737 MAX Flight Crew Operations Manual
Fire Protection -
System Description
8.20.4
MN-FLT-OH-201
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 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 fire warning bell sounds
• both master FIRE WARN lights illuminate
• the APU fire switch illuminates
• the APU automatically shuts down
• the wheel well APU fire warning horn sounds, (on the ground only), and
the wheel well APU fire warning light flashes.
APU Fire Extinguishing
The APU fire extinguisher system consists of one APU fire extinguisher bottle, an
APU fire switch, an APU BOTTLE DISCHARGED light, and an EXT TEST
switch. The APU ground control panel located in the right main wheel well also
contains an APU fire warning light, an APU BOTTLE DISCHARGE switch, an
APU fire control handle and APU HORN CUTOUT switch.
The APU fire switch is normally locked down to prevent inadvertent shutdown of
the APU. Illumination of the APU fire switch unlocks the switch. The switch may
also be unlocked manually.
Pulling the APU Fire switch up:
• provides backup for the automatic shutdown feature
• deactivates the fuel solenoid and closes the APU fuel shutoff valve
• closes the APU bleed air valve
• closes the APU air inlet door
• trips the APU generator control relay and breaker
• allows the APU fire switch to be rotated for discharge
• arms the APU fire extinguisher bottle squib.
March 1, 2021
