Exam Revision Flashcards
What is a wire and what is it’s purpose?
- Single insulated electrical conductor
- Distributes power
What is meant when referring to a wires shielding? What does it do?
Generally copper shielding material prevents electromagnetic interference (EMI) between sensitive circuits
What is a cable?
Two or more insulated electrical wire conductors
What is the ‘norm’ for a bundle common of wires?
May be twisted together or contained in common outer covering of insulation or shielding material, over which can be additional outer insulating jacket
What does a co-axial cable do?
Supply RF energy signals from antenna to avionics systems
What are some systems that use the co-axial cable?
VHF COMMS, LRRA, DME, ATC, VHF Nav
Is co-axial cable one way or two way signal?
Two way signal
What does a thermocouple wire do?
Constructed of special materials required for use in EGT systems
Why are cables twisted?
To prevent extraneous magnetic field interference between sensitive circuits or systems
Why are there multistranded wires?
If one wire breaks, it is still connected
Where is tin coated conducted wire not used? Why?
- Fuel circuits due to fuel corroding wire
- In areas where it can be exposed to operations and elevated temperatures, corrodes.
What sort of conductors are used in conditions of elevated temperatures?
Nickel plated copper conductors
What is the advantage of using multistranded aluminium conductor wires?
Saves weight
Where would you not use aluminium conductor wires?
Areas where there is high vibration
Where are aluminium conductor wires usually used?
Within the pressurised fuselage
What sort of wiring insulation material must be used?
Must be such that no toxic fumes evolve in event of wire failure
What is Flemmings Law?
Cancels out magnetic field by having two wires with return path back to source
Why don’t companies ‘hot stamp’ wires/cables anymore?
Hot stamping can potentially damage wires/cables
How do Boeing number stamp their wires?
- Specific bundle ID number (e.g. W242)
- Specific sequential number (e.g. 001 to 999)
- Gauge of wire (e.g. 22 or 20) W242-001-22
What sort of system does Airbus use to number stamp its wires?
Air transport Association (ATA) system
How does Airbus number it’s wires?
Unique code dedicated to different system (i.e. 23 = comms, 10=HF/VHF) followed by sequential number of wire followed by gauge. 2310-899-22
What questions should be asked before installing a wire?
- What wire gauge
- Is it in a SWAMP area (severe wind and moisture problem)
- Is it subject to corrosion (fuel or hydraulics)
- Does need additional insulation
- Does it need to be shielded temperature range exposed to (-65 to 150)
What should be done with regards to installing bundled wires?
- Prevent electrical interference with other bundles
- Not interfere with any moving control systems
- Have at least 12 mm clearance from any hydraulic line/pipe
- Clipped to secure bundles to airframe or power plant to establish clearance
- Installed and protected in metal conduits or fibre glass raceways to prevent chaffing
- Sleeving to prevent chaffing from vibration
- Wires routed so bends dont exceed minimum bend radius of wire (10x outside diameter of largest wire/cable in bundle)
How are wires/cables etched with id number?
UV laser etching or plastic sleeving that are indelibly
Why is polyvinal chloride insulation (PVC) not used on wires/cables?
Releases toxic fumes when wire broke
What is a disadvantage of using kaplon wiring/cable insulation?
Can exhibit circumferential cracking when acceptable bend radii exceeded.
What are good types of insulators for wires/cables?
- PTFE
- Kapton
- ETFE
What is the first thing you should check before replacing a wire?
The wire diagram manual to find preferred wire/cable type
What would you need to do if you wanted to use a wire not in the WDM?
Seek approval from CASR 21.009 and/or CASR 21.437 CASA delegate
When replacing a wire, what should you do with regards to the WDM?
- Check the wire list (contains info on every wire used in the aircaft as well as manufactures preferred type of wire/cable)
- If wire/cable isn’t on wire list, check the Alternate wires/cables section
- If not on that list, need to get CASR 21.009 and/or CASR 21.437 approval
What must be checked with the connectors/receptacles?
That the pins & sockets are the correct size
Why are solder connections not used any more?
It can run down the wire and lead to wire losing flexibility
What is grounding?
Terminations that require connection to airframe
What considerations are needed for clamping?
- wire bundles properly supporte
- properly sized and correct material type
- spaced correctly for environement
- mounted correctly
What is routing?
Adds wires needed to connect components
Why is routing important?
- Eliminates chaffing
- Makes sure correctly located with respect to proximity to fluid lines
- Correct bend radii
- Protection against cargo damage and/or maintenance damage
- Adequate access for repair/maintenance
What must be placed inside metallic conduit?
Teflon sleeving over wires
Why is an AC (alternating current) preffered to a DC (direct current) system?
- It is simpler, cheaper and more efficient than DC generator to produce same voltage level
- permits the use of transformers to change voltage (either higher or lower)
- Frequency of oscillation (more efficient)
What is the CSD?
- Constant Speed Drive
- Hydromechanically operated gear transmission system that couples engines accessory gearbox to Generator
What does the CSD do?
- converts variable engine speed of accessory gear box to constant speed for driven generator to produce AC electrical power at constant frequency
How does the CSD maintain constant speed?
It has a governor to maintain approx 6000rpm
In the event of CSD/generator malfunction, what can pilots do?
Disconnect generator drive from engine accessory gear box via switch
After disconnection, when can we reconnect CSD/Generator?
Only on the ground
How can we monitor the ‘health’ of the CSD?
Low Press (pressure) and Temp indicator warnings in the cockpit
What sort of power does the generator provide?
115 +/- 3 Volts
400 +/- 5 Hz
Why do we use 400Hz?
- Chose for weight saving purposes.
- frequencies above 400Hz cause skin effects (current flow not evenly distributed)
How is the generator cooled?
Cooling supplied to Generator by ducting from engine fan air
What are the advantages of three phase vs single phase in ac generation systems?
- Can be operated at slower speeds to produce same power
- Peak voltages seperated by 120 degrees and not 360 = steadier power output
In the generator, what is the ‘neutral’ point connected to?
Connected to the ground
What is the volt ampere rating?
Max amount of energy (current) that generator can supply to demands of electrical and avionic services
How much volt ampere can generator supply?
45kVA, note the maximum load is 50kVA however this is never demanded from generator.
What is the purpose of the Generator Control Unit (GCU)?
- Provides automatic operation of control functions of each generator system
- provides signal to ‘trip’ the generator control breaker (GB) (trip = open circuit/take off line)
- regulates generators voltage and limits amplitute of generators phase voltage under fault conditions
What does the GCU protect the generator from?
- Overvoltage (127-133 volts- gb trips)
- Undervoltage (within 5-9 s of dropping below 93-103 volts, gb trips)
- Overfrequency (425-435Hz, gb trips)
- Underfrequency (360-370, gb trips)
- Differential feeder faults (difference between phases exceeds 20 amps, gb breaks)
- Over current (if total load on Generator exceeds 170-175amps, after time delay, GB trips)
What does “BUS OFF” indicate?
When either GEN or APU GEN (aux power unit) is not supplying the GEN BUS
What does GEN OFF BUS light indicate?
When GEN is not supplying its GEN BUS
What does TRANSFER BUS OFF indicate?
When transfer bus is not powered by GEN BUS
What is a relay?
Electromagnetically operated mechanical switch, when current applied to coil, arm moves and completes circuit.
What happens if any generator fails?
- That generators bus won’t be supplied with 115volts
- APU can be started and supply that GEN BUS
Why do we have automatic bus transfer system?
It increases electrical power reliability, i.e quicker response time in supplying failed bus
Why does APU not have a CSD?
After completion of APUs starting cycle, drives generator at constant speed of approx 6000rpm
What is the main difference between 747 and 737 power generation systems?
Very similar, it has four 115V ac 3 phase, 400Hz except it can supply up to 90kVA loads
How many integrated drives and generators (IDG’s does 747 have and how are they connected?
4 and in parallel
How many engine driven generators does an A380 and what is their output?
4 and 150kVA each
How many APU generators does an A380 have? And with what output capability?
Two and 150kVA
While the A380 power system is fully computerised, how does 737 and 747 GCU control their generating systems?
Transistor circuits
How many ac generators does a 787 have?
2 per engine 250kVA each
2 on APU 250kVA each
What can static electricity interfere with?
HF and Nav (ADF in particular)
What does bonding do?
- Ensures aircraft is of same electrical potential
- Metal parts electrically connecte to conductive structure
- Non metal parts bonded to ac structure via conductive return path (conductive paint, aluminium strips, stainless steel mesh)
What in the aircraft is bonded?
- All control surfaces
- Avionics racks
- Circuit ground terminals
- Radio/NAV antenna
- Galley
- Fuel lines
- Hydraulic lines
What can inproper bonding lead to?
- Radio/comms interference
- injury
What is the purpose of bonding?
- Protect from lightning strikes
- Current return paths for electrical/avionic circuits
- prevent development of RF potentials
- protect from electric shock
- prevent interference with radio
- prevent accumulation of static charges
What is the basic bonding process?
- Clear area of paint/con-conductive finishes and corrosion
- Protect area using alodine
- Establish bond
- Measure bonding resistance
- Surface around the bond re-painted
What is the purpose of the HF Comm system?
- Provide comms link between flight crew and ground
- HF Data Links (HFDL)
At what hert does HF comm system transmit/receive?
2-30 MHz
What is HFDL?
- HF Data Links
- Worldwide comms between ac and ground
- works over north pole
- Lower cost than SATCOM
What do a HF Comm system consist of?
- Receiver transmitter (RT)
- Antenna
- Antenna Coupler
- Control Panel
- Remote Electronic Unit
What does the RT do?
- Provides transmit signal and power to antenna at selected frequency
- Processes received signal from signal at selected frequency
- provides AM, Single Side Band Mods, Voice and Data Comms
What does antenna do?
- Radiate transmitted RF energy it receives from RT
- Receives RF energy directed to ac from ground
What determines the efficiency of the antenna?
Length and/or postion of antenna
What sort of aircraft is a fixed wire antenna attached to?
Slower turbo prop and jets
Piston ac
What are the advantages of notch/shunt antenna over fixed wire?
- It’s flush and has no impact on structure leading toreduced drag and less weight
- Doesn’t require lightning protection
Where is the antenna coupler located?
Vertical Stabiliser
What does the antenna coupler do?
Matche antenna impedance to RT over HF range
What does the Antenna coupler tune the antenna to?
50 ohms
What does the control pannel consist of?
- On/Off switch
- Mode Selection (USB/LSB or AM)
- Frequency selection (1 and 10 kHz)
- RF sensivity
What is the signal/noise ratio?
Ratio between magnitude of signal to background noise
Which is better, high or low signal to noise ratio?
High
a low s/n has hiss and static
Which is more efficient, AM or USB/LSB and why?
- USB/LSB, they use less power to carry modulating signal
- Provides greater range (uses all transmitter power to convey voice info)
- Clearer speech
What does Amplitude modulation consist of?
Carrier frequency and corresponding side band frequencies
What does Upper side band consist of?
Carrier frequency plus modulating frequency (FC + FM)
What does Lower side band consist of?
Carrier frequency minus modulated frequency (FC-FM)
What does the REU do?
Process all radio audio signal from each crew station
How is the HF propagated?
- HF Signal leaves antenna and remains near surface (ground wave)
- Sky wave leave antenna and goes up into ionosphere, then reflects signal back to certain frequencies (skip)
Define Maximum usable frequency?
- Highest transmitted frequency that is reflecte back to earth
- Signal absorbed the least
- gives greatest reflection back from ionosphere
Should you use HF when refuelling?
No
What frequency should you use during day and night?
10-30 during day
2-10 during night
How does VHF operate?
Uses line of sight (no ionosphere propagation)
What frequency range does VHF go from?
118.000 to 136.975 kHz
What sort of spacing is there between channels?
25kHz spacing = 760 channels
Some parts of Europe and US, 12.5kHz spacing 1520 channels
How does VHF transmits its radio waves?
AM techniques
What are some advantages of VHF over HF?
- Offers least amount of interchanel interference
- Smaller antenna
- more efficient power trasnmitted from RT to ground receiver
What sort of antenna is both HF and VHF?
Omnidirectional
What range does VHF have?
120nm @ 10000 feet to 245nm @ 40000feet
Where are VHF Comm 1 and VHF Com 2 located?
VHF Comm 1 on top (incase of belly landing)
VHF Comm 2 on belly
What is VHF Comm 3 normally dedicated to?
ACARS (Aircraft Communication Addressing and Reporting System)
What is ACARS?
Simple digital messages to and from aircraft
Out/Off/On/In messages
What does the following mean?
ADO733/0745 EA 0836 SYD
Out ADO0733
Off 0745
ETA for Sydney 0836
How does HF data link differ from ACARS?
- Provides coverage when out of VHF Range (i.e. oceanic and polar routes)
- Transmitted to ground via SATCOM (or other satellite)
How many volts are provided to DC system?
28 volts DC
How is the 28 dc volts provided to DC power systems?
- 3 transformer rectifier units (TRU)
- Battery (nickel cadmium)
- Battery charger
- Cockpit control and indicator panels
What does the TRU do?
Supplied with 115 Vac and converts it to 28 Vdc
What is the advantage of 3 phase ac then rectifying and filtering?
- Much shorter dc wave with less ripple voltage
- Output of TRU is regulated and ensures constant voltage
What is elements is the battery made up of?
Nickel and Cadmium (Ni Cd)
What is the amp hour capacity on the average Ni Cd battery?
36 amp hour capacity
What are the advantages of Ni Cd batteries over Lead acid type?
- Better weight to energy ratio (NiCd lighter)
- Can supply higher current demand and withstand higher charges
- Has Longer life
- more robust
What are the disadvantages of the Ni-Cd battery? And what is done to assist?
- Temperature sensitive during charging, so is fitted with a temperature monitoring device that goes off above 58 degrees
Which is the negative and which is the positive element in a Ni-Cd battery?
Ni = positive
Cd = Negative
What is ‘charging’ of battery?
Conversion of electrical energy into stored chemical energy
What will a 25 ampere hour battery deliver?
25 ampere for a minimum of 1 hour
What is the service life of a Ni-Cd battery?
250 hours
What is supplied by the battery power?
- Ground service ops (lighting) during refuelling
- Starting of the APU
- Last resource of electrical energy in event of total power failure
What is electric load analysis?
- Determines loads battery must supply
- TIme battery endurance under emergency conditions is approx 40 mins
How is the battery charged?
Supplied from 115 Vac Ground Service Bus (GSB)
Sensing circuitry determines charge state and provides rapid charge followed by pulsing charge
When does the charge current cease? And why doesn’t it continue to charge?
When battery is considered by circuitry to be charged. Doesn’t continue to charge because otherwise cell voltage will rise with charging process and charge current requirements will fall
What do TRU1 and TRU2 provide power to?
BUS1 and DC BUS 2
Are TRU1 and TRU2 in parallel?
Yes
What does TRU3 do?
- Is a back up for either TRU1 or TRU2 in event of failure (provided BUS TRANSFER is in AUTO)
- Main function to supply power to 28 Vdc battery bus (providing BAT SW ON)
What is the alternate supply when 115V Main AC BUS 2 not powered?
Hot Battery Bus
What happens when battery switch is selected to on?
Guarded switch goes down
28 Vdc batt bus normally supplied by TRU3 if 115 V Main AC bus 2 is powered
What happens when battery switch is off?
- Guard is lifted
- No power is supplied unless Standby Power is in ‘BAT’ position
- Prevents ignition of fumes
What is it called when the battery is unservicable?
Total Discharge State
Where is the Auxiliary Power Unit started from?
AC Battery on ground
How much AC power via generator does Auxiliary Power Unit supply on ground and in air?
60kVA on ground and 50kVA in air
What is Auxiliary power Unit powered directly from?
28 Vdc Hot Battery Bus
How is the Auxilary power unit started?
- Battery charger disconnected from battery bia APU start interlock relay
- This realy prevent battery charger supplying any dc current via 28 Vdc hot battery bus for APU start
In the event of an emergency, what systems powered by the battery?
- VHF Com 1
- Engine and APU Fire detectors and extinguishers
- STBY FLT Instruments (ALT, IVSI and ADI)
- STBY RUDDER FLT Control Hyd Valves
- APU Starting
- No1 IRS
- Engine Fuel Shut off an Cross feed valves
- Engine Wing anti ice
- Landing gear indicator and aural warning
- Engine ignition system via ac standing bus