Figure 9-10.  Discharge valve (left) and cartridge (squib) (right). Figure 9-11.  Discharge indicators.  These valves are required in a two-shot system to prevent  the agent in a reserve container from backing up into the  previous emptied main container. Valves are supplied with  either MS-33514 or MS-33656 fitting configurations. Discharge indicators provide immediate visual evidence  of container discharge on fire extinguishing systems. Two  kinds of indicators can be furnished: thermal and discharge.  Both types are designed for aircraft and skin mounting.  [Figure 9-11] The thermal discharge indicator is connected to the fire  container relief fitting and ejects a red disk to show when  container contents have dumped overboard due to excessive  heat. The agent discharges through the opening created when  the disk blows out. This gives the flight and maintenance  crews an indication that the fire extinguisher container needs  to be replaced before the next flight. If the flight crew activates the fire extinguisher system, a  yellow disk is ejected from the skin of the aircraft fuselage.  This is an indication for the maintenance crew that the fire  extinguishing system was activated by the flight crew, and  that the fire extinguishing container needs to be replaced  before the next flight. Fire switches are typically installed on the center overhead  panel or center console in the flight deck.  [Figure 9-12] When the fire switch is activated, the following happens: the  engine stops because the fuel control shuts off, the engine is  isolated from the aircraft systems, and the fire extinguishing  system is activated. Some aircraft use fire switches that need  to be pulled and turned to activate the system, while others  use a push-type switch with a guard. To prevent accidental  activation of the fire switch, a lock is installed that releases the  fire switch only when a fire has been detected. This lock can  be manually released by the flight crew if the fire detection  system malfunctions.  [Figure 9-13] Visible and audible warning systems are installed in the cockpit  to alert the flight crew. A horn sounds and one or several warning  lights illuminate to alert the flight crew that an engine fire has been  detected. These indications stop when the fire is extinguished. Fire detector sensing elements are located in many high- activity areas around aircraft engines. Their location, together  with their small size, increases the chance of damage to  the sensing elements during maintenance. An inspection  Fire Detection System Maintenance  Warning Systems Thermal Discharge Indicator (Red Disk) Fire Switch Yellow Disk Discharge Indicator Discharge Indicators 9-10