2A-26-30: Fire Detection and Warning System
1. General Description:
The fire detection and warning system provides a means to detect and indicate
the presence of an overheat or fire condition in the engine areas and APU
enclosure. A dual channel system, it will remain functional should a single channel
fail or one of its power sources be lost.
The system receives power upon selection of the battery switches to ON. It will
remain operable at all times provided the Essential DC bus is powered and all
associated circuit breakers are closed. Prior to APU start, the APU fire detection
system is tested by the flight crew. Likewise, prior to engine start, the engine fire
detection system is also tested. Any failures or faults during engine portion of the
fire detection system testing are annunciated.
The fire detection and warning system remains active in sensing its respective
areas during all phases of flight. The engine portion of the fire detection system
continuously monitors self-health, annunciating any failures or faults.
During shutdown, the fire detection and warning system remains operable until
selection of the battery switches to OFF.
2. Description of Subsystems, Units and Components:
A. Engine Fire Detection System:
A dedicated continuous-wire fire detection system monitors engine fire
zone temperature with two parallel dual loops. The parallel dual loops,
LOOP A and LOOP B, which are approximately one inch apart, provide
independent, yet related, fire detection. Each loop consists of three serieswired
sensing elements that are attached to the lower side of the engine
combustion area, forward fixed cowl and aft fixed cowl. Each engine’s two
loops then connects to a digital engine fire detection control unit located in
the tail compartment.
In response to an engine fire, the fire detection control unit triggers the
appropriate visual and aural warnings. If the detection system
malfunctions, the control unit triggers the appropriate CAS message and
illuminates the overhead panel fault lights.
(1) Sensing Loop and Elements:
The engine’s dual loops consist of two element assemblies parallelmounted
to rigid stainless steel rails. Each rail conforms to the
shape of the area for which fire detection is provided. Each loop
consists of three series-wired sensing elements, commonly called
firewire segments.
A firewire consists of a thin stainless steel sheath containing a
coaxial wire. The coaxial wire is insulated from the sheath by a
semi-conductor material. Resistance of the semi-conductor varies
proportionally with temperature, falling as sensor temperature
increases. Simultaneously, capacitance of the sensor increases,
providing a basis for logic discrimination between fire or fault. Both
ends of each coaxial wire is connected to its respective portion of
the fire detection control unit, while the sheath connects to the
aircraft’s electrical ground.
If either loop should fail or become shorted or fail for any reason, a
fault alert illuminates. The flight crew may then select the faulty loop
OPERATING MANUAL
PRODUCTION AIRCRAFT SYSTEMS 2A-26-00
Page 7
January 31/02
Revision 6
off, leaving the remaining loop to function as a single loop system.
Should a loop ever be severed, it will continue to operate normally,
but will indicate as faulty during testing.
(2) Fire Detection Control Unit (FDCU):
The FDCU is a digital device that contains separate but identical
circuit boards for the LOOP A and LOOP B fire detection loops.
Each circuit board contains a voltage regulator, fire and fault
warning logic circuits, fire and fault warning output circuits, a firewire
driver and a fire warning comparator and reference selector.
Supplied with 28 VDC power from the Essential DC bus through the
L/R FIRE DET circuit breakers, the FDCU generates a 400 Hz
squarewave that is supplied to the fire detection loops. The FDCU
uses this squarewave output and the resulting feedback signal to
provide fire and fault detection.
(3) Test Control Panels:
Two system test control panels are provided for the flight crew to
verify proper system operation. They are located on the cockpit
overhead panel and are labeled FIRE TEST (Figure 4) and FIRE
DETECTION (Figure 5). With the Essential DC bus powered, testing
of the system is possible.
(4) Fire Annunciations:
When exposed to high temperatures, (e.g., engine fire), semiconductor
resistance drops and capacitance rises. If this occurs in
both loops (except when a failed loop is isolated) and the changes
are within the predetermined bounds as caused by heating of the
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