Fire Detection and Warnings Flashcards
Overheat Warning Systems
Triggered by hot gases
Risk of damage other than by fire
Designated Fire Zones
Area where failure or leakage may pose a fire risk requiring protection
Engines and auxiliary power units protected by a firewall which will prevent further spreading
Location of Fire Detectors
Rapid warning of a localised fire or overheat condition Rear of engine Wheel wells Auxiliary power unit Cargo compartments
Fire Warnings in the Cockpit
Red lights
Audible warnings
Unit Type Detectors
At points most likely to be affected by fire
Thermocouple detectors
- In engine bays
Bimetallic switch
- When heated two different metals expand at different rates
Continuous Type Detectors
Continuous loop (continuous wire loop) Resistance type Capacitance type
Fire Detection Systems
Fenwall: tube with a single wire
Kidde: tube containing 2 wires embedded in a thermistor core (will sense average and hot spots)
Smoke Detectors
Cargo holds, equipment bays Air from compartment is routed through smoke detectors Show which compartment is affected Photo-electric cell Visual smoke indicators
Fire Extinguishing Equipment
Fixed (permanently installed)
Mixed
Portable
Extinguishants
Methyl bromide (MB) Bromochlorodifluoromethane (BCF) Carbon dioxide (CO2) Water Dry chemical
Methyl Bromide (MB)
Low boiling point (approx 5 degrees celcius)
Protection of engines
Toxic (not used in cabins)
Bromochlorodifluoromethane (BCF)
Electrical and flammable liquid fires
Semi-toxic: vent cabin
Carbon Dioxide (CO2)
Electrical and flammable liquid fires
Takes away heat and oxygen
Water
Combustible materials only
Not suitable against electrical of flammable liquid fires
Typical Fixed Systems
Circuits controlled by switches or handles
Triangle of Fire
- Heat or source of ignition
- Fuel (material that will burn)
- Oxygen
Without one of these components fire will go out
Differential Expansion Detectors
Consists of a pair of contacts mounted on a spring bow assembly fitted within an expansion tube
When heat is applied, the tube expands at a greater rate than the bow drawing the contacts together, providing power to the warning circuit
Short time delay before the warning is activated to prevent false warnings
Fire Wire Systems
Continuous Loop Fire Detectors
Consists of a tube, with a central electrode insulated by a core of temperature sensitive material
Core has either a negative coefficient of resistance or a positive coefficient of capacitance
Will continue to work with a single break as the loop is connected at both ends
React to isolated temperature increases or if the temperature of the whole detector increases
Positioned around the engine fire zones as a continuous double loop and both must detect a fire to initiate the warning
Resistance Continuous Type Detector
As the temp increases, the resistance of the insulating material decreases causing the current to flow between the electrode and the outer tube to increase, until the warning system operates
A short circuit could give a false warning
Capacitance Continuous Type Detector
Core material is dielectric
The case and inner electrode are poles connected across an AC supply
Will not give false warnings
As temp increases, the properties of the inner core material change, increasing the capacitance of the detector and the feedback current will reach a level and activate the fire warning
Gas Filled Detection System
Consists of a number os sensing elements
Tube has a central core giving off hydrogen gas when heated
Helium gas is forced into the tube under pressure and sealed
Pressure monitored by two switches
Uses the double loop system
The Integrity Switch
Closed at the tubes under normal pressure
If damage occurs and helium gas is released the pressure decreases and the switch will open indicating a fault
The Alarm Switch
Open under normal pressure
If heated locally, hydrogen gas will be released, pressure will increase and the switch closes initiating the fire warning
If exposed to a hot gas leak, helium pressure will increase and close the switch
