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System Description
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(WARNING)
RED MESSAGE (CAUTION)
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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Overview

The engine fire detection system is a dual-loop fire detection system. These loops are connected to the FIREX control unit (FIREX-CU). Each engine has two fire detection loops (A and B). Each engine loop has four fire detection elements connected in series. Fire detection loops A and B share the same mechanical mount and are electrically isolated. The two ends of each loop are electrically connected to the FIREX-CU. This makes each loop single-fault tolerant against open circuits. Each detection loop is attached to the aircraft with clamps. The engine fire sensing elements are located near the critical power plant structure elements. The engine fire detection system interfaces with the SYSTEMS TEST control panel.

Core Sensing Element

The core sensing element is on the core of each engine. It senses the heat from a burn through the core of the engine.

The alarm trip temperature (476 °C [888.8 °F]) is given when approximately 100 °C (212.0 °F) is added to the maximum temperature of operation (382 °C) [719.6 °F]) to which the sensing elements are usually exposed. The 100 °C (212.0 °F) value decreases the risk of incorrect or irregular alarms from unusually high temperatures.

The alarm temperature value is in relation to the sensing element length, core material and the properties of other elements in the loop. The alarm condition goes out when the temperature decreases to <468 °C (874.4 °F).

300_2612_001

11/16/15

Forward Nacelle Sensing Element

This element is on the forward nacelle of each engine near the wet bay. The wet bay is the area of the engine where the fluid lines connect in the pylon. It senses the heat from a fire that occurs in the wet bay area of the engine. The alarm trip temperature (218 °C [424.4 °F]) is given when approximately 100 °C (212.0 °F) is added to the maximum temperature of operation (121 °C [249.8 °F]) to which the sensing elements are usually exposed. The 100 °C (212.0 °F) value decreases the risk of incorrect or irregular alarms from unusually high temperatures.

The alarm temperature value is in relation to the sensing element length, core material and the properties of the other elements in the loop. The alarm condition goes out when the temperature decreases to less than 212 °C (413.6 °F).

300_2612_004

Pylon Sensing Element

This element is on the pylon firewall of each engine. It senses the heat from a fire that occurs on the structural mounts for the engine. The alarm trip temperature (208 °C [406.4 °F]) is given when approximately 100 °C (212.0 °F) is added to the maximum temperature of operation (121 °C [249.8 °F]) to which the sensing elements are usually exposed. The 100 °C (212.0 °F) value decreases the risk of incorrect or irregular alarms from unusually high temperatures.

The alarm temperature value is in relation to the sensing element length, core material and the properties of other elements in the loop. The alarm condition goes out when the temperature decreases to <202 °C (395.6 °F).

300_2612_005

Gearbox Sensing Element

This element is on the gearbox of each engine. It senses the heat from a fire that occurs near the gearbox or the systems attached to it. The alarm trip temperature (214 °C [417.2 °F]) is given when approximately 100 °C (212.0 °F) is added to the maximum temperature of operation (80 °C [176.0 °F]) to which the sensing elements are usually exposed. The 100 °C (212.0 °F) value decreases the risk of incorrect or irregular alarms from unusually high temperatures.

The alarm temperature value is in relation to the sensing element length, core material and the properties of other elements in the loop. The alarm condition goes out when the temperature decreases to <208 °C (406.4 °F).

300_2612_006

03/30/22

Operation

The FIREX-CU receives 28 VDC power from the aircraft left and right essential buses for detection and monitoring functions. The FIREX-CU discrete signals and ARINC 429 bus signals interface with the data concentrator unit (DCU) for alarm and status reporting functions. The FIREX-CU continuously monitors the resistance to ground of each loop. In fire conditions, the loop resistance decreases and supplies a loop fire indication at the FIREX-CU. The FIREX-CU includes short circuit discriminator circuits to identify the difference between a loop fire and loop failures.

The FIREX-CU has two modes of operation, dual-loop and single-loop logic detection. In the dual-loop logic detection mode, the A and B loop signals are necessary to the FIREX-CU to cause a zone fire alarm. In the single-loop logic detection mode, only one fire signal from an element is necessary to cause a fire alarm. The FIREX-CU automatically changes to the single-loop logic detection mode in failure conditions (loss of a single channel or loop). When zone fire conditions are met, the FIREX-CU transmits the appropriate fire warning to the EICAS through the ARINC 429 and hardwired discretes. Faults on the detector loops are identified by the FIREX-CU with indications supplied to the EICAS through the data concentrator unit (DCU). If there is a fire sensed, the following visual indications and alarm sounds occur:

  • FIRE flag shown on the applicable engine fan rpm display (N1 scale) on the EICAS
  • L (R) ENGINE FIRE warning message on the EICAS
  • WARNING light on the WARNING/CAUTION panel illuminates
  • L (R) ENG FIRE arming pushbutton annunciator (PBA) on the engine control panel illuminates
  • Triple chime sounds
  • LEFT (RIGHT) ENGINE FIRE” aural message sounds

The L (R) ENGINE FIRE warning message will show when one of fire detection loops on the left (right) engine has sensed a fire.

The L (R) FIRE DET FAIL caution message will show when the left (right) engine fire detection system has failed.

The EICAS messages that follow are related to the engine fire detection system:

EICAS MESSAGE(S) LEVEL (COLOR)
L ENGINE FIRE WARNING (red)
R ENGINE FIRE WARNING (red)
L FIRE DET FAIL CAUTION (amber)
R FIRE DET FAIL CAUTION (amber)

System Test

A test of the fire detection system can be done with the rotary test switch on the SYSTEMS TEST control-panel set to the FIRE DET position. When the test switch is momentarily pushed (approximately 1 second), it starts a test of the entire fire, overheat and smoke detection system.

10/15/20

Component Location Index

Component Location Index
IDENT DESCRIPTION LOCATION IPC REF
A95/A99 CORE SENSING ELEMENT (LH) LOOP A, 
LOOP B		    ZONE(S) 430 26-12-01
A95/A99 CORE SENSING ELEMENT (RH) LOOP A, 
LOOP B		    ZONE(S) 440 26-12-01
A96/A100 FORWARD-NACELLE SENSING ELEMENT 
(LH) LOOP A, LOOP B		    ZONE(S) 430 26-12-05
A96/A100 FORWARD-NACELLE SENSING ELEMENT 
(RH) LOOP A, LOOP B		    ZONE(S) 440 26-12-05
A97/A101 PYLON SENSING ELEMENT (LH) LOOP A, 
LOOP B		    ZONE(S) 412 26-12-09
A97/A101 PYLON SENSING ELEMENT (RH) LOOP A, 
LOOP B		    ZONE(S) 422 26-12-09
A98/A102 GEARBOX SENSING ELEMENT (LH) LOOP A, 
LOOP B		 ZONE(S) 432BB 26-12-13
A98/A102 GEARBOX SENSING ELEMENT (RH) LOOP A, 
LOOP B		 ZONE(S) 442BB 26-12-13
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