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LOWER CARGO COMPARTMENT SMOKE DETECTION -LOWER CARGO SMOKE DETECTOR AND FANS -INTRODUCTION

Purpose
The smoke detectors monitor air in the cargo compartments for smoke. The fans bring air from the cargo compartment into the smoke detector. Only one fan on each detector operates at a time.
Physical Description
The forward and aft lower cargo smoke detectors are identical. The smoke detectors have these components:
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Two electrical connectors

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Four air inlet ports

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Four shock mounts

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Two fans

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Four airflow sensors (internal)

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Four smoke detection chambers (internal)

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Two electronic control channels (internal).

The smoke detectors use light emitting diode (LED) optical smoke detectors and acoustic air flow sensors.
Location
There is a smoke detector in the forward and aft lower cargo compartments. The detectors are behind the cargo compartment sidewall lining opposite the cargo doors.
Training Information Point
The smoke detector is an LRU and the fans are LRUs. The smoke detector has no internal LRUs. There is no scheduled maintenance of the cargo smoke detectors or fans. Alert messages or status messages show if there is a fault condition.
The forward lower cargo smoke detector and the aft lower cargo smoke detector are interchangeable. Four bolts attach the smoke detector to the structure.
Each smoke detector channel has its own electrical connector. The two connectors are different so you cannot cross-connect the wires. Program pins in the connectors give the detector this information:
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Location in the airplane, forward or aft

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If a remote smoke detector is connected

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The active air inlet ports.

Clamps attach the fans to the smoke detector. The length of the wire connected to the fan is such that you cannot connect the wire to the wrong connector on the smoke detector.
The lower cargo smoke detectors are onboard software loadable LRUs. You use the maintenance access terminal (MAT) to load software into the smoke detectors.
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LOWER CARGO COMPARTMENT SMOKE DETECTION -LOWER CARGO SMOKE DETECTOR AND FANS -INTRODUCTION

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General
The smoke detector has two channels that control and monitor these components:
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Airflow sensor

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Optical smoke detector

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Fans.

Airflow Sensors
The airflow sensor has these parts:
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Transmitter

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Receiver.

The sensor monitors the amount of air that goes through each air inlet port. The smoke detector sends a fault message on an ARINC 429 bus for these conditions:
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Too much airflow

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Not enough airflow

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Failed airflow sensor.

Optical Smoke Detectors
The optical smoke detector has these parts:
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Two source light emitting diodes (LEDs)

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Two intensity monitor photodiodes

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Two scatter detector photodiodes.

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One set of diodes connects to channel 1. The other set of diodes connects to channel 2. Only one set of diodes operates at a time.
Inside the smoke detection chamber, air flows between a source LED and a scatter detector photodiode. Usually, only a small amount of light from the lED gets to the scatter detector. If the air has smoke in it, the smoke particles reflect more light on the scatter detector. This causes an alarm signal.
The intensity monitor photodiode makes sure that the source LED is on and keeps the output of the source LED constant. This configuration also finds contamination of the LED and photodiodes. A defective diode or contamination causes the detector to change to the other set of diodes. The detector sends a fault message.
Condensation from the in-line water separators flows through one of the chambers. The airflow sensor and optical smoke detectors in this chamber are not active.
Channel 1 And 2 Circuits
The two channels operate independently. If one channel fails,
the other channel can  control all functions of the smoke 
detector. 
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LOWER CARGO COMPARTMENT SMOKE DETECTION -LOWER CARGO SMOKE DETECTOR -FUNCTIONAL DESCRIPTION
One channel does continuous BITE while the other channel controls the smoke detector. Channel 1 controls during one electrical power cycle, channel 2 controls during the next power cycle. If the control channel or its diodes fail, the other channel takes over. The detector sends a fault message.
　
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