On the cover, the situation seems pretty-straightforward, right? Aircraft Rescue Fire Fighters (ARFF) respond to the scene and witness smoke or fire coming from the main landing gear. But, understanding why these types of incidents occur is critically important to have well-educated and seasoned firefighters operating on multimillion dollar aircraft.
Understanding the Why
In the same way as propulsion or lift happens, braking is one of those critical functions that an aircraft must do to transport passengers safely. Thankfully, the braking systems are able to make the aircraft come to a stop after landing, thus allowing a safe taxi to the gate. Or during a rejected takeoff, it can stop the aircraft in the event of an actual emergency.
Whether you are pedaling a bicycle or are riding in a car or on an aircraft, the problem is exactly the same. Kinetic energy of a moving vehicle has to be absorbed first and then transformed into heat in order to allow the dissipation of said energy. For instance, stopping a large commercial aircraft like an Airbus A340 or a Boeing 777 travelling at 186 mph or 300 km/h in just a few thousand feet means absorbing more than one billion joules of kinetic energy in a few short moments of rollout. In either case, the temperature can reach 1,832°F or 1000°C. Although the risk is only one in one million during takeoffs, the most extreme case a firefighter will see regarding wheel and brake emergencies will be that of an aircraft which suffers an unexpected, rejected takeoff. This is because during a rejected takeoff, the other aircraft systems like the reverse engine thrusters, or flap settings, which normally assist the braking system are not in play during an extreme emergency hard stop.
The heating of aircraft wheels and tires presents a potential explosion hazard to the ARFF firefighter, and it is greatly increased when fire is also present. In order to not endanger the members of your department, it is important to distinguish the difference between “hot brakes” and “brake fires.” Hot brakes happen daily throughout the airline industry, and most will normally cool themselves without the intervention of the ARFF Department. Most aircraft operating manuals for propeller-driven aircraft recommend that the flight crew keep the forward propeller of the hot brakes rotating to provide airflow over the affected wheel-brake assembly. Most wheel assemblies of jet aircraft have fusible plugs, which will melt at about 350° F or 177° C and deflate the tire before the increased tire pressure causes the tire to explode.
ARFF Considerations on Arrival
When responding to a hot brake or wheel assembly fire, ARFF personnel should initiate a mandatory size-up of the hazards from a safe distance. The use of fixed-mount or portable handheld thermal imaging cameras can provide the exact location of the seat of the heat or fire or a comparison of other wheel brake assemblies that are not affected and any potential exposure considerations with regards to heat and fire to the wing.
ARFF Considerations Tactical
The ARFF firefighter must be wearing all of his or her PPE and SCBA. You should have the understanding that this type of emergency can be dynamic. Investigating in your station wear is completely inappropriate. You should approach the wheels with extreme caution in a fore or aft direction, NEVER approach from the side (in line with the axles) The main safety concern is a tire explosion. The sidewall and tread of a commercial aircraft tire are the weak points for directional explosion or failure. The second hazard is the fusible plug that is designed to deflate the tire in a safe manner to prevent catastrophic failure of the aircraft rim and/or the tire. The plug, as you can see in the picture, is safety wired, but don’t fall victim to assuming the wire will hold the plug from becoming a bullet. And lastly, should catastrophic failure occur, the flying shrapnel will puncture the wing and/or the wing’s fuel tank causing a three-dimensional running fuel fire.
ARFF Ventilation Tactics
Since the kinetic energy/heat is transferred to the entire wheel assembly, it is critically essential that firefighters cool the hot brakes with natural, negative-, or positive-pressure fans. The initial temperatures of the wheel brake assembly should be acknowledged and reported to the incident commander. Fireground operations should commence within the identified incident safety area depicted in the supplied picture.
ARFF Extinguishing Agents
Should a wheel-brake fire be located on ARFF arrival, then personnel should commence with a rapid and swift attack using the appropriate extinguishing agent. For small wheel and brake fires that have minimal exposure hazards, your first option should be a potassium-based, Purple-K dry chemical extinguisher. This agent of choice provides rapid knock down and extinguishment. But, firefighters should understand that this agent of choice provides no cooling effect in regard to reignition or rekindle. Upon fire extinguishment of this area, combined operations should commence with an added cooling effect from a box fan, positive-pressure fan, or from the hydraulic ventilation features of a fog stream nozzle. Special note: should you choose this hydraulic ventilation tactic, the fog stream should be directed away from any extremely hot components. Cold water application to these hot components can cause the metal to crack or explode similar to concrete spalling. The choice of extinguishment application depends on your ARFF apparatus capabilities and resources. For example, a dual-agent hydrochem handline works excellent for fire attack using the dry chemical agent and has the secondary punch of AFFF for cooling/vapor suppression effect.
For larger area wheel and brake fires, should the fire be impinging on the aircraft’s wing and fuel storage above, the agent of choice should be copious amounts water and AFFF. If you on have an extendable reach turret with the capabilities of hydrochem application, then you could apply the dry chemical agent first and immediately switch to AFFF without ever leaving the cab of the apparatus. The primary application should include direct application to the seat of the fire (wheel brake assembly) while repetitively applying AFFF to any high heat exposure areas of the wing and fuselage. During these types of incidents, a small wheel and brake fire has the potential to destroy and totally consume the entire aircraft should the heat and flames cause catastrophic failure of the wing fuel tank assembly.
During my 26 years of ARFF work experience, I have witnessed the overly aggressive use of water to rapidly cool a hot brake/wheel assembly, especially if it is isolated and localized. This type of behavior may cause explosive failure of the wheel assembly. Solid streams of water should not be used except as a last resort. Water fog or mist can be used but intermittent application of short bursts (5 to 10 seconds) every 30 seconds is highly recommended. Once the aircraft tires have been deflated by either the fusible plug or tire explosion, there should be no further danger of an explosion hazard with the wheel assembly.
IFSTA Aircraft Rescue and Fire Fighting manual and the FAA Aircraft Rescue and Firefighter Training CBT discusses that hot brakes should be allowed to cool naturally. If housekeeping wise, wheel-bearing grease, hydraulic fluid and/or the tires themselves catch fire, then the use of AFFF to cool while extinguishing fire is highly recommended.
Hot brakes or wheel assembly fires on the cover can seem like a minor emergency to the less than experienced Aircraft Rescue Fire Firefighter. It is our job as ARFF Fire Chief’s, Training Officers and professional educators to maintain a high-level of operational readiness. This should include wearing all of our assigned PPE and SCBA on these types of responses and incidents. Having the (KSA’s) knowledge, skills and abilities to handle these types of incidents is paramount for firefighter safety and ultimately to the safety of the passengers on board that aircraft.
Until next month Brothers and Sisters. Train Hard. Chief Billy
WILLIAM GREENWOOD is a 26-year veteran of the fire service. He is currently the Assistant Fire Chief of Training at the Manchester-Boston Regional Airport. He is a Senior Staff Instructor for the New Hampshire Fire Academy and owns Fire Emergency Training Consultation Services, FETC provides advanced firefighter and leadership training throughout the United States. He is also a national speaker for FDIC International and has been published in Fire Engineering and Fire Rescue.