11.8 - Fire Protection Flashcards
What are the 3 requirements for a fire?
- Oxygen
- Fuel
- Heat
What type of fire is a ‘‘class D’’ fire?
Fires in which some metal burns.
Typical occurences include brakes & wheels
List 5 requirements for a fire detection system.
- Must not give false warnings
- Must give a rapid indication and accurately identify it’s location
- Must accurately indicate when a fire has been extinguished
- Must sound a warning if a fire re-ignites
- Must continue to indicate the presence of a fire as long as the fire exists.
What is the difference between a fire detection system and an overheat detection system?
A fire detector system warns the flight crew of the presence of a fire that raises the temperature of a particular location to a predetermined high value.
An overheat detector initiates a warning when there is a lesser increase in temperature over a larger area.
Explain the operation of a Thermoswitch Type fire detection system?
There are a number of bimetallic thermoswitch type spot detectors in a circuit. If a detector is heated in the presence of a fire, strips distort and close the contacts, completing the circuit between loop and ground.
What does a thermocouple measure?
A thermocouple system monitors a specific location and triggers the fire warning if temperature rises a great deal faster than the entire compartment.
What materials are used in Thermocouples?
A thermocouple is two different types of wire welded together and the point where they are joined together is called the junction.
Materials of wires are typically iron and constantan
Describe the operation of a Fenwall Fire detection system?
Single-conductor element has a center conductor supported in a thin-wall inconel tube by ceramic beads.
An electrical connection is made to the conductor, and the outside tube is grounded to the airframe.
The space between the beads is filled with a eutectic (low melting-point) salt whose resistance between the center conductor and the outside tube drops, and signal current flows to initiate a fire warning. Salt will solidify when fire stops.
In a thermoswitch type spot detector, what happens when the test switch is closed?
Whent the test switch is closed, it provides a path to ground through the test relay. This will energise the relay and cause it to switch contacts which will then provide another path to ground which goes through the fire-warning light and fire bell, activating both.
What is the function of the reference junction in a thermocouple type fire warning system?
The reference junction is a thermocouple that is surrounded by insulation preventing it’s temperature from changing rapidly.
In the event of a fire, all other thermocouple temperatures will rise rapidly and this will create a voltage difference between the reference junction and other junctions which will allow current to flow and close activate a sensitive relay closing a fire warning circuit.
The test switch of a pneumatic type loop system is pressed but nothing happens, what is likely to be the fault?
The likely fault is a break in the tube resutling in a loss of the gas (helium) pressure.
This type of fault will result in the integrity switch contained within the housing being opened as opposed to it’s normally closed state. The helium pressure is what keeps the integrity switch normally closed.
Describe the operation of a photoelectric type smoke detector.
Air is drawn through the detector chamber and a light source shines a light beam in the chamber. If smoke is present in the chamber, light is reflected into the photocell (also installed in the chamber) which changes it’s conductivity.
The photocell is in a bridge circuit and any change in the amount of current it conducts will trigger a smoke-warning signal.
What are the advantages of using HALON in a fire extinguishing system?
HALON fire extinguishing agents are non corrosive, evaporate rapidly, leave no residue and require no cleanup or neutralization.
In addition, Halon 1301 does not requie any pressurising agent.
What is the function of the safety plug in a High Rate Discharge(HRD) extinguishing system?
The safety plug in a HRD extinguishing system allows the gas to vent to atmosphere if the temperature of the compartment where the bottle is mounted rises enough to become dangerous.
The plug does this by melting under the temperature increase. The gas blows out a red indicator disk indicating bottle discharge due to an overheat condition.
What specific items would you inspect on an installed High Rate Discharge(HRD) extinguishing system?
Aircraft Maintenance Manual. I would also inspect inspect the fusible safety plug to ensure it has not melted due to excessive temperature in the form of an overheat condition. Finally,
I would also check for the indicator disks. I would check for the red indicator disk indicating discharge through an overheat condition and for the yellow indicator disk which indicates discharge through normal operation of the disk.
The red blow-out disk visible near the extinguishing bottle is missing
This means:
That the fusible safety plug has melted and allowed the gas to escape due to an overheat condition.
What is the requirements for a fire?
- Oxygen
- Heat
- Fuel
In thermoswitch fire-detection systems using several thermoswitches and a single indicator light, the switches are wired in _______ (parallel / series) with each other, and the entire combination of switches is in ______ (parallel / series) with the light
Parallel
Series
The fire detection system that can operate properly when there is either an open or a short circuit in either of its two loops is the ______ (single-terminal or two-terminal) thermoswitch system.
two-terminal
The fire detection system that initiates a fire warning when there is an excessive rate of temperature rise in the monitored compartment is the _______ system
(thermocouple or thermoswitch)
Thermocouple
Current flows in a fire-warning thermocouple circuit because of the voltage produced by the difference in the _____ of any of the junctions.
temperature
A pneumatic fire detection system _____ ( does or does not) detect a general overheat condition as well as a local hot spot
does
(pneumatic fire detection system = Systron-donner)
Some of the air in an ionization-type smoke detector is ionized by a tiny piece of ______ material
radioactive
Describe the operation of a Walter Kidde detection system?
Walter Kidde Detection System = two conductor loop
This system has two parallel wires embedded in a thermistor material whose resistance decreases as temperature increases.
One wire is grounded to tube, and the other is connected to a control unit which continually measures the total resistance of the loop. Can detect overheat condition + single hot spot.
How does a Systron Donner type detector work?
(Systron Donner type detector - Pneumatic-type Continuous Loop System)
Tube filled with helium gas under pressure with titanium wire in the middle.
When the pressure of the helium becomes great enough due to fire it closes a set of normally open contacts initiating fire warning.
How do you check the contents of a HRD bottle?
A gauge shows the pressure of the agent and the gas in the container
