BY FRANCOIS JACQUET
Aircraft firefighters sel-dom face major crashes. Instead, they usually respond to a myriad of incidents such as engine fires, cargo fires, unsafe landing gear, hot brakes, and auxiliary power unit (APU) fires. This article focuses on the dangers firefighters face when confronted with a Boeing 727™ APU fire.
Modern commercial aircraft are equipped with an APU, a turbine engine usually located in the tail section of the plane, or, in some planes like the Boeing 727, in a central position between the main wheel wells (photo 1).
The APU acts as a generator, providing power to the aircraft electrical system while the plane is on the ground. It also supplies compressed air through ducts called bleed lines to the main engines for startup or the air-conditioning system. Like any turbine engine, an APU is equipped with an air intake; a fuel distribution line; a combustion chamber; exhaust ducting; and, as mentioned, bleed lines.
In the Boeing 727, the APU air intake and the main unit are located in the left main landing gear wheel well. The exhaust gas duct exits through the right main wheel well and comes out on top of the right wing (photo 2).
 (1) Auxiliary power unit in left wheel well. (Photos by author.) |
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 (2) The APU exhaust port can be seen on top of the right wing. |
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The location of the exhaust port, visible from the main passenger cabin, creates unwarranted passenger-initiated evacuation when the APU experiences torching on startup. On October 14, 1993, the National Transportation Safety Board issued a letter of recommendation indicating the following: “The highest percentage of unwarranted passenger-initiated evacuations have occurred on 727 airplanes. The Safety Board believes that these frequent occurrences are linked to the location of the 727 APU exhaust outlet, which is clearly visible in the right over-wing area.”
The APU mounted on the 727 is equipped with a fire extinguishing system capable of discharging halon into the unit. That system can be activated from two locations: in the cockpit on the flight engineer’s panel and remotely in the left main wheel well in between the main landing gear strut and the wing root.
FIREFIGHTING
When an APU catches on fire, the same tactical priorities used in structure fires apply to the aircraft: life safety, exposure protection, fire control, and overhaul. As with any building protected by a fixed fire protection system, the aircraft fire protection system should be used first; manual firefighting should be the last resort. The incident commander should confirm whether the flight crew had activated the APU fire extinguishing system or firefighters had activated the fire suppression system from the outside. When the APU fire extinguishing system has been activated, the fuel valve feeding the APU from tank #2 (central), the air intake, and bleed lines will close.
Apparatus roof-mounted turrets are ineffective in extinguishing 727 APU fires because of the recessed location. One and three-quarter-inch hoselines backed by a dual-agent (dry chemical and AFFF) line should be used. Dry chemical hoselines should be available to extinguish quickly any three-dimensional fires originating from the wheel well. Firefighters should use the reach of their hoseline stream to knock down any fire from a safe distance and avoid working underneath the aircraft.
In the overhaul phase of an APU fire, access to the APU compartment is essential. Access to the APU is obstructed on both sides of the plane by the two wheel well doors. In the “normal” configuration, these doors are retracted (photo 3). To unlock and bring them down (photo 4), there are two handles (one for each door on each side) located in between the landing gear strut and the wing root (photo 5).
Firefighters entering the wheel well compartment when the doors are in the “down” position should ensure that the door release handles are in the “locked” position. Failure to do that will result in the sudden and unexpected closing of the doors, crushing anyone standing in the well if the flight crew or would-be rescuers are reactivating the hydraulic system.
 (3) Closed wheel well doors (normal landing configuration). |
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 (4) Wheel well doors are in the extended position (sign of hydraulic problems). |
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 (5) Wheel well door release handle. |
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 (6) This shrouded fuel line (curved and square shaped) is another danger that confronts firefighters. |
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Another danger that ARFF firefighters will confront in the wheel well of the 727 is a shrouded pressurized fuel line (photo 6). During an APU fire, this fuel line should be protected at all times. Immediately above the APU lies the passenger floor deck, and fire penetration in the main cabin is very likely if the fire is not promptly controlled.
The plane’s main batteries energize the APU electrical system. Disconnecting the batteries will cut off power to the unit but also to the fire extinguishing system. Thus, disconnect the batteries only after having carefully evaluated the fire situation.
The airplane’s fixed fire extinguishing system controls the vast majority of APU fires. On those occasions when it does not, firefighters should be prepared to act quickly, but any firefighting activity taking place in the wheel wells should be accomplished safely. Before entering the wheel well, firefighters should ensure that the plane is immobilized. The IC should confirm with the flight crew that the aircraft parking brakes have been set. Firefighters should also chock the main landing gear wheels. Wear full protective gear, and do not forget hearing protectors. Jet engines are really loud!
If possible, the IC should communicate with airline aircraft mechanics from the onset of the incident. Their technical expertise and in-depth knowledge of the aircraft systems will greatly increase the efficiency of firefighting operations. The fire officer in charge should be cautioned. Aircraft mech-anics specialize in very specific fields of activity (power plants, avionics, airframe, and so on), and it would be wise to ascertain if the problem on hand is within their field of expertise by simply asking them.
Firefighters should be warned that working in, around, and especially underneath an aircraft during an emergency is extremely dangerous. Especially avoid walking or working underneath the plane near the nose gear, as it has a tendency to retract unexpectedly unlike the rear main gear.
If there is an airport in your jurisdiction, take the time to learn more about the airplanes flying above you. One day you might have to deal with one up close and personal.
FRANCOIS JACQUET has been a member of the Ottawa McDonald-Cartier International Airport Fire Department (Ontario) for eight years and has been a captain for the past six years. He began his career 14 years ago with the Yellowknife Fire Department (Northwest Territories). He has a bachelor’s and a master’s degree in economics from Laval University.
