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DUCT LEAK AND OVERHEAT DETECTION -BODY DUCT LEAK DETECTORS

Purpose
Body duct leak detectors monitor the pneumatic distribution ducts for leaks in these areas:
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Main wheel wells

*
Outboard of the aft and bulk cargo compartments, left side

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Fuselage from the aft pressure bulkhead to the APU firewall

*
Air driven pump area.

Physical Description
The airplane has a dual loop body duct leak detector circuit. Each loop in the circuit has twelve elements that connect in series. Clips along the length of the assembly attach the elements to the structure.
Each element has an outer conductor around an inner conductor. The separator between these conductors is an electrical insulator at temperatures less than the overheat set point (255F, 124C).
Functional Description
When the temperature is more than the set point, the separator conducts electricity. The electrical continuity between the inner and outer conductors gives an input to the DLODS control card.
Location
The body duct leak detectors are near the pneumatic distribution ducts in the body areas. The detectors extend from the main wheel wells to the APU firewall.
Training Information Point
Where loop 1 uses a male connector, loop 2 uses a female connector to prevent cross connection of the loops.
The body duct leak detection system has a fault location feature for the detector loops. Maintenance messages include a zone number to help you find the location of a duct leak, short circuit, or open circuit.
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DUCT LEAK AND OVERHEAT DETECTION -BODY DUCT LEAK DETECTORS

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777 AIRCRAFT MAINTENANCE MANUAL

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DUCT LEAK AND OVERHEAT DETECTION -FUNCTIONAL DESCRIPTION

General
The three DLODS control cards monitor the detector loops. Each DLODS control card uses ARINC 429 buses and two ARINC signal gateway (ASG) cards as an interface with the ARINC 629 systems buses. The DLODS control cards use this configuration to send data to each other and to other systems and LRUs.
Detector Status
Each detector loop has three possible conditions:
*
Usual -low impedance in the inner conductor loop circuit and high impedance between the inner and outer conductors

*
Open circuit -high impedance in the inner conductor loop circuit

*
Short circuit -low impedance between the inner and outer conductors.

A loop in the usual condition has no electrical defects and no overheat condition. A loop with an open circuit is broken or disconnected from the card. A loop with a short circuit is damaged or has an overheat condition.
Control Card Logic
The cards compare the condition of both loops of a detector to find an overheat condition. The cards transmit an overheat signal if both loops have a short circuit. The cards also transmit an overheat signal if one loop has a short circuit while the other loop has an open circuit.
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If a DLODS control card fails, the remaining cards do not get information about the loops connected to the failed card. The operating cards transmit an overheat signal if one loop has a short and the other loop connects to a failed card.
Interfaces
A wheel well fire causes an EICAS warning message and master warning indications. The fire/overheat test switch does a test of the wheel well fire detection part of DLODS.
Duct leak or overheat conditions cause EICAS caution messages and master caution indications. The duct leak or overheat input to the ASCPCs causes pneumatic system valves to close and isolate the leak. The input to the APUC causes the APU surge valve to open if it is necessary. A fan case overheat input to the ACIPS cards causes the engine anti-ice valve to close.
Training Information Point
The AIMS shows a loop failure status messages if a loop is not in the usual condition. The message specifies the detector circuit and the loop. The message shows for loops with open or short circuits. If there is an overheat condition, loop failure messages show for both detector loops. If a DLODS control card fails, loop failure messages show for all the loops that connect to that card.
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DUCT LEAK AND OVERHEAT DETECTION -FUNCTIONAL DESCRIPTION

The central maintenance computing system also shows messages for loops not in the usual condition. The maintenance message specifies the detector circuit, loop, and if the problem is an open or short. The message also specifies the zone location for these failures:
　
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