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. otheraPu 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 ambercaution 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 components handle and fuel‑shutoff switch for an engine entire fan cowl and thrust reverser inner installed in the fire or flammable fluid leak‑ fire. on airplanes with engine‑indication‑ cowling are designed to be fireproof for age zones are required to be explosion and‑crew‑alerting‑system (eicaS) capability, in‑flight conditions. the upper quadrant proof. in the event that flammable fluid or eicaS messages are shown on the inte‑ of the fire zone compartment (typically vapor seeps into a cavity of the component, grated flight‑deck displays. an eicaS +/‑ 45 degrees of the engine core compart‑ it is required to contain the fire should the message also appears in the event of a ment) will prevent burn‑through to the flammable fluid ignite. the components are detector system failure. adjoining strut and wing structures for the also required not to overheat and become ground condition. in addition, fireproof aPu fire detection. the typical aPu fire‑ an ignition source. each component type is feedthroughs, boots, seals, and sealants detection system consists of two or three tested in a laboratory environment to dem‑ are used to seal penetrations and gaps on detectors connected in series by airplane on strate that it meets these requirements. firewalls and barriers to prevent flammable wiring, each with a redundant detector

18 aero quarterly qtr_04 | 10 figure 6: fire-extinguishing control controls for all airplane fire‑extinguishing systems are located on the flight deck. this control module for the 747 shows controls for all four engines, the aPu, and the cargo hold. the respective fire handle illuminates red if an engine fire condition is detected. Pulling the fire handle arms the fire‑ extinguishing system and shuts down the engine. it also shuts down the fuel furnished to that engine, the pneumatic system, the hydraulic system, and the electrical system associated with that engine. rotating the fire handle discharges extinguishing agent into the engine.

element (referred to as a detector loop) with on some airplane models, the bottles teSting of fire P rotection both ends connected to the control card. are installed in the left and right wing lead‑ SyStemS anD comPonentS the aPu detection system uses the same ing edge (see fig. 5). on other models, they type of detector elements, circuitry, opera‑ are mounted in the fuselage or in the strut Boeing conducts airplane ground and tion, testing methods, and certification or pylon. each engine fire zone is required flight testing of various aspects of the procedures as the engine fire‑detection to be protected by two indepen dent engine and aPu fire‑protection systems. system. the fire detection system will extinguishing‑agent bottles, each capable testing includes: automatically shut down the aPu when a of extinguishing a fire within the zone. ■■ Fire detection system. fire is detected. the fire detector assem‑ aPu fire extinguishing. the aPu fire‑ ■■ Fire‑extinguishing‑agent concentration. blies are contained entirely within the aPu extinguishing system located in the aft ■■ Fluid drainage, both on ground and compartment in locations selected to fuselage is a single‑shot system provided to in‑flight. provide maximum coverage of the com‑ extinguish a fire in theaPu compartment. ■■ temperature margin verification for part ment where fire could occur and to the system consists of the extinguishing‑ fire detectors. ensure prompt fire detection. agent bottle, distribution tubing, discharge ■■ laboratory qualification of all system aPu fire‑detection displays and controls nozzle, and necessary flight‑deck controls components. are located on the flight deck. When a fire and displays. the fire‑extinguishing‑agent ■■ laboratory fire tests for firewalls, is detected, the aPu fire handle illuminates, bottle is located on the forward side of feedthroughs, fire seals, installation the maSter Warning lights illuminate, the aPu firewall. the single discharge line components, and flammable fluid warning and status messages are displayed directs agent to a nozzle located in the carrying components, in simulated on eicaS, and the aural warning is acti‑ aPu compartment. installed conditions. vated. an external aPu fire horn that is the aPu‑extinguishing‑agent bottle resettable from the flight deck (or from the design is similar to that of the engine. aPu fire/shutdown panel near the wheel SUmmary However, the aPu bottle has a single well or away from the aPu) is also provided. discharge outlet rather than two, and the Boeing places the highest importance engine fire extinguishing. engine fire‑ service pressure, amount of agent, and on designing and certifying fire protection extinguishing systems consist of safety relief valve opening pressures differ. systems on an airplane. the engines extinguishing‑agent high‑pressure bottles, Some aPu installations have options for and aPus incorporate extensive fire distribution tubing, nozzles, and flight‑deck a second aPu bottle, for automatic fire protection, including detection and extin‑ controls and displays. two fire‑extinguishing‑ extinguisher discharge, and for the use of guishing systems. agent bottles containing Halon 1301 and an engine bottle in place of the standard For more information, please contact Sham interconnecting tubing are installed in a aPu bottle. the fire‑extinguishing systems Hariram at [email protected]. location where they can serve two engines, for both the engines and aPu are controlled although there are airplane models that from the flight deck (see fig. 6). have two independent bottles serving each engine.

19 WWW.Boeing.com/commercial/aeromagazine