MD Helicopters CSP-900RMM-2, Servicing And Maintenance

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CSP − 900RMM − 2 MD Helicopters, Inc.
Rotorcraft Maintenance Manual
Page 1
Revision 4
26-10-20
DECK AND ENGINE AREA FIRE/OVERHEAT DETECTION SYSTEM
DESCRIPTION AND OPERATION
1. Deck and Engine Area Fire/Overheat
Detection System
(Ref. Figure 1)
The fire/overheat detection for the transmis­
sion deck and engines consist of detection
cables, control units, and installation brackets.
The upper deck detection cable is routed down
the front center panel of the firewall assembly,
under each engine accessory assembly, and
around the upper deck under the edge of the
transmission.
The engine detection cables are routed on the
engine compartment inboard firewall and
inboard half of the aft firewall. Each detection
cable is connected to a separate fire control
unit located on the aft cabin compartment
bulkhead.
2. Deck and Engine Area Fire/Overheat
Detection Components
A. Fire/Overheat Detection Cable
The fire/overheat detection cable is a 0.070
inch (1.778 mm) outside diameter, double wall,
flexible stainless steel tube. The tube contains
a single wire center conductor in a highly
compacted manganese oxide semi-conductor
material.
A double steel sheath is used for the cable to
be stronger and more flexible. The cable can
bend to a minimum installation radius of
0.750 inch (19.1 mm).
All metal surfaces within the systems electri­
cal connectors are ceramic coated or surface
coated to eliminate the possibility of false
alarm due to a bridging action of conducting
materials. The use of a continuous loop
enables the system to be monitored for open
circuit condition. In the event of a single cable
break, the system remains operational.
The cable is a temperature averaging device,
its absolute or base resistance being a function
of the surrounding ambient temperature. As
the cable temperature rises, its resistance
drops until it reaches the predetermined set
point of the control unit.
B. Fire Control Unit
The fire control units are solid state assem­
blies. When the average temperature rises, the
control unit will sense the drop in resistance
until it reaches the set point and sends a
signal to the Integrated Instrument Display
System (IIDS) displaying a fire warning
(
FIRE ).
Two channel discriminators, fault and fire, are
interconnected and time-phased in order to
discriminate against false fire warnings
resulting in a short in cable circuitry. Resis­
tance used in the system is 75 ohms, short
discriminator actuating resistance is 60 ohms.
These values assure ample margin between
fire alarm resistance and short circuit resis­
tance.
C. Installation Brackets
Standoff brackets, stainless steel positive
locking cable clamps, and high temperature
silicon rubber grommets are used for installa­
tion of the detection cables.