9.5 Dealing With Emergencies: Fire Classes, Extinguishing Agents & First Aid

9.3 Emergency Response, Fire Classes, Extinguisher Chemistry & First Aid

Aviation maintenance hangars and flightline aprons present an extraordinarily volatile fire environment. High-octane aviation fuels, atomized hydraulic sprays, flammable degreasers, composite curing heating blankets, and energized high-amperage electrical circuits coexist in continuous close proximity. A single maintenance error—such as an ungrounded fuel line, a hot brake rotor contacting hydraulic mist, or an electrical short—can initiate an uncontrollable conflagration within seconds. Licensed certifying technicians must master combustion dynamics, the chemical properties of extinguishing media, emergency hangar deluge procedures, and immediate life-saving first aid interventions.

Fire Dynamics: The Fire Triangle and Fire Tetrahedron

Historically, combustion was conceptualized via the classic Fire Triangle, consisting of three components:

1. Fuel: A combustible substance in solid, liquid, or gaseous phase.
2. Heat (Thermal Energy): Sufficient energy to raise the fuel to its flash point and autoignition temperature.
3. Oxygen (Oxidizer): Ambient air containing at least 15% oxygen by volume to sustain rapid oxidation.

Modern fire science has expanded this model into the Fire Tetrahedron, introducing the fourth critical dimension: the uninhibited chemical chain reaction. During the combustion of organic fuels, thermal pyrolysis generates highly reactive chemical intermediates known as free radicals (predominantly hydroxyl $OH^\bullet$ and atomic hydrogen $H^\bullet$). These free radicals react exothermically with oxygen in rapid propagation cycles. Extinguishment occurs by disrupting any one of the four faces of the tetrahedron:

• Cooling: Absorbing heat energy faster than it is produced (e.g., water on Class A fuels).
• Smothering: Diluting or excluding oxygen below the concentration required for combustion (e.g., carbon dioxide or aqueous film blanket).
• Starvation: Cutting off the supply of fuel (e.g., closing an emergency fuel shut-off valve).
• Chemical Flame Inhibition: Introducing chemical agents that scavenge free radicals, breaking the self-sustaining chain reaction (e.g., clean gaseous agents).

Classification of Fires: Classes A to D

Aviation maintenance training uses the fire classes defined by the US National Fire Protection Association (NFPA) and repeated in FAA maintenance handbooks and advisory circulars:

Class A: Ordinary Combustible Materials

Wood, cloth, paper, upholstery, rubber, and plastics. Cooling with water or water-based agents is most effective.

Class B: Flammable Liquids

Fuels, oils, greases, solvents, paints, and hydraulic fluids (FAA guidance also groups flammable gases here). These need an agent with a blanketing effect, such as foam, carbon dioxide ($CO_2$), dry chemical, or Halon. Water is not recommended, because most petroleum products float on it and a burning liquid can spread.

Class C: Energized Electrical Equipment

Fires in wiring and electrical equipment, where the agent must be electrically non-conductive. First remove electrical power, remembering that capacitors and coils can hold residual energy, then use $CO_2$, Halon, or another non-conductive agent. Water and foam are not acceptable on energized equipment. Once the equipment is de-energized, agents suitable for Class A or B fires can be used.

Class D: Flammable Metals

Magnesium, titanium, and similar metals, typically in wheels and brakes or as workshop swarf. Class D fires are usually started by a Class A, B, or C fire. Never use water: it reacts violently with burning metal and can scatter molten metal. $CO_2$ is also ineffective, because burning magnesium can continue to burn in carbon dioxide. Use dry powder, following the manufacturer's recommendations.

Lithium Battery Fires

Lithium-ion batteries in portable electronic devices need a different approach. EASA and FAA guidance is to extinguish the flames with the available extinguisher and then cool the device with water or another non-alcoholic liquid to stop thermal runaway spreading to other cells and re-igniting. Do not try to pick up or move a burning or smoking device. For installed aircraft batteries, follow the aircraft maintenance manual.

European Extinguisher Ratings

Portable extinguishers sold in Europe are rated to EN 3-7 against standard test fires: A (wood crib), B (liquid heptane), and F (cooking oil and fat). These ratings describe what an extinguisher can put out; they are not the Class A to D fire classes above.

Extinguishing Media: Chemistry, Applications, and Aircraft Damage

Selecting the correct extinguishing medium is vital not only for fire suppression, but to prevent secondary destruction of sensitive airframes and avionics.

Water and Foam (AFFF)

• Water Extinguishers: Feature immense cooling capability due to water's high latent heat of vaporization ($2.26\text{ MJ/kg}$). Restricted strictly to Class A fires. Highly conductive; never use on live electrical equipment.
• Aqueous Film Forming Foam (AFFF): Discharges a surfactant foam that floats across burning hydrocarbon liquids, creating an airtight film that suffocates the fire and seals flammable vapors. Ideal for large apron fuel spills.

Carbon Dioxide ($CO_2$)

• Discharged as a cold gas and snow, $CO_2$ dilutes the oxygen around the fire below the level needed for combustion, smothering Class B and electrical fires. Because it leaves zero chemical residue, it is safe for delicate aircraft instruments.
• Hazards: $CO_2$ expels from the horn at approximately $-78.5^\circ\text{C} (-109^\circ\text{F})$. Direct skin contact causes severe cryogenic frostbite and cold burns; horns must be gripped only by insulated handles. In confined areas (cockpits, cargo bays), $CO_2$ induces rapid asphyxiation; personnel must evacuate immediately.

Dry Chemical Powder

• Formulated from sodium bicarbonate, potassium bicarbonate (Purple-K), or monoammonium phosphate (ABC powder). Powders provide rapid flame knockdown by chemically coating the fuel and scavenging free radicals.
• Severe Aircraft Hazard: Dry chemical powder is severely corrosive to aircraft aluminum alloys, copper wiring harnesses, and avionics. When exposed to atmospheric moisture, the fine chemical salts form aggressive acids that penetrate airframe lap joints and destroy delicate electrical connections. Furthermore, the fine dust causes severe respiratory distress and impairs visibility. Avoid dry chemical powder inside cabins and avionics bays where a suitable alternative is available, and report any use so the residue can be cleaned and the equipment inspected.

Clean Gaseous Agents (Halon Replacements)

Historically, Halon 1211 (BCF) and Halon 1301 were the universal standard in aviation due to their non-conductivity, zero residue, and powerful chemical inhibition of combustion. Halon production has ended under the Montreal Protocol because of its ozone-depleting effect. Regulation (EC) No 1005/2009 allows Halon in aircraft only as a critical use with cut-off dates, so it still protects many aircraft while replacement agents are introduced. Ground facilities often use clean agents such as fluoroketones (FK-5-1-12) or hydrofluorocarbons (HFC-227ea), which leave no corrosive residue and do not conduct electricity.

Hangar Emergency Response and Deluge Systems

Aircraft hangars are equipped with automated fire protection systems to combat high-energy fuel fires.

High-Expansion Foam Deluge Systems

In the event of a large fuel spill fire, automated overhead foam generators discharge vast volumes of high-expansion foam (expansion ratios of 500:1 to 1,000:1). The foam can cover the hangar floor to a depth of several metres within minutes. While the foam is non-toxic, immersion causes total loss of visibility, severe spatial disorientation, and muffled hearing. Trapped technicians must not run. Personnel must cup their hands over their nose and mouth to create an air pocket and navigate systematically along peripheral hangar walls to marked emergency exit doors.

ARFF Liaison

Flightline personnel discovering a fire must immediately raise the alarm via manual call points, alert air traffic control, and evacuate the danger zone. When Airport Rescue and Firefighting (ARFF) vehicles arrive, maintenance personnel must yield command, providing responders with critical intelligence regarding fuel loads, pressurized oxygen bottles, and hazardous chemical locations.

Workshop First Aid: Electric Shock, Chemical Burns, and Thermal Trauma

Prompt, disciplined first aid response is the difference between survival and death in maintenance accidents.

Electric Shock Response Protocol

1. Isolate Power First: Never touch an electric shock victim who remains in contact with an energized circuit. Doing so makes the rescuer part of the electrical pathway. Immediately trip the circuit breaker, pull the GPU quick-disconnect plug, or use a non-conductive rescue hook (fiberglass pole or dry wooden broom handle) to pry the victim away.
2. Assess ABC (Airway, Breathing, Circulation): 400 Hz electrical shocks frequently induce fatal ventricular fibrillation or respiratory arrest. If the victim is unresponsive and not breathing normally, immediately summon emergency services, deploy an Automated External Defibrillator (AED), and begin continuous Cardiopulmonary Resuscitation (CPR) at a ratio of 30 chest compressions to 2 rescue breaths.
3. Secondary Survey: Check for electrical entry and exit burn wounds, and monitor for delayed cardiac dysrhythmias even if the patient regains consciousness.

Chemical Contamination and Burns (Skydrol, Battery Acids, Strippers)

• Exposure to phosphate-ester hydraulic fluids, sulfuric acid, or potassium hydroxide requires immediate, aggressive decontamination.
• Irrigation Protocol: Instantly transport the victim to an emergency eye-wash station or safety deluge shower. Irrigate affected skin and eyes with copious, low-pressure clean running water for a minimum of 15 to 20 minutes.
• Carefully remove all contaminated clothing, coveralls, and leather boots while flushing.
• CRITICAL: NEVER ATTEMPT CHEMICAL NEUTRALIZATION. Applying an alkaline solution to an acid burn (or vinegar to an alkali burn) induces an intense exothermic chemical reaction that generates extreme heat, inflicting severe secondary thermal burns on the patient. Seek emergency medical care immediately.

Thermal Burns Management

• Cool thermal burns under clean, cold, slow-running water for 10 to 20 minutes to stop heat progression into deeper dermal layers.
• Remove loose clothing and constricting jewelry (rings, watches) before tissue edema (swelling) sets in. Never pull away clothing that has melted and adhered to the burn wound.
• Cover the burn loosely with a clean, sterile, non-adherent dressing or clean medical plastic wrap applied in flat sheets (never wrapped tightly around limbs).
• Never apply butter, grease, ointments, or adhesive bandages, and never burst blisters, as intact skin provides an indispensable sterile barrier against lethal infection.

Comparative Analysis: Fire Classes & Extinguishing Agents

Worked Maintenance Scenario: Hot Brake Fire on the Apron

After a high-energy rejected take-off test, an aircraft taxis to the run-up apron with extremely hot brakes. A hydraulic line near one brake fails and sprays fluid onto the brake, which ignites: a Class B fire at a wheel that also contains magnesium alloy parts.

A junior technician grabs a water extinguisher and runs towards the wheel. The senior engineer stops them:

1. Why not water: Suddenly cooling very hot brakes and wheels can cause thermal shock and wheel failure, and water on burning magnesium would make the fire worse.
2. Approach direction: Hot wheels can burst, and their fusible plugs are designed to release tyre pressure, so the team approaches from the front or rear of the wheel, never in line with the axle.
3. Isolate the fuel: The engineer asks the flight deck to shut down the hydraulic pumps so fluid stops feeding the fire, and raises the alarm.
4. Knock down the fire: The team uses a wheeled dry chemical extinguisher on the hydraulic fluid fire while keeping everyone clear of the sides of the wheel.
5. Hand over to the fire service: When the airport fire service arrives, they take command and manage cooling of the brake and wheel assembly. The engineer briefs them on the fluid, the fuel load, and the wheel hazards.

The lesson: identify the fire class, isolate the fuel or energy source, choose the agent that suits that class, and respect the particular hazards of hot wheels.

Exam Pitfalls / Common Traps

• Trap 1: Using water or $CO_2$ on metal fires and hot wheels. Burning magnesium is a Class D fire and reacts violently with water, and it can keep burning in $CO_2$, so dry powder is used. Rapid cooling of hot brakes and wheels also risks thermal shock and wheel failure; approach from the front or rear, never in line with the axle.
• Trap 2: Using dry chemical powder in cockpits or electronic equipment bays. Dry powder leaves corrosive, hygroscopic residues that can damage avionics, wiring, and aluminium structure, so avoid it where a suitable alternative exists and report any use so the area can be cleaned.
• Trap 3: Attempting to neutralize chemical burns. Technicians often mistakenly believe acid burns should be washed with alkali, or alkali burns with vinegar. Neutralization creates an exothermic chemical reaction that burns the patient. Flush exclusively with copious clean water for 15 to 20 minutes.
• Trap 4: Touching an electric shock victim before de-energizing the circuit. Attempting to pull an electrocution victim away with bare hands electrocutes the rescuer. Always isolate the power source first or use an insulated rescue hook.