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 C.  The fire detection systems are each composed of four dual-loop detectors. Thus, each loop of a fire detection system has four detector elements, one from each detector. The nacelle overheat detection systems are each composed of one dual-loop detector.
 D.  Each detection loop has a control card, and each engine has a fire/overheat logic/test card. All four systems have one fire/overheat test panel.
 E.  All four systems operate in AND logic. Thus, both loops of a system must sense a fire/overheat or fault condition in order for an alarm to be given.
 2.  Component Details (Fig. 1)_________________
 A.  Engine Fire and Overheat Detectors
__________________________________
 (1)  AIRPLANES WITH SYSTRON DONNER DETECTORS;
 Refer to the data that follows:

 (a)  
A detector consists of two sensing elements which are attached to a support tube by quick-release mounting clamps. Each sensing element contains an inert gas, a gas-emitting core material, and has a responder on one end. The responder contains two pressure switches, and provides the electrical interface with the airplane wiring.

 (b)  
The inert gas in a sensing element expands as a function of average gas temperature. The gas-emitting core material expells gas due to high localized temperatures. Both actions cause an increase in pressure in the element, which causes an alarm pressure switch (in the responder) to activate. Both actions are completely reversible - as the temperature decreases, the pressure decreases, and the alarm switch deactivates. When a sensing element is damaged allowing the inert gas to leak out, an integrity pressure switch deactivates. The pressure switches provide alarm and integrity signals to the appropriate detector control card.

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757
MAINTENANCE MANUAL

MAIN EQUIPMENT CENTER
Engine Fire Detection Component Location Figure 1 (Sheet 1)
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MAINTENANCE MANUAL

SENSING
ELEMENTS

Engine Fire Detection Component Location
Figure 1 (Sheet 2)

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MAINTENANCE MANUAL

 (c)  The fire and overheat detectors are all identical except for the normal resistance value (at the electrical interface), the length, and a factory-set alarm point (temperature) for the alarm switch to activate.
 (2)  
AIRPLANES WITH GRAVINER DETECTORS; Refer to the data that follows:

 (a)  
A fire detector consists of two sensing elements which are attached to a support tube by quick-release mounting clamps. Each sensing element is a resistor-capacitor network, with resistance varying as a function of temperature.

 (b)  
At low temperatures, the impedance of the sensing element is mainly resistive. As temperature increases, the resistance drops, thus the impedance becomes more reactive. The detector card senses the change as a fire signal. A pure resistance will not be sensed by the detector card as a fire, but as a fault.

 (c)  
An overheat detector consists of two sensing elements which are attached to a support tube by quick-release mounting clamps. Each sensing element contains an inert gas, a gas-emitting core material, and has a responder on one end. The responder contains two pressure switches, and provides the electrical interface with the airplane wiring.

 (d)  
The inert gas in a sensing element expands as a function of average gas temperature. The gas-emitting core material expells gas due to high localized temperatures. Both actions cause an increase in pressure in the element, which causes an alarm pressure switch (in the responder) to activate. Both actions are completely reversible - as the temperature decreases, the pressure decreases, and the alarm switch deactivates. When a sensing element is damaged allowing the inert gas to leak out, an integrity pressure switch deactivates. The pressure switches provide alarm and integrity signals to the appropriate detector control card.
　
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