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Engine fire sensor informs the pilot engine fire and triggers fire extinguisher to release their payload. However, fire extinguisher valves are controlled through pressure operated switches or electrical switch. These valve controls require either pressurized fluids or simple dc currents, not the digital data transmitted from the engine fire sensor to the Engine Information display in the cockpit. Could you tell me what kind of signal is outputted from engine fire sensors?

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  • $\begingroup$ When the engine fire detection system detects (via heat sensor) a fire it sends a signal to the cockpit normally illuminating the involved (one for each engine) cockpit fire extinguisher handle and also an aural warning is heard. The flight crew must activate the fire extinguisher system for the engine involved. Usually pulling the illuminated fire handle and rotating it to the left or right (2 bottle system), shutting off fuel etc. to the engine and deploying the extinguishing agent. Does not happen automatically. $\endgroup$
    – user22445
    Commented Nov 18, 2021 at 2:04

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From a generic system configuration standpoint, the fire sensing loops don't normally trigger the fire bottles; only the cockpit indicators and alarms. The bottles are fired by separate controls normally on the glareshield or overhead panel.

The pilot needs to be able to fire the bottles at his/her discretion during the engine fire drill. For example, if a fire alarm goes off right after takeoff, the pilot may delay calling for the drill until the airplane is in a more stabilized phase.

The bottles are fired by electrical squibs (an explosively fired plunger) wired through the firewall shutoff valve switches and normally powered by a very basic battery source bus, so that closing the shutoff valves arms the bottles. You'll normally have guarded pushbutton lights, or handles that you pull and turn to do the arm and fire function.

The fire sensing loops are really just a kind of thermal switch that sets off a fire alarm. You use a small stainless steel tube with a wire down the center, snaking around the engine, with a voltage applied to the center wire, and a compound insulating the center wire from the outer tube, that becomes conductive above a certain temperature. High temperature applied at any point along the length of the loop closes the circuit.

So to answer your question, whatever monitor unit or central computer is looking for a fire, in most cases the unit will be supplying 28 volts to the loop, and triggering the alarm if 28 volts appears on the return side of the circuit (because there is a hot spot somewhere on the loop that completes the circuit).

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  • $\begingroup$ why didn't it activate the release mechanism automatically though? wouldn't it be faster and reduce pilot workload? Also, would you mind provide some sources for your description of engine fire sensor set up $\endgroup$
    – Faito Dayo
    Commented Nov 18, 2021 at 3:08
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    $\begingroup$ Because you may not want it to activate as soon as the alarm goes off. On a departure, you worry about flying the plane, then deal with the fire when you can divert your attention to the procedure, at least no sooner than autopilot engagement. Also you normally operate the firewall shutoffs and wait, and only fire a bottle if the fire is still going after an interval. $\endgroup$
    – John K
    Commented Nov 18, 2021 at 4:46

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