Fire protection systems are installed on aircraft to detect and protect against an outbreak of fire. You will find these systems near the engines and in the fuselage.
These systems monitor the conditions which could lead to a fire and are comprised of smoke detectors, heat sensors near engines or hydraulic systems and visible and audible warnings in the cockpit.
The chance of a fire in small general aviation or experimental type aircraft are very remote but at least the engine compartment should have some form of detection as this is the area where heat, air and a combustible fluid are together in close proximity.
You as the builder of an experimental aircraft have the option to install an fire extinguisher under the cowling, space permitting of course.
There are two basic type of unit fire detectors: thermocouple and a bi-metallic switch.
A thermocouple consist of two dissimilar materials which will generate a small voltage when its heated. They are also used in exhaust gas temperature (EGT) or cylinder heat temperature (CHT) sensors.
The bi-metallic switch detects a temperature change as each of the two dissimilar materials have a different expansion thereby deforming the metal arm and contacting a switch, thereby opening or closing it depending on the type of switch that is used.
Fire can also be detected by infrared sensors connected to a circuit where a threshold is set to indicate a temperature that has gone up and action of the pilot might be required.
A different method of fire detection uses a continuous loop device. This is a thin wire inside a capillary tube insulated with a thermistor type material. This material will become a conductor when a certain high temperature is reached.
When the loop is triggered (by heat) a small current will flow and this trips a detector circuit and warning device. After the temperature drops (fire is extinguished) the wire will stop conducting and the system is reset and can be used again.
In fixed installations the gas is held in pressurized bottles ready to be expelled by either an electric trigger or automatically by inertia switches in the event of a crash.
The gas is released from the bottle and routed through tubing to nozzles to be expelled in the engine or cargo compartment. Older installations use a one shot (one time use only) or a double shot system.
To put out a fire you will need to take away one of the three items that sustain the fire: air, temperature or the combustible agent. A fire extinguishing agent replaces air with an inert gas so that combustion can no longer take place. Water cools the temperature with the same results.
Carbon Dioxide (CO2)
This type is commonly used in cargo/ freight compartments. Also in small handheld extinguishers in the cabin, but the size must be in proportion with the cabin to allow for sufficient breathing by the occupants after releasing of the agent.
Methyl Bromide (MB)
Commonly used in engine bay compartments, much more effective with an equivalent weight compared to CO2 agents. It is also toxic and thus unusable in the cabin or cockpit.
Usually found in handheld extinguishers in the galley areas. It is a powder and very effective against liquid, electrical and the usual combustibles such as paper and fabrics.