This month let’s take a look at auxiliary power unit (APU) fires. The APU itself provides power to start the main engines. Heavy turbine compressors must be turned to a significant speed to start the engine. Before these engines are to be turned though, the APU must be started, generally done by a battery or hydraulic accumulator. Once the APU is up and running, it can then provide power, be it electric, pneumatic, or hydraulic, to start the aircraft’s main engines.
For the main engine to start, this rotation of the turbine is needed as well as AC electrical fuel pumps, and an electrical ignition source is needed to ignite the fuel. As the turbine is rotating, the front inlet fans are also rotating. After ignition, both fans and the turbine speed up their rotation. As combustion stabilizes, the engine only needs enough fuel to run at idle. The now running engine can replace the APU if needed to start up more engines.
APUs are also used to run accessories while the engines are shut down. This allows the cabin to remain comfortable while the passengers are boarding before the aircraft’s engines are started. Electrical power is used to run systems for preflight checks. Some APUs are also connected to a hydraulic pump, allowing crews to operate hydraulic equipment (such as flight controls or flaps) prior to engine start. This function can also be used, on some aircraft, as a backup in flight in case of engine or hydraulic failure.
Location of the APU
On most larger aircraft, the APU is installed in the rear of the airplane. It is up in the tail cone just outside of the passenger compartment. This design feature and location provides a high level of isolation from the passengers. The APU compartment is further isolated from the forward sections of the airplane by a fireproof compartment firewall. Now in some smaller regional jets, the APU installation might not be in the tail cone. These are also located in a self-contained compartment with firewall structure surrounding them on all exposed sides of the airplane.
Activating Fire-Protection Systems
The typical engine fire-detection system includes both fire and overheat detectors. Each detector location has two heat-sensing elements, as discussed in our previous cargo fire article, along with associated support tubes, brackets, and electrical connectors. Sufficient coverage area is required to ensure prompt detection of a fire within a fire zone. The detector elements of the fire or overheat detection system are configured to form two redundant loops, with each detector loop monitored by a separate control card or a controller. Signals from the detectors are processed through an automatic fire, overheat, and test system to generate flight-deck displays and audible warnings to alert the flight crew in the event of an engine fire. Alerts are displayed in the form of lights, a red MASTER WARNING for fire and an amber CAUTION for overheat in a Boeing Aircraft, together with the simultaneous illumination of the associated engine fire handle and fuel shutoff switch for an engine fire.
Remote Firefighter Access Controls
The Boeing Manufacturing Company have provided ARFF personnel with an easy access panel located on the front landing gear strut. This panel, depicted in our picture below taken at a recent firefighter training session, shows the panel is located on the (right) or co-pilot’s side of the strut.
On the top of the panel, there is a red lens that, if illuminated, shows if there is a fire in the APU. The control panel on P62, has a red and black push activated button. The red button will shut down the APU, and the black button will discharge the onboard halon extinguishing agent directly into the APU. With this remote panel, firefighter action allows a big margin of safety for the firefighters on the ground.
Firefighter Incident Action Plan
If the flight crew hasn’t already activated the onboard controls to extinguish an APU fire, then firefighters should access the control panel as described previously. The steps for fireground operation are more of a local “authority having jurisdiction” protocol, but I will describe what I expect to occur on my incident.
We must make sure the incident aircraft does not move or rollaway once we start our firefighting tactics. Sounds silly but the slightest grade can allow an aircraft to roll if not properly parked. The force of firefighter streams can push an unstable aircraft. Therefore, we should chock the wheels of the incident aircraft to assure it will not move. This can be achieved by using our ARFF apparatus wheel chocks or wood cribbing from our rescue equipment cache. A firefighter or the fire officer should then complete a thorough size-up using a thermal imaging camera. Locating the seat of the fire and/or locating a high heat source is critical. If it is determined that the APU is still burning after flight crew activation of suppression agent or after the use of the firefighter remote access control panel, the firefighters must gain access to the cockpit to confirm APU shut down while simultaneously locating the APU intake on the exterior of the aircraft. Shutting down the APU will often remove most of our problems with an APU fire. Locating the APU intake from the exterior, the port that provides the air required to support fire combustion, also provides a great access point for firefighting extinguishing agent.
The agent of FIRST choice should be a clean agent, halon, or environmentally friendly halon alternatives. These agents reduce further damage to the aircraft engine. But if the fire is starting to breach the APU nacelle or the fireproof containment box, then the agent of choice should be whatever you have in the greatest quantity to provide a rapid loss stop. In my response area, this would be water mixed with Mil Spec 3% AFFF foam applied directly into the intake port. This can be achieved from the ground with a handline, from a roof turret, or the preferred choice: articulated boom. Opening the rear service doors closest to the APU will provide good, all natural cross-ventilation. Seeing that the passengers will be coming out of the main 1L cabin door, positive pressure ventilation (PPV) front to back is not an option. Applying your PPV fan to the rear service door area with the other door directly across open will create a good negative pressure or vacuum effect of the main cabin.
Cabin evacuation should be done but in a very controlled manner. Communication is the key. If attached to a jet bridge, then the incident commander should request that the flight crew deplane the passengers through their normal procedures with an emphasis that they will need to leave quick and efficiently as the fire department is checking on something out back. Remember that the APU is located within a fireproof compartment. Any odor and smoke in the cabin is often more of a nuisance and from our extinguishing efforts. Once the thermal imagining camera determines that the APU fire has been knocked down, gaining access to the APU compartment must be achieved to assure that no further hazards exist. Once overhaul has been completed, then the incident can be placed under control. Remember that your FBO or Airline Maintenance Facility can be a great source of knowledge, education, and experience. These maintenance professionals work on these aircraft daily and will most likely be the ones who are going to repair this aircraft. I will often request the shop foreman or supervisor to the scene to assist the incident commander with on-site reference.
Remember these types of incidents often occur while sitting at the gate. Passengers are not in the deplaning mode. They have just boarded the aircraft and are getting settled into their seats. They are absolutely not thinking about getting off the aircraft immediately. Communication between ARFF personnel, the flight crew, and the passengers are essential to have a positive flying experience.
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.