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(WARNING)
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AMBER MESSAGE (ADVISORY)
CYAN MESSAGE (STATUS)
WHITE MESSAGE Require immediate crew attention. These messages are accompanied by flashing master warning, triple chime attenson with or without voice message or dedicated tone. Require immediate crew awareness and future crew action may be required in the form of alternate system selection or performance limitation. These message are accompanied by flashing master caution and a single chime. Indicate safe or normal system operation which require crew awareness, over and above the dark cockpit philosophy. Indicate minor failures or reduction in systems capability, which require no crew action.

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03/16/26

Message Overview:

Message Name:

SHORT CIRCUIT

Message Code:

B3-008143

Associated CAS:

FIRE SYS FAULT
(Advisory)

Reporting LRU: R ENG FIRE LOOP A
System Description: 26-12-00 - Engine Fire Detection System
Schematic Diagram: SSM 26-12-00-101 - Engine Fire-Detection System
Wiring Diagram: 26-12-05 - Fire Loops Engine (RHS) - ALL

Message Description:

Open circuit on right engine element engine core loop (A95), forward nacelle loop (A96), pylon loop (A97) or gearbox loop (A98) wire open, or FIREX Control Unit (A91) internal failure.

Possible Causes:

  • Core Sensing Element Loop A (RH) (A95)
  • Forward-Nacelle Sensing Element Loop A (RH) (A96)
  • Pylon Sensing Element Loop A (RH) (A97)
  • Gearbox Sensing Element Loop A (RH) (A98)
  • FIREX Control Unit (A91)
  • Associated Wiring

Troubleshooting Tips:

Advisory Wire/Service Bulletin: None

Full Throttle Blog/Forum Articles/Infoservice/Newsletter: None

Flight Operation Notifications Manual (FONM):None

NOTES:

Operation

  • The sensing element is used to find a fire. It consists of an Inconel tube with two electrical conductors in a thermistor material. One conductor is welded to the terminal lug ring and the other, the ground conductor, is welded to the shell of the sensing element. The resistance of the thermistor material decreases when its temperature increases.
  • At normal ambient temperatures the resistance of the sensing element is high. The resistance decreases quickly as the temperature of the sensing element increases. When the resistance of the sensing element decreases to a pre-set point that is the same as the specified alarm temperature, the detection circuit gives an alarm signal.

Continuity Test

  • The continuity test measures the maximum resistance between each terminal lug. The maximum resistance for each sensing element is given in the table.
  • Do the test as follows:
  • Connect the test leads of the multimeter to the terminal lugs at each end of the sensing element.
  • Measure the resistance of the sensing element. Make sure the resistance of the sensing element is not higher than the specified maximum resistance given in the table.

    Fire Detector Part No Sensing Element Part No Sensing Element Length (Inch) Sensing Element Length (Cm) Max. Wire Resistance Ω
    446700-3 21802950 29.50 74.93 2.46
    446700-5 25806340 63.40 161.04 5.28
    446700-6 21802310 23.10 58.67 1.93
    446700-7 21802450 24.50 62.23 2.04

NOTE: To help you find an intermittent malfunction, tap the sensing element lightly while you monitor the multimeter. Replace a sensing element if the resistance is higher than the given value in troubleshooting recommendations.

Insulation Resistance Test

  • This test measures the resistance between the conductor of the sensing element that is not grounded and the conductor of the sensing element that is grounded. Refer to Table 102, Minimum Insulation Resistance. Do the test as follows:
  • Connect the negative lead of the megohmmeter to the shell of the sensing element.
  • Connect the positive lead of the megohmmeter to one terminal lug of the sensing element.
  • Apply the applicable voltage given in table to the sensing element.
  • Monitor the megohmmeter indication while you apply the voltage. The resistance value must be higher than the value given in Table.

NOTE: To help you find an intermittent malfunction, tap the sensing element lightly while you monitor the megohmmeter. Replace a sensing element that has a lower resistance than that given in this table.

Minimum Insulation Resistance

  • Minimum Resistance in Megohms Vs Temperature (°F/°C)

Fire Detector Part No Sensing Element Part No Minimum Resistance, (In Megohms), Vs Temperature (°F/°C) Test VDC
65/18 70/21 75/24 80/27 85/29 90/32 95/35 100/37
446700-6 21802310 0.19 0.16 0.14 0.13 0.11 0.10 0.08 0.07 10
446700-7 21802450 0.18 0.16 .014 0.12 0.10 0.09 0.08 0.07 10
446700-3 21802950 0.15 0.13 0.1 0.10 0.08 0.07 0.06 0.06 10
446700-5 25806340 142 121.29 103.47 88.33 75.24 64.20 54.73 46.68 500

Quick Links:

General Setup for Maintenance
Standard Aircraft Configuration for Maintenance AMM 12-00-00-867-803
Connect Electrical Power to the Aircraft AMM 24-00-00-861-801
Remove Electrical Power from the Aircraft AMM 24-00-00-861-802
Electrical/Electronic Safety Precautions AMM 24-00-00-910-801
Electrostatic Discharge Safety Precautions AMM 24-00-00-910-802
Maintenance Tasks
Removal of the Fire-Detection-and-Extinguishing (FIREX) Control-Unit AMM 26-11-01-000-801
Installation of the Fire-Detection-and-Extinguishing (FIREX) Control-Unit AMM 26-11-01-400-801
Removal of the Core Sensing Element AMM 26-12-01-000-801
Installation of the Core Sensing Element AMM 26-12-01-400-801
Removal of the Forward-Nacelle Sensing Element AMM 26-12-05-000-801
Installation of the Forward-Nacelle Sensing Element AMM 26-12-05-400-801
Removal of the Pylon Sensing Element AMM 26-12-09-000-801
Installation of the Pylon Sensing Element AMM 26-12-09-400-801
Removal of the Gearbox Sensing Element AMM 26-12-13-000-801
Installation of the Gearbox Sensing Element AMM 26-12-13-400-801

Troubleshooting Recommendations:

  1. Check the FIREX Control Unit (A91) circuit breakers CB1-A11, CB2-A11 and CB5-A3, CB5-A4 pull and reset.
    Re-check MDC for the fault.

    1. If system checks are good, do close out.
    2. If fault remains, continue with next step.

  2. Check the FIREX R ENG FIRE Loop A for continuity and no shorts to ground, ring from A91P1 pin 17 to A91P1 pin 18. The resistance of the pylon loop (A97) about 2.46Ω, the engine core loop (A95) about 5.28Ω, the gearbox loop (A98) about 2Ω, and the forward nacelle loop (A96) about 1.93Ω with a total resistance about 11.67Ω.

    1. If the resistance value is good, go to step 11.
    2. If the resistance value is out of tolerance, continue with next step.

  3. Gain access to loop A97 and check continuity at loop; it should be 2.46Ω.

    1. If resistance value is 2.46Ω, go to step 5.
    2. If resistance value is not 2.46Ω, continue with next step.

  4. Replace loop A97, do close out.
  5. Gain access to loop A95 and check continuity at loop; it should be 5.28Ω.

    1. If resistance value is 5.28Ω, go to step 7.
    2. If resistance value is not 5.28Ω, continue with next step.

  6. Replace loop A95, do close out.
  7. Gain access to loop A98 and check continuity at loop; it should be 2.46Ω.

    1. If resistance value is 2Ω, go to step 9.
    2. If resistance value is not 2Ω, continue with next step.

  8. Replace loop A98, do close out.
  9. Gain access to loop A96 and check continuity at loop; it should be 1.93Ω.

    1. If resistance value is 1.93Ω, go to step 11.
    2. If resistance value is not 1.93Ω, continue with next step.

  10. Replace loop A96, do close out.
  11. Inspect the R ENG FIRE Loops A: pylon loop (A97), engine core loop (A95), gearbox loop (A98), and forward nacelle loop (A96) for proper installation and adjust as required.
    Reset CB2-A11 and CB6-A4 and re-check MDC for fault.

    1. If fault is no longer posted in MDC, do close out.
    2. If fault remains, continue with next step.

  12. Replace FIREX Control Unit.
  13. Do close out.

apu_fire_loop13_apu_firewall_detection_harness

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