Now I was told that the way that it works is that some engine fire 'loops' are basically constructed in a way that has a central core of conducting wire, this is then sheathed by a normally non-conducting (dielectric) material, which is itself is then sheathed by another layer of conducting material.
The ends of the conducting materials are linked to the fire detection unit, and an electrical signal/potential is applied to each.
Now clever bit is that with an increase of temperature the intermediate dielectric 'breaks down' and becomes conductive - allowing current to flow between the inner and outer conductors - and one would imagine that it's the level of current flow sensed between the two that triggers either the overheat or fire annunciation.
But perhaps the cleverest bit is that even if the wire is cut (i.e. the loop is broken) the system will still work.
For those who fly the B737-3/4/500, you'll know that there are two detector loops and two levels of fire/overheat detection…. HOT, and HOTTER.
If a HOT situation exists, you get an OVERHEAT indication.
If HOTTER, you get the full blown FIRE indication.
During NORMAL operation, both of the detectors must sense an alert before it is displayed.
Now the detection system needs to be able to handle internal faults and to that end it has a FAULT DETECTOR SYSTEM which automatically deselects a faulty detector…. and here's the rub... the system will NOT turn on the fault light to tell you that you have lost a detector loop, unless you do an OVHT/FIRE test.
The subsequent test is a tricky thing to do: If the OVHT DET switch is NORMAL and you get a FAULT light... you have a failure of both detector loops on one or both engines.
If you then push the OVHT/FIRE test switch and you get a FAULT light... how do you determine which loop is the one with the fault ? Well, push OVHT DET switch to A (or B) and if the light comes on, that is the broken one.
Uhm, but can the system still detect a fault ? Well place the FAULT/INOP and OVHT/FIRE test switch to the FAULT/INOP position and if the FAULT light comes on you're in luck it's still working !
Of course please do check this in your Technical Manual and / or with a Training Captain.
Ps. Any engineers out there got some more info on how these systems really work ?