15:20 Fire Protection Systems Flashcards
Fire detection systems are always installed in areas where technical failure or human error are most likely to produce a fire. Where are these areas
Engine and APU areas
All fire detection systems contain what 3 main components
- A sensor located in the monitored area
- A control unit, primarily located in the avionics compartment which receives the signals from the sensor
- Warning panels, which are mostly located in the cockpit
Other than detecting fires what do the detection sensors also detect
They are used for overheat detection
The fire detection system finds a fire by detecting what?
The heat of the fire
What does the spot sensor use to detect fires
Usually a bi-metallic switch
How does a bi-metallic switch work
It triggers an alarm when it closes due to the heat
This type of sensor is used for pneumatic ducts because many individual sensors are needed for larger areas and because defective sensors are hard to detect
What are continuous loop sensors usually used on
Engines
What are the two different types of continuous loops
- The semiconductor loop
- The pneumatic pressure loop
They look almost identical but work differently
What is the pneumatic pressure loop called
A Systron-and Donner loop after the manufacturer
How does the pneumatic pressure loop work
It consists of a steel tube which contains gas. When the pressure in the tube increases due to higher temperature it operates a pressure switch which, when closed, triggers the alarm via the control unit
This switch is located in the thicker end of the loop and is called the RESPONDER
What are the other names of semiconductor called after their manufacturers
Kidde, Graviner, Edison or Fenwal
How does a semiconductor loop work
Each loop has a metal tube that contains a semiconductor material. This material decreases its electrical resistance with increasing temperature. An alarm is triggered when the resistance between the central wire and the tube decreases below a predetermined threshold
What is the purpose of the control unit
The control unit identifies the normal and alarm conditions of the sensor with a measurement of the input resistance
What is the local fire warning
When an alarm condition from the sensor is detected, the fire warning of the specific system is triggered, illuminating a red fire warning light on the overhead panel
How long does a local warning stay on in the cockpit
As long as the fire detection loop detects a fire
What type of systems are all essential fire warning systems
Dual loop
How does a duel loop type of sensor work
A fire warning is only triggered if both sensors detect the fire at the same time. This AND logic prevents a false fire warning if one of the two sensors is set to the alarm condition due to a failure
What type of detection system is not ideal for fire warning systems
The bi-metallic switch as it is easy to trigger and alarm
What 3 main functions do all fire detection control units usually have
- They receive electrical signals from the sensors and identify the normal and alarm condition from this input signal. From the alarm condition the control unit activates the output signals which usually cause a fire warning
- The control unit in most systems can also identify fault conditions and activate the necessary functions
- It controls the test functions
When would it be necessary to perform an operational test on the fire detection system to ensure it is working properly
Before each engine or APU start
What does a fire detection operational test do
Simulates a fire or overheat condition for the control unit and ensures that the sensors are connected correctly
How could you find a fault in a fire detection system
The central maintenance computer system can be used in modern aircraft
What does each semiconductor loop consist of
A stainless steel tube filled with a semiconductor ceramic material with a wire in the centre
The ceramic material acts like an endless number of resistors. The resistors have an NTC characteristic, because of the semiconductor material
How are all resistors connected in a semiconductor loop
In parallel on one side by the internal wire that is connected to the control unit. On the other side they are connected by the tube that is connected to the ground
What manufacturer is the semiconductor loop type which contains 2 internal wires called
Kidde
What manufacturers make the single internal wire loop type
Fenwal, Graviner or Edison
What is the normal total resistance between all parallel resistors
Between 1mega ohms for long loop and 10mega ohms for short loops
If there is a fire or overheat the resistance in the loop decreases below what resistance
500 ohms then the alarm is triggered
If one loop detects a fault on a double loop system what would type of warning would a pilot receive in the cockpit
A loop fault message to identify a failure
When installing new loops what maximum bend radius must not be exceeded
Not more than 3.2mm maximum, and no more than a minimum of 25.4mm must be obeyed
What does a modern gas pressure loop consist of
Has two pressure switches and a resistor
The first one is the monitor or integrity switch, it is normally closed due to the high gas pressure in the loop
What is the amount of pressure found in a modern gas loop system
About two times higher than the ambient pressure at sea level
What is the second switch in the pas pressure loop for
The alarm switch and closes at higher gas pressure, caused by overheat and fire
What is the purpose of the resister inside the transponder on a gas pressure loop
It allows the control unit to identify which pressure switch is open or closed
At what temperature does high gas pressure close the alarm switch for it to be triggered
When the complete loop is more than 200 Celsius or one small part is over 400 Celsius
What type of indication would a pilot receive if a loop is damaged
The gas pressure decreases and the monitor switch opens
This will trigger the fault warning
What are the main components of a basic fire extinguishing system
- The extinguisher bottle
- The discharge head with the cartridge
- The piping with the spray nozzle
The system also needs electrical circuits for system control and monitoring
What are fire extinguisher bottles made from
It is a spherical stainless steel container
What fire extinguishing agent do the fire extinguishing bottles contain
Halon 1301 in a liquid format
How is the extinguishing agent pressurised in the bottles
It is pressurised by nitrogen at around 600psi at 20 degrees Celsius
This guarantees the complete discharge of the bottle when it is used
What does the size and weight of a fire bottle depend on
The amount of halon that is necessary to extinguish a fire in a compartment
Where are the largest fire bottles located
Cargo compartments - weight around 20KG
Engine bottles - Weight around 10KG
APU - Weight around 5KG
When would the weight of a fire bottle be checked
During a C-check
What is the purpose of weighing the extinguisher bottle
The measured bottle gross weight must be the same as marked on the bottle. The bottle is measured with out the discharge head. A correct result would indicate there has been no leakage
How is the halon discharged from the bottle
Through the discharge head and an electrically controlled cartridge that is installed in the bottom of the discharge head
Is it possible for a bottle to have more than one discharge head
Usually only one discharge head is found however ones with two and three do exist. The amount of discharge heads represent how many individual cylinders the bottle has. The heads will be identical
How much explosive powder does the discharge cartridge in the centre of the discharge head contain
Around 400 milligrams of explosive powder
How does the powder in the discharge cartridge explode
When an electric current heats up a wire inside the cartridge. The wire is named the squib. The explosion of the cartridge is normally controlled by the electrical firing circuit
Who is permitted to handle the discharge cartridge
A person licensed to work with class C explosives
When removing the discharge cartridge what must be placed on it to prevent from exploding due to static electricity
An approved electrostatic protective cap is always installed to prevent any explosion due to static electricity
What is the maximum service life of a discharge cartridge
6 years
What is the purpose of the pressure switch on fire bottles
It controls a light on the fire panel in the cockpit when the bottle pressure decreases below 50% of the normal pressure
Some bottles have a pressure gauge for maintenance checks. When reading the gauge what must an engineer be mindful of
The bottle pressure changes with temperature
In the case of an emergency the frangible disk on fire bottles has a pressure relief function. At what temperature will the disk brake
At around 1700psi
The fire extinguisher bottle has an extra overpressure relief valve when the bottle and the spray nozzle are in different compartments. How does this work and where would you find it
On the APU.
The relief valve is a thermal fuse which melts when the temperature exceeds a safe value
What is the purpose of the yellow disk
It is on the fuselage near the red disc
The yellow disc is pushed out by a small piston if the bottle discharges via the spray nozzles
Systems must be isolated when there is a fire, which is done automatically and which is done manually
The APU is done automatically and the engines must be done manually via the fire handle, this cuts off fuel and hydraulic supply to the engines
To make the firing circuit as safe as possible what is done to make sure there is always power supplying the circuit
It is connected to the hot battery bus which is always connected by the batteries
How could you ensure the squib is still serviceable
A squib test can be carried out in the cockpit either on the squib panel or the fire panel.
If the squib light comes on the system is ok and if there is no light there is a brake in the system
Why must you be careful when replacing the squib bulb in the cockpit
If a bulb which requires a higher current is used the current could be enough to fire the squib
For engine fire systems what do regulations state with regards to fire bottles
They require a ‘two shot’ system. This means that the content of two fire extinguisher bottles can be discharged in to one engine
Before discharging the extinguishing agent in an engine what must be done first
Engine shut down is required and isolate it from all of its supplies
When the fire button is pressed what functions happen
- The engine driven generator Is switched off because the generator control relay and the generator power relay open
- The bleed air valve closes
- The low pressure fuel shut off valve closes
- The hydraulic valve closes
- The central warning system gets a signal and updates the ECAM displays
- The fire bottles become armed
What is different about each bottle on a 4 engines aircraft
They have 2 heads as they supply the engines in pairs
Where is the APU fire bottle located
In the stabiliser in front of the APU firewall
Only a 1 shot system is required for the APU
Where is the APU fire test done
On the CMC to ensure the bottle doesn’t fire
