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If an engine has a fire, the fire warning light comes on and the solenoid energizes to release the switch. When the solenoid is energized, the fire switch can be pulled. 

When the fire detection system malfunctions or the solenoid is defective, and the flight crew wants to extinguish an engine fire, someone must push the fire override switch. The fire override switch allows the fire switch to be pulled when the solenoid is not energized. When the fire switch is pulled, the push-pull switch contacts operate electrical circuits that stop the engine and isolate it from the airplane systems. With the switch pulled, it can be rotated to left or right to a mechanical stop at the discharge position. The rotary switch contacts close and operate the fire extinguishing system. 

When the fire switch is pulled, the switch isolates the following aircraft systems from the engine: 

• Closes the fuel spar valve 

• Deenergizes the engine fuel metering unit (FMU) cutoff solenoid 

• Closes the engine hydraulic pump shutoff valve 

• Depressurizes the engine driven hydraulic pump valve 

• Closes the pressure regulator and shutoff valve 

• Removes power from thrust reverser isolation valve 

• Trips the generator field 

• Trips the backup generator field [Figure 9-27] 

## Engine Fire Operation 

If an engine has a fire, the engine fire detection system gives a fire warning in the flight deck. The engine fire warning lights come on to identify which fire switch to use to extinguish the fire. The solenoid in the fire switch energizes and releases the switch so that the fire switch can be pulled. If the solenoid does not energize, push the fire override switch to release the fire switch manually. When the fire switch is pulled, it stops the engine, and the fire switch isolates the engine from the airplane systems. 

If the fire warnings do not go away when the switch is pulled, position the switch to the DISCH 1 or DISCH 2 position, and hold the switch against the stop for one second. This fires the squib in the fire extinguisher container and releases the fire extinguishing agent into the engine nacelle. Ensure that the engine bottle discharge light comes on. If the first bottle does not extinguish the fire, the switch must be placed to the other DISCH position. This fires the squib for the other bottle. 

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## Figure 9-24. Location of fire extinguishing bottles. 

