OPERATIONS MANUAL

Fire Protection
Fire Warnings:
When a fire is detected, the fire bell on the flight deck rings every five
seconds. The first ring is louder than the subsequent rings. In case
of an APU fire, the APU fire warning horn sounds also; the horn is
installed in the right body gear wheel well. Both the bell and the horn
can be silenced by extinguishing the APU fire, or by pulling the APU
fire switch. In any case of a fire—in the APU, engine, cargo, or wheel
well—, pushing a master warning reset switch silences the flight deck
fire bell and turns off the master warning lights. Fire warnings are
entirely reset when the respective fire is extinguished.
Fire warnings are inhibited during part of the takeoff: the inhibit starts
at V1 if the FMC provides a valid V1 speed, else when the aircraft pitch
rises through 5°. The inhibit ends after 25 seconds or when climbing
through 400 ft.
Squibs and Fire Extinguisher Bottles:
Squibs are small, electrically triggered explosive devices mounted on the
sealed outlets of the fire extinguisher bottles. When a squib detonates,
it tears the outlet open, and pressurized halon gas in the bottle will
discharge through a duct system into the area where the fire is located.
The main hot battery bus provides the trigger power; the power may be
routed to a squib using the respective fire switch. Using a test switch on
the maintenance panel, the power can be routed to the squib through
a test lamp; the lamp acts as a resistor and reduces the current so that
the squib will not explode during the test. Lights that do not illuminate
during the test will indicate the respective power line to the squib is
broken, or not powered, or the squib has been fired.
Each test switch refers to the squibs on the A or B engine bottles and the
APU bottle,—or to cargo bottle squibs,—or to both. For example, on a
non-ER passenger aircraft, test switch 1 refers to all A bottle squibs and
all cargo bottle forward squibs; and test switch 2 refers to all B bottle
squibs and all cargo bottle aft squibs. Other aircraft include two more
test switches that refer to the cargo bottle squibs only.
The test light itself incorporates another switch; when that switch is
pushed and the light does not illuminate, the test lamp is broken, or
power is removed. That is a plain lamp test and cannot be influenced by
a failed squib.
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