FIRE DETECTION SYSTEM on B737-200 FREIGHTER.
[Mr Meljoe Ferreira]
The Securaplane ST 3000 Fire Detection System detects fire originating in the lower cargo compartments. To minimize the quantity of wiring required, decrease system weight and increase reliability, radio communication links replace wiring between various limits of the system.
The System consists of 9 smoke detectors - Four in the Forward cargo and Five in the Aft cargo compartments - mounted in the ceiling of each lower cargo compartments, One central control unit [CCU] located in the Aft wall of the FWD cargo compartment with a configuration module [CM], one control display unit[CDU] on P-5 Overhead panel, Two circuit breakers at P-18 panel and associated wiring.
The System meets FAA required Class "D" to Class "E" cargo conversion. The System will detect smoke anywhere within any cargo compartment within 60 seconds at at-least two SDU locations.
This Unit is located in the cockpit overhead P-5 panel and is used as the control interface for the system to the maintenance crew and provides flight crew with smoke detection alarm reporting. The CDU also reports system operational status and has built in test capabilities. The CCU sends system status to the CDU. The CDU displays system information in the form of a scrolling, ten character alphanumeric display and indicator lights. Display information includes alarm, maintenance information and system status. The CDU also has discreet output to activate the Aircraft Aural Warning [Fire Bell] and Master Fire Warning lights. CDU receives its power from CCU.
Fig #1 CDU
A] Mx Test Button - Activates maintenance display, steps through system messages on Mx display and is also used to reject sensor in set up mode.
B]Mx LED - Amber LED illuminates to indicate a system component needs maintenance.
C] Mx display - 10 character, scrolling, alphanumeric display shows system status when Mx test button is pressed. Also displays Fire and Location if smoke is detected.
D] Mx set up Button - Button initiates system set up mode, also used to accept sensors in set up mode.
E] System OK LED - Green LED illuminates when system is functional.
F] FIRE indication lamp - Red lamp illuminates to show that smoke has been detected in a cargo bay.
G] Lamp Test Button - Button illuminates all CDU lamps, Fire bell, Master caution fire warning lamps.
2.Central Control Unit [CCU]
Fig #2 CCU
The dual Central Control unit [CCU] houses the spread spectrum receiver which receives status from the Smoke Detector Units [SDU]. The CCU also contains the detection logic which determines presence of smoke, located on the aft wall of Forward Cargo compartments. CCU receives two 28vdc supply from circuit breakers located on P-18 panel. The configuration module [CM] is mounted on the CCU with two screws and permanently secured to the airframe by a steel lanyard. These units store the details of smoke detectors with serial numbers and location. This unit stays with the aircraft even when the CCU may be removed.
3.Smoke Detector units [SDU]
SDU's are installed through the overhead cargo liners and attached to the aircraft floor beams via aluminum mounting brackets. Units monitor for the presence of smoke and/or heat +150degF. Two detectors must detect smoke or one detector must detect smoke and another detector to detect heat above 150 degF in a single compartment, to annunciate the lower cargo smoke LED on CDU.
Fig #3 SDU
Each SDU is powered by two 3.6 voltLithium Thionyl batteries connected in series located behind the SDU grill. The SDUs automatically transmit/report their individual serial numbers, battery condition and status to the CCU receiver every 30 seconds using radio signals once the ID plug is installed. The CCU monitors each SDU to ensure at least one RF transmission is received within 10 minutes at power on and every 10 minutes thereafter.
This SDU is called Infrared smoke detector which is not actually a smoke detector!. It is a particle detector. So any particle fine enough to enter the central chamber will scatter the infrared light into the IR sensor causing an alarm as long as the particles are still present. For the system to declare an alarm, two smoke sensors must be alarming in the same compartment.
Identification plug is installed on the rear of the unit and is permanently secured to the Airframe by a steel lanyard. The Identification plugs are used to differentiate various cargo compartment smoke detector locations. They are colour banded on the connector, Black for FWD, Red for AFT. The lanyard is used to secure the identification plug attached to the aircraft when the SDU is removed.
FIG#4 Block Diagram
Smoke detection and alerting occurs when photo electric smoke detector units detect smoke or smoke and High temperature. The sensors are particle detectors and set to sense particles with micron size of smoke consistency. When smoke is detected it is transmitted to the CCU describing the condition and location of the detected smoke [bay].
The CCU normally wait until Two smoke detector units in the same cargo bay have reported smoke before declaring a fire alarm. The CCU will also declare a fire condition if a single smoke detector unit declares with a smoke and a high temperature. If any smoke detector unit in a cargo bay fails and any one remaining smoke detector unit in that cargo bay reports smoke, the system will also declare an alarm. When the CCU has declared a fire condition, it sends messages to the CDU to illuminate "Lower Cargo Smoke" annunciator, activate activate aircraft fire warning bell and illuminates the master fire warning lights on glare shield panels. The fire warning bell and master fire warning lights can be cancelled.
On application of power, a system self test runs each time aircraft power is interrupted. If system is OK and no faults detected, the green "SYS OK" LED on the CDU will illuminate. The Mx display will be blank and all other annunciators will be off. If a fault is detected the "Mx LED" will be illuminated. If a fault which will prevent system operation is detected the "SYS OK" LED will be extinguished and the "Mx LED" remains illuminated. If the "Mx LED" is on depressing the "Mx TEST" button will display a specific fault message. If no faults are found, the "Mx Display" will read "SYSTEM OK".
LAMP TEST :-
Pressing and holding the Lamp Test switch on the CDU, tests the CDU display and annunciators and also the CDU interface with aircraft fire warning system. The aircraft fire bell should sound and both fire master warning lights illuminate and the Mx display should be illuminated in a checker board pattern. Pressing either master warning lights or Bell cut out switch during the Lamp test should reset the lights and the fire bell.
Mx TEST BUTTON:-
If no system faults are currently detected, pressing the "Mx TEST" button should result in a "System OK" message on the display. If Mx annunciator is illuminated, one or more fault messages should display when the switch is pressed.
[SDU] Smoke Detector P/N 100-0649-01 - Qty 9.
[CCU] Central Control Unit P/N 100-0127-03 - Qty 1.
[CDU] Control Display Unit P/N 100-0120-03 - Qty 1.
[C/B] Circuit Breaker P/N 2TC6-1 - Qty 2.