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• Top left side (APU accessory section) (450° F [232° C])
• Top center forward (load control valve) (600° F [316° C])
• Bottom aft right corner (450° F [232° C])
• APU air inlet duct (450° F [232° C])
The four thermal switches are parallel connected; therefore, any one of the
four switches reaching its trip point will trigger the visual and aural APU fire
annunciations. Accordingly, failure of one switch does not render the
system inoperative. When a thermal switch closes, 28 VDC from the
Essential DC bus (through the APU FIRE DET circuit breaker) closes a
logic relay, causing the following visual and aural APU fire annunciations:
• Red APU FIRE warning capsule on APU control panel illuminates
(Figure 6)
• Red APU FIRE warning message is displayed on CAS
• Red APU FIRE capsule illuminates on the SWLP, if the SWLP is in
manual mode, or if in automatic mode coupled with CAS failure
• Both red MASTER WARN lights illuminate, with corresponding aural
warning tone
• Nosewheel well warning bell (or tone speaker) sounds (if aircraft is
on the ground)
• The APU FIRE checklist is displayed on the lower center portion of
the copilot’s navigation display (DU 5) in MAP or COMP or PLAN
mode (airplanes SN 1144 and subsequent and SN 1000 through
1143 with ASC 178 incorporated)
• If the logic relay closes due to an actual fire warning (not a test), the
APU will also be made to flame out
When the temperature drops below the thermal switch trigger point, the
switch opens to de-energize the logic relay, deactivating the fire warnings.
Testing the APU fire detection system is accomplished using the APU FIRE
OPERATING MANUAL
2A-26-00 PRODUCTION AIRCRAFT SYSTEMS
Page 10
January 31/02
Revision 6
TEST switch, located on the cockpit overhead panel and shown in Figure
4. With Essential DC bus power available and the switch depressed and
held, all of the previously-listed annunciations will occur, except the APU, if
running, will not be made to flame out. Releasing the switch removes all
annunciations.
C. Engine Overheat Detection:
Low pressure engine bleed air cools various parts of the engine. After
cooling the engine, the air exhausts overboard through a duct on the
bottom of the engine cowling. A thermal switch in this duct monitors cooling
air temperature as it is exhausted overboard.
If engine cooling air temperature exceeds 860 ±25° F (460 ±15° C), the
thermal switch closes to trigger the following annunciations:
• Red L-R ENGINE HOT warning message is displayed on CAS
• Red L (or R ) ENGINE HOT capsule illuminates on the SWLP, if the
SWLP is in manual mode, or if in automatic mode coupled with CAS
failure
• Both red MASTER WARN lights illuminate, with corresponding aural
warning tone
Like the fire detection systems, when the temperature drops below the
thermal switch trigger point, the switch opens, deactivating the overheat
warnings. Also like the fire detection systems, the overheat detection
system receives power from the Essential DC bus.
D. Other Overheat Detectors:
Although not included in the engine or APU fire / overheat detection and
warning system, the following overheat sensing components and their
resultant annunciations bear discussion here. For more information on
these overheat sensing components, see their respective system
description.
(1) Aft Equipment Compartment:
A thermal switch is installed on each side of the aft equipment
compartment (tail compartment), adjacent to the bleed air manifolds.
The two switches, wired in parallel, close at 200 ±5° F (93 ±3° C) to
trigger a red AFT EQUIP HOT warning message on CAS. A red AFT
EQUIP HOT capsule will illuminate on the SWLP, if installed.
(2) Engine Pylons:
Three thermal switches installed in each engine pylon adjacent to
the high stage bleed air pressure regulator and precooler. If any of
the thermal switches reaches 300 ±5° F (149 ±5° C) the switch
closes to trigger a red L-R PYLON HOT warning message on CAS.
A red L (or R) PYLON HOT capsule will illuminate on the SWLP, if
installed.
(3) Bleed Air Ducts:
A thermal switch is installed downstream of each bleed air precooler
heat exchanger. The switch closes when air temperature
downstream of the precooler reaches 200 ±5° F (93 ±3° C),
triggering an amber L-R BLEED AIR HOT caution message on CAS.
(4) Nose Cowl Anti-Icing:
OPERATING MANUAL
PRODUCTION AIRCRAFT SYSTEMS 2A-26-00
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