Citation X Auxiliary Power Unit Systems 

APU Fire Detection and Extinguishing System Description

System Description
The fire detection system warns the flight crew if an APU overheat condition is present. The fire extinguishing system allows the flight crew to discharge a fire bottle into the APU enclosure to extinguish a fire.
System Operation
The APU fire detector is a pneumatically-operated device and is routed around the exterior of the APU. If an overheat condition occurs, the pressure within the detector increases and closes a pressure-operated switch. The switch signals the APU Engine Control Box (UT018) and the APU Logic Module (NK102), then the following occurs:
  • The red APU FIRE annunciator illuminates.
  • The APU fire double chime sounds.
  • APU FIRE will be displayed on EICAS.
  • The APU Engine Control Box (UT018) shuts down the APU.

Pushing the red APU FIRE switch/light discharges the squib in the APU fire bottle. The extinguishing agent (Halon) is directed into the APU enclosure.

CAUTION: The fire bottle discharges when the APU FIRE switch is pushed even if the APU FIRE annunciator is extinguished and the APU MASTER switch is in the OFF position.

The fire detection system is tested by pushing the APU TEST switch. The red APU FIRE annunciator illuminates, and the red APU FIRE and FIRE BOTTL LOW APU will be displayed on EICAS during a system test.