APU Fire Protection

APU fire protection consists of these systems:

  • APU fire detection powered by the battery bus.
  • APU fire extinguishing powered by the hot battery bus.

APU Fire Detection

A single fire detection loop is installed on the APU. As the temperature of the detector
increases to a predetermined limit, the detector senses a fire condition. The APU fire
warning switch remains illuminated until the temperature of the detector has decreased
below the onset  temperature. The system contains a fault monitoring  circuit. If the loop
fails, the APU DET INOP light  illuminates indicating the APU fire detection system is
inoperative.

The indications of an APU fire are:
The wheel well APU fire warning horn sounds, (on the
ground only), and the wheel well APU fire warning light
flashes.
Wheel Well APU Fire Control
  • The fire warning bell sounds.
  • Both master FIRE WARN lights illuminate.
  • The APU fire warning switch illuminates.
  • The APU automatically shuts down.