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Fire Detection

Fire Protection

Fire protection systems are installed on aircraft to detect and protect against an outbreak of fire. You will find these systems in and near the engine compartment and in the fuselage to protect the occupants.

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 is 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 your own experimental aircraft have the option to install an small fire extinguisher (remotely controlled) under the cowling, space and weight permitting of course.

Fire Detectors

Thermocouple

There are two basic type of unit fire detectors in use, this is the thermocouple and a bi-metallic switch.

A thermocouple sensor consist of two dissimilar materials which will generate a small voltage when its temperature is raised. Thermocouples are also used in exhaust gas temperature (EGT) or cylinder head temperature (CHT) sensors for use in engine management systems and indication in the cockpit.

Bimetallic switch

The bi-metallic switch detects a temperature change as each of the two dissimilar materials have a different expansion rate. Thereby deforming the metal arm and thus contacting a switch, and opening or closing it depending on the type of switch that is implemented.

Fire can also be detected by infrared sensors connected to a circuit where a threshold is set to indicate any temperature that has gone up or out of range. The pilot may then decide if any action might be required to handle the situation.

Detection loop

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 of material. This material will become a conductor when a certain preset 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.

Fire Extinguishers

Fire extinguisher bottles

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 of the aircraft.

The gas is then 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.

Extinguishing agents

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 reduces the temperature with the exactly the same result: the fire can not sustain itself.

Carbon Dioxide (CO2)

This type is commonly used in cargo/ freight compartments. It is also found 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)

It is commonly used in engine bay compartments, it is much more effective with an equivalent weight compared to CO2 agents. It is also toxic for humans and thus unusable in the cabin or cockpit.

Dry Powder

Usually found in handheld extinguishers in the galley areas. It consists basically of a powder and is very effective against liquid, electrical fire and the usual combustibles such as wood, paper and fabrics. Perfect for this application!

Written by EAI.


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