Because of the importance of engines to safe flight, it is critical that they incorporate extensive and reliable fire‑protection systems. 14 aero quarterly qtr_04 | 10 Fire Protection: engines and auxiliary Power units the engines and auxiliary power units (aPus) on Boeing airplanes incorporate extensive measures for fire protection, including fire detection and extinguishing systems. By Sham Hariram, technical Fellow, Boeing Propulsion engineering; Paul Philipp, Senior Propulsion engineer, Boeing Propulsion engineering; and Dave Dummeyer, associate technical Fellow, Boeing Propulsion engineering This article is the first in a series exploring to effect this separation, isolation, and porate extensive and reliable fire‑protection the implementation of fire protection on control, Boeing uses both passive and systems. the aPu utilizes similar systems. transport category airplanes. active systems. Passive systems include the this article describes how Boeing provides use of noncombustible or self‑extinguishing fire protection for the engine pods, also Fire protection is given one of the highest materials; separation by routing, compart‑ termed “engines,” and for aPus. an engine considerations at Boeing in airplane design, men tation, isolation, ventilation, and pod consists of the engine, the inlet, the testing, and certification. drainage; and bonding and grounding. nacelle, the thrust reverser, the exhaust in designing an airplane’s fire protection active systems include fire and overheat section, and the strut or pylon. systems, Boeing uses the principles of detection systems, fire‑extinguishing separation, isolation, and control. these systems, temperature sensing, air and fuel principles involve separating the three PaSSive fire-Protection SyStemS shut‑off means, and automatic shutdown essentials for creating a fire (i.e., fuel, of nonflight critical systems. Fire protection ignition source, and oxygen), isolating engine zones. the engines and strut or systems on Boeing airplanes meet all potential fires from spreading to other parts pylon structures on Boeing airplanes form aviation regulatory requirements as well of the airplane, and controlling a fire should compartments, each of which is isolated as internal Boeing design requirements. one occur. by basic structure and ancillary surfaces. Because of the importance of engines each engine nacelle or strut compartment to safe flight, it is critical that they incor‑ 15 WWW.Boeing.com/commercial/aeromagazine figure 1: compartmentation in a typical engine pod engine pods incorporate zones designed to minimize the probability of a fire and to isolate a fire, should one occur. Side view cross Section Fan compartment Fuel and Hydraulic and thrust reverser System Services aft Fairing transcowl cavity upper Strut cavities compartment 45° 45° Strut aft Strut Drain engine core cowl core compartment Dry Bays engine core Fan Duct thrust See cross‑Section lower Strut Surface and upper reverser View at right 90º min. of engine core cowl transcowl lower Bifurcation cavity main engine Drain core compartment including engine through lower Power and accessory Sections Bifurcation Flammable Fluid leakage zone Fire zone Dry Bay zone Firewall figure 2: a typical aPU compartment the aPu compartment firewall isolates the aPu from the rest of the airplane. Strut‑mounted Fire Detector inlet Door element exhaust Bulkhead aPu compartment Firewall Firewall‑mounted exhaust Fire Detector element Flow Fire Bottle exhaust Drain combustor and Door‑mounted Fire oil cooler Drain Detector element exhaust Flow Fire Bottle and Fire Detector elements aPu Firewall 16 aero quarterly qtr_04 | 10 figure 3: a typical strut or pylon is designated as a zone, such as fire zone, Strut structures incorporate numerous firewalls of various materials to flammable fluid leakage zone, or dry bay isolate strut and wing areas adjacent to engine fire zones. zone, according to the potential for the presence of flammable fluids and ignition sources (see fig. 1). only the compartments that contain thumbnail Fairing Fwd Fairing ignition sources and the potential for flammable fluid leakage are classified as fire zones. examples are the engine case around the compressor, combustor, and turbine sections of the engine. the gearbox underwing Fairing Fan cowl Support Beam and its accessories are also considered potential ignition sources during failure Fwd lower Vapor Barrier Skirt Fairing conditions that could cause temperatures to exceed the auto ignition temperatures of Side Fairing fluids that may be present in the compart‑ Fire Seal ment. the areas adjacent to the engine fire zone — such as the engine fan com‑ Heat Shield partment, strut or pylon, and strut heat shield — are isolated by firewalls. other compartments are isolated by bulkheads and vapor barriers. Boeing mitigates fire hazards in engine and nacelle compartments by: titanium aluminum composite nickel alloy ■■ minimizing the potential for ignition. ■■ using compartmentation and isolation. ■■ routing flammable fluid‑carrying lines figure 4: typical engine fire and overheat detector locations away from electrical wires and hot nacelle compartments contain multiple fire or overheat detector elements. pneumatic ducts. ■■ using nonflammable construction Fire Detector lower materials. Spar of Strut ■■ utilizing firewalls. ■■ Providing a means for detecting and extinguishing fires in fire zones. ■■ Providing a shut‑off means for flammable fluids into and out of the fire zone. ■■ minimizing the accumulation of Fan case overheat flammable fluids and vapors through the temperature Sensor use of drainage and ventilation. aPu compartment. the aPu compartment overheat Detector upper Forward is by definition a fire zone (see fig. 2). it is isolated from the rest of the airplane by a firewall. the aPu installation uses the same Fire Detectors fire hazard mitigation principles as the core aft annular engines listed above, plus: ■■ automatic shutdown of the aPu. ■■ automatic shutoff of air source. Flammable fluid drainage. the engine nacelle and aPu installations are designed to drain Fire Detector flammable fluids overboard. these drainage lower Bifurcation provisions include drain holes, hoses, and tubing for capturing and safely discharging Fire and overheat Detectors engine Fan engine core flammable fluid leakage overboard. 17 WWW.Boeing.com/commercial/aeromagazine figure 5: engine fire-extinguishing Wing Front system Spar on this airplane model, two fire‑extinguishing‑agent bottles and associated tubing are installed in the leading edge of each side of the wing. Fire outboard engine extinguishing extinguishing Bottles tubing inboard engine Strut engine Fire or Pylon extinguishing tubing Ventilation. in the engine pod, ventilation fluid, vapor, or the propagation of fire to active fire-Protection SyStemS air is provided in fire and flammable fluid adjacent zones. leakage zones to minimize the accumulation most aPu installations are located in the engine fire detection. the typical engine of flammable vapor. all ventilation flow is airplane tailcone outside of the pressure fire‑detection system includes both fire designed to exit safely without being vessel or passenger compartment. this and overheat detectors (see fig. 4). each reingested. in the engine core fire zone, provides a high level of isolation. the aPu detector location has two heat‑sensing controlled cooling flow provides a source compartment is further isolated from the elements along with associated support of core ventilation. Some installations forward sections of the airplane by a tubes, brackets, and electrical connectors. have dedicated ventilation inlets. the aPu fireproof compartment firewall. other aPu Sufficient area coverage is required to compart ments are ventilated by either a installations not located in the airplane ensure prompt detection of a fire within the mechanically driven fan or a passive tailcone are in self‑contained compartments fire zone. the detector elements of the fire‑ eduction cooling system. any flammable with firewall structure surrounding them on or overheat‑detection system are configured vapors are forced out through vent openings all exposed sides within the airplane. to form two redundant loops, with each or through the eduction exhaust system. detector loop monitored by a separate Bonding and grounding. all Boeing the airflow driven by either of these two control card or a controller. Signals from airplanes incorporate electrical bonding or systems prevents the accumulation of the detectors are processed through an grounding provisions for electrical system flammable vapors and provides cooling automatic fire‑and‑overheat‑logic‑and‑test components and structure to protect for hot surfaces. system to generate flight‑deck displays and against static electricity, sparking, or the aural warnings to alert the crew in the event Firewalls. the strut and nacelle areas arcing that occurs between surfaces of an engine fire. alerts are displayed in the adjacent to the engine fire zones are resulting from electrical system faults or form of lights (i.e., a red maSter Warn ing isolated by a firewall extending from the lightning strikes. the bonding or grounding for fire and an amber caution for over‑ engine fan frame to the exhaust nozzle (see prevents conditions that could ignite heat), together with the simultaneous fig. 3). For engines with fan compartment flammable vapors. illumination of the associated engine‑fire mounted gearbox and accessories, the explosion proofing. electrical