Engine fire detector (lower FWD) Engine fire detector (upper) Engine section view (looking forward) Engine section side view Engine fire detector (lower) Thermal relief outlet No. 1 engine APU No. 2 engine Directional control valve Forward bottle AFT bottle Figure 9-7.  Large turbofan engine fire zones. Figure 9-8.  Typical fire extinguishing system. In addition to the engine and nacelle area zones, other areas  on multiengine aircraft are provided with fire detection  and protection systems. These areas include baggage  compartments, lavatories, APU, combustion heater  installations, and other hazardous areas. Discussion of fire  protection for these areas is not included in this section, which  is limited to engine fire protection. Commuter aircraft certificated under 14 CFR part 23 are  required to have, at a minimum, a one-shot fire extinguishing  system. All transport category aircraft certificated under 14  CFR part 25 are required to have two discharges, each of  which produces adequate agent concentration. An individual  one-shot system may be used for APUs, fuel burning heaters,  and other combustion equipment. For each “other” designated  fire zone, two discharges (two-shot system) must be provided,  each of which produces adequate agent concentration.  [Figure 9-8] The fixed fire extinguisher systems used in most engine fire  protection systems are designed to dilute the atmosphere with  an inert agent that does not support combustion. Many systems  use perforated tubing or discharge nozzles to distribute the  extinguishing agent. High rate of discharge (HRD) systems  use open-end tubes to deliver a quantity of extinguishing  agent in 1 to 2 seconds. The most common extinguishing  agent still used today is Halon 1301 because of its effective  firefighting capability and relatively low toxicity (U.L.  9-8 Engine Fire Extinguishing System Fire Extinguishing Agents