatic fire protection systems do.)

Surprisingly, in most cases there are no cockpit indications of lavatory smoke or fire. This makes it critical for the cabin crew to inform the flight crew of any such situation as soon as possible. Once the cabin crew begins to investigate the source of any smoke or fire, they’re trained to first alert the entire cabin and flight crews, and then fight the fire. In larger aircraft with more than one flight attendant, the attendant who discovers the smoke or fire is to fight the fire and delegate another flight attendant to alert the rest of the crew. In the flight-attendant preflight briefing, many captains also request that a flight attendant remain on the cabin-cockpit interphone continuously throughout any smoke or fire event to keep a clear line of communication open. With the cockpit door closed (a requirement since 9/11 even in smoke or fire situations), the interphone is the only way the flight crew can be alerted to and kept informed about any cabin smoke or fire situations.

waste chute

automatic waste container fire bottle

lavatory smoke detector (battery operated)

trash

FIGURE 6.11 | Lavatory fire protection.

Portable Fire Extinguishers

Fire extinguishers are classified into four different types, with each designed for a different kind of fire. It is important (and an FAA requirement) that flight crew members know the locations and types of all fire extinguishers on board and which to use on a specific fire.

Fire Extinguishers: • Type A: designed for fires containing ordinary combustibles such as wood or paper. • Type B: designed for fires containing flammable liquids such as hydraulic fluid. • Type C: designed for electrical fires. • Type D: designed for combustible metals such as magnesium, which is found in some aircraft structures. (Magnesium is so combustible that it’s used in making flares.)

This memorization task is no longer as daunting as it used to be, since most aircraft are now equipped with Halon fire extinguishers, which can be used to extinguish Type A, B, and C fires, thereby covering most fires one might confront on an aircraft. Many aircraft also carry water extinguishers, which are only used on Type A fires, so be careful when using this type—you wouldn’t want to dump water on an electrical or magnesium fire and make conditions worse. A simple way to remember which type of extinguisher is used for which type of fire is: • Type A = Ash: used on a fire that makes an ash (paper, wood) • Type B = Bang: used on a fire that makes a bang (flammable liquid) • Type C = Charge: used on a fire having a charge (electrical fire) • Type D = Dent: used on a fire that makes a dent (fire involving metal)

Pilot Masks and Goggles

A challenge you’ll face during training, but hopefully never in real life, is donning pilot oxygen masks and smoke goggles under simulated emergency conditions and shooting an instrument approach to minimums while wearing them. Seeing out of smoke goggles is restrictive even under the best of conditions, and as a result of recent smoke- and fire-related accidents, new designs are being tested. Oversized smoke hoods under development reach almost to the instrument panel, in an effort to improve visibilty and, therefore, pilot performance under dense smoke conditions.

Portable Breathing Equipment (PBE)

Portable breathing equipment, or PBEs, are basically “see-through” hooded bags designed to protect flight and cabin crew members from the effects of smoke or harmful gases. Small chemical oxygen generators in the rear of each hood may be activated by the user, providing about a 10 minute supply of oxygen and allowing the crew member to fight an in-flight fire or otherwise manage an emergency without being overcome by hazardous gases. One or more PBEs must be readily accessible in the cockpit, with additional units sometimes located elsewhere in the cabin.

Auxiliary Power Unit Fire Protection

Since auxiliary power units (APUs) are remotely located engines with their own fuel lines, they are always monitored for fire and normally have automatic self-extinguishing systems. Upon fire detection, these automatic systems perform basically all the functions of the fire handles associated with the main powerplants. They cut off fuel to the APU engine, close air intake doors in many cases, and trigger warning lights. Many aircraft are equipped with external APU fire control panels so that ground crew members can suppress APU fires without running to the cockpit.

Cargo Compartment Fire Protection

Due to a number of cargo fire incidents and accidents, and the continuing fire risk posed by dangerous goods that might be present in passenger baggage or other freight, cargo compartment fire protection systems have become standard equipment on most large aircraft. The most common of these systems utilize a series of fans that continuously draw air from individual cargo compartments through a dedicated smoke detector. When a cargo compartment fire warning is received in the cockpit, cargo

compartment extinguishing systems are manually activated by the flight crew.

Similar to the engine fire bottles we’ve previously discussed, cargo compartment fire bottles are designed to discharge extinguishing agent throughout the entire compartment where they are located. Many systems are equipped with multiple bottles per compartment. The first bottle is discharged fully in hopes of an instant or “knock-down” extinguishing effect. The remaining bottles can subsequently be gradually discharged either manually or automatically for periods of up to an hour, to prevent possible fire re-ignition or flare-ups. This additional capability is designed to give the flight crew more time to get the aircraft safely on the ground (Figure 6.12).

cargo fire bottle 1 cargo fire bottle 2

smoke detector A cargo compartment

cargo smoke blowers

FIGURE 6.12 | Cargo compartment fire protection.

smoke detector B

cargo smoke blowers