Aircraft fire protection systems Flashcards
What are the principles of extinguishing fires?
- Cool the fuel below its ignition point
- Remove the oxygen supply
- Separate the fuel from the oxygen
What are class A extinguishing agents?
Responds best to water or water type which lower the fuel below its ignition point
What are class B extinguishing agents?
Responds to carbon dioxide, halogenated hydrocarbons, and dry chemicals, all which displace the oxygen supply making combustion impossible.
- Dry powder
- Halo
- CO2
What are class C extinguishing agents?
- Respond best to CO2 which displaces the oxygen
- Must have a non-mettalic horn to prevent static electricity and transmission of electricity to operator from the electrical source.
- Halons or dry chemicals can be used but this could lead to further contamination and damage of electrical systems.
What are class D extinguishing agents?
Responds best to dry powder which prevents oxidation and the resulting flame.
Where are aircraft required to have protection systems?
In designated fire zones and potential fire zones
Smoke detection and Gas detection principles?
- Used for anticipating a fire that produces smoke before significant temperature changes
- Typically in cargo/baggage and lavatories
- Light refraction type (photoelectric cell)
- Ionisation type
- Solid-state type
Types of fire/overheat detection systems?
Thermo switch thermocouple Tubular type - Fenwal - Kidde -Pneumatic type
Explain the thermo switch system?
- A circuit where one or more thermal switches are connected in parallel, and then to a warning horn and an aural alarm system
- (spot detector) works by the expansion of the outer casing unit
- When exposed to heat the casing becomes longer, causing the two contacts inside to meet closing the circuit
- Closing the circuit activates the warning system on the flight deck.
Explain the thermocouple system?
- Rate of the rise detection system
- A circuit where one or more thermocouples are connected in series to activate an alarm when there is a sufficient temperature increase at the sensor
- Thermocouples made of two dissimilar metals which are twisted together inside an open frame
- Allows for flow through the probe not allowing damage to the wires.
Explain the fenwall system?
- Continous loop system
- Consists of a small lightweight, flexible Inconel tube with pure nickel wire conductor running throughe middle
- The space between the nickel conductor and tubing wall is filled with porous aluminum oxide, ceramic insulating material
- Any voids or clearances are saturated with a eutectic salt mixture which has a low melting point
- Tube is hermetically sealed at both ends, with insulating materials and threaded fittings.
How does the fenwall system detect heat?
- When heated, current can flow between the centre wire and the tube wall because the eutectic salts melt, and the resistance drops rapidly.
- The increased current flow provides a signal. which is used in the control unit to sound an alarm.
- Once the fire is extinguished or overheat condition is corrected eutectic salt increases its resistance and the system returns to stand by mode.
Explain the kiddie system?
- Continous loop system
- Uses an Inconel tube with a thermiresistor material embedded with two electrical conductors (which helps to prevent redundancy)
- One conductor is grounded to the outer shell at the end of the tube
How does the kiddie system detect heat?
- When heated the thermistor material drops in resistance
Explain the Pneumatic System?
- Continous length system
- Sensing element consists of a stainless steel tube containing two separate gases plus a gas absorption material in the form of a wire in the tube.
- Tube under pressure and filled with helium gas
- The titanium wire centre, which is a gas absorption material, contains hydrogen gas.
- Wire wrapped in helical fashion with inert metal tape for stabilization
- Gaps between the turns of tape allow for rapid release of hydrogen gas from the wire when the temperature reaches the required level
