representative of systems found in modern Navy
de-energizing the warning circuit and turning out
the L FIRE warning indicator lights. Normal
aircraft. The system is of the continuous-element,
temperature conditions energize relay coil K2-A.
resetting type. The sensing element consists of two
When the temperature rises, the sensing
conductors separated by a semiconductor. The
element resistance decreases, shunting the current
outer conductor is at ground potential, and the
from resistor R2 through diode CR1, turning
center conductor connects to an amplifier input
transistor Q1 off. This switches transistor Q2 on,
in the fire detector control unit. The semi-
transistor Q3 off, and transistor Q4 on. With
conductor portion of the element has an inverse
transistor Q4 on, relay coil K2-B energizes, and
temperature coefficient; as the temperature
relay coil K2-A de-energizes. This transfers
increases, the resistance of the sensing element
(switches) contacts of relay K2, energizing the
decreases.
warning circuit. Then, 28 volts of dc powers the
The fire detector control unit continuously
warning circuit through pin K of the detector,
monitors the electrical resistance of the fire
turning on the L FIRE warning indicator lights.
detector system's sensing element. The control
If the temperature drops, the warning circuit
unit activates a fire warning light (in some units
de-energizes, causing the fire warning indicator
an audible warning is also given) when one of the
lights to go out.
following conditions exist:
Transistors Q5 and Q6 make up the short
1. The sensing element resistance decreases to
discriminator circuit. Its principle of operation
the predetermined level (established by the fire
works on the rate of change of sensor resistance.
alarm setting) due to an increase in temperature.
In a fire, the resistance rate of change is slow. In
2. The sensing element resistance decreases
an electrical short condition, the resistance rate
at a predetermined rate due to the rate of
drops abruptly. There are two timing circuits--
temperature increase in the sensing element.
one is made up of resistors R5 and R6 and
capacitor C2, and the other of resistors R15 and
NOTE: In the past, flights have aborted and
R16 and capacitor C5. These circuits are preset.
crewmen actually ejected because of a fire warning
This allows a slow change of sensor resistance to
light illumination caused by a short circuit in the
let transistor Q1 switch transistor Q2 before
system. Modern aircraft fire warning systems have
transistor Q5 switches transistor Q6. A fast change
a short discriminator circuit that differentiates
of sensor resistance allows transistor Q5 to switch
between an overtemp (fire) and a short circuit.
transistor Q6 before transistor Q1 can switch
If there is a short in this system, the fire warning
transistor Q2. If transistor Q6 switches first,
light will NOT illuminate.
transistor Q2 is unable to switch. This action holds
The rest of this section contains a description
of a dual jet engine aircraft fire warning system.
Both systems operate identically and are similar
in operation to fire warning systems found on
other Navy aircraft. The left side is discussed in
this section.
Look at figure 5-12. Electrical power from the
L FIRE DET circuit breaker supplies the left fire
detector and sensing element circuit through pin
A of the detector control unit. The left fire-sensing
element loop connects to pins L and C of the
detector control unit. This completes the sensing
circuit through normally closed contacts of the
de-energized relay K1. At normal temperatures,
the sensing element resistance is high, reverse-
biasing diode CR1. This allows current through
resistor R2 to turn transistor Q1 on. With Q1 on,
the base current at Q2 is shut off, turning Q2 off.
With Q2 off, the current flows into the base of
Q3. This turns Q3 on and Q4 off. Transistors Q3
and Q4 are relay-driving. With Q3 on, relay
Figure 5-13.-Container and dual valve assembly.
coil K2-A energizes, opening K2 contacts,
5-22
