aircraft, but a replacement is under development. Halon 1301  cannot be produced anymore because it depletes the ozone  layer. Halon 1301 will be used until a suitable replacement  is developed. Some military aircraft use HCL-125 and the  Federal Aviation Administration (FAA) is testing HCL-125  for use in commercial aircraft. Fire extinguisher containers (HRD bottles) store a liquid  halogenated extinguishing agent and pressurized gas  (typically nitrogen). They are normally manufactured from  stainless steel. Depending upon design considerations,  alternate materials are available, including titanium.  Containers are also available in a wide range of capacities.  They are produced under Department of Transportation  (DOT) specifications or exemptions. Most aircraft containers  are spherical in design, which provides the lightest weight  possible. However, cylindrical shapes are available where  space limitations are a factor. Each container incorporates a  temperature/pressure sensitive safety relief diaphragm that  prevents container pressure from exceeding container test  pressure in the event of exposure to excessive temperatures.  [Figures 17-12  and  17-13] Containers  Discharge Valves Discharge valves are installed on the containers. A cartridge  (squib) and frangible disk-type valve are installed in the  outlet of the discharge valve assembly. Special assemblies  having solenoid-operated or manually-operated seat-type  Figure 17-12.  Built-in non-portable fire extinguisher containers  (HRD bottles) on an airliner. Figure 17-13.  Diagram of fire extinguisher containers (HRD bottles). 17-12 No. 1 Engine Container No. 2 Engine Container Double check tee valve (2) Explosive cartridge Relief valve vent Relief valve Pressure gauge Pressure switch and gauge To number 1 engine To number 2 engine To number 1 engine fire-pull switch To number 2 engine fire-pull switch Second shot to number 1 engine Second shot to number 2 engine To cockpit light