Part 6 - Communication Systems Flashcards
What are the band of VHF frequencies used by aircraft?
118.000 MHz to 137.000 MHz
What is the channel spacing for VHF comms above FL195?
8.33 kHz
What is VOLMET?
Weather information transmitted from ground stations on specific frequencies in the VHF band
What is the propagation method and theoretical range of VHF?
VHF uses line of sight space wave transmissions with a theoretical range of 123 NM between aircraft at a height of 10.000 feet and a ground station at sea level
What is speech AM used for?
Radio telephony transmissions which are vertically polarised transmissions with a range of speech frequencies being from 300 Hz to 3 kHz
In a VHF transceiver, what happens when a PTT is initiated?
The PTT signal is routed directly to the microprocessor system, and routed out of the transceiver as the KEY EVENT signal
This signal provides the SELCAL reset and the key-event time recording on the DFDR
What is the typical output power of a VHF system?
30 watts
What does a VSWR of greater than 2:1 indicate?
The transmission line is unserviceable
How is sealing usually achieved between the aircraft skin and an aerial base of the fin variety?
Suitable gasket or a sealing agent of the zinc chromate variety or a combination of both
What type of aerials are of ridig construction, with a typical thickness to chord ratio of 10:1?
Blade
What are the band of frequencies normally used for HF comms?
3 - 30 MHz
What type of electrical connector is used to connect the aerial to the transmission line in a comms system?
Co-axial
Why is the left VHF/UHF transceiver always fed from the standby bus?
If all generators fail, including the APU, the pilot has about 30 minutes to get the aircraft down, and they must be able to carry out communication with ATC
In the event of total aircraft power failure, with only battery power available, which of the following will not be sharing battery power - standby horizon, HF system or the emergency lights?
HF system
With VHF frequency tolerance of 25 kHz channel spacing, what is the tolerance of the operating frequency?
30 parts per million
To how many decimal places should a VHF system using phase locked loop (PLL) and a voltage controlled master oscillator (VCMO) be accurate to?
4 decimal places
Where will forward power be at a maximum in a VHF system?
Close to the radio rack at the Tx/Rx
Why might HF be installed on aircraft, as well as VHF?
The aircraft might be flying routes where, for some part of the flight, aeronautical radio stations are out of VHF range
HF is provided for long distance communication where aircraft are flying over wide areas of water or sparsely populated land masses
HF systems normally use what kind of transmission?
SSB
How do HF DSB systems increase the frequency of the oscillator?
By non-linear (class C) frequency multiplication
What frequency multiplication do HF SSB system use?
Must be linear, and class AB1 is the best available
What are the advantages of SSB over DSB?
- Uses less than half the bandwidth
- Uses less power
- S/N ratio is 9dB better with SSB
- Selective fading will not destroy the intelligence and there is no carrier to lose
Where is AGC derived in a HF DSB receiver?
Detection of the carrier RF
Where is AGC derived in a HF SSB receiver?
Detection of sideband RF since there is no carrier present
How are SSB transmitters usually power rated and how is this defined?
Peak envelope power which is defined as the RMS power at the peak or crest of the envelope
In DSB HF, where is sidetone usually taken from?
The Power Amplifier (PA) output to the aerial circuit
What ARINC specification does HF conform to?
ARINC 719
What would a typical HF antenna impedance be?
50 Ω
What is the purpose of the coupler unit and what is particular about the unit?
Matches the HF antenna with the output circuit (50 ohms) of the HF transceiver
It is a pressurised sealed box
What type of aircraft might make use of Notch Antennas?
High speed aircraft
On HF wire antennas, what would happen where the antenna load exceeds 190 lbs?
A copper pin will be sheared, allowing a 3/16” extension of overall antenna length, resulting in a reduction of tension
The movement also exposes a yellow warning band on the unit
What is the most common technique to minimise ‘cross-talk’?
Attenuate the unselected path by use of resistive potential dividers
Which intercommunication system allows the aircrew to talk to each other without any ambient noise or interruptions?
Flight interphone
What is service interphone?
Intercom between the flight deck and maintenance points
Where would a headset socket be located for the service intercom?
- Engine nacelles
- Refuelling points
- Avionic and hydraulic bays
- APU
- Nosewheel
What would extinguish automatically if not cancelled by the RESET key on the ACP?
MECH or ATT call lights
What is the correct order of priority of PA messages?
- Flight crew announcements
- Cabin crew announcements
- Pre-recorded announcements
- Boarding music
What will be the result of a successful self-test of the CVR system?
You will hear 4 sequential tones, each of 600 Hz
A corresponding deflection on the meter from red to green for each tone
What two precautions should be followed with regards to removing and installing a ULB battery?
Do not clamp the ULB in a vice or damage to the unit will occur
Incorrect polarity will permanently damage the ULB
What is the purpose of the shorting plug for the SELCAL system?
Determines the aircraft SELCAL identification code
What are the outputs of the detector in the SELCAL system?
- Alert lamps - each channel outputs a discrete (ground) to an alert lamp on the pilot’s call panel whenever the tone combination is recognised
- Chime - The decoder can output a discrete (ground) to the Warning Electronic Unit (WEU) to sound a single HI (587 Hz) chime for any channel activated
What is ACARS?
Aircraft Communication Addressing and Reporting System enhances air-ground communications and reduces flight crew work load through the use of high-speed , digital data link messages
Also allows for ground monitoring capability of aircraft engines and other parameters
In a CVR, what are the two most critical tracks of a four-track tape?
Tracks 2 and 3
Relate to captain and area microphone
What component would cut all electrical supplies to the CVR?
Inertia switch
What is the frequency and length of battery for the ULB?
37.5 kHz and 90 days
What are the two modes which ACARS operates in?
- Demand
- Polled
What is polled mode in ACARS?
Allows only the ground processor to initiate communications
What is demand mode in ACARS?
Allows either the airborne station or the ground processor to initiate communications
What will happen in demand mode with ACARS if the message sent is not free from errors?
No acknowledgement signal (ACK) will be sent and the airborne system will retransmit the message
After six attempts without ACK, the airborne system will alert the aircrew
How can a SELCAL alert be cancelled?
- Pressing the illuminated light/switch
- Associated radio’s PTT mic switch is pressed
What type of fading is the HF band prone to?
Selective fading
What facilities does the AIS provide?
- Service selection
- Flight interphone
- Service interphone
- Cabin staff interphone
In the AIS, what stops 2 channels from being selected simultaneously?
Only one transmission can be selected at a time, because the buttons are mechanically interlocked
What is the purpose of the flight interphone?
Provides a means for communication between flight crew positions
What are the internationally agreed requirements of a CVR?
- The last 30 minutes of flight deck communications (internal and external) and ambient noises will be retained on an endless magnetic tape
- The recording will survive physical extremes
What are the four channels of input for a CVR?
Channel 1 - Flight engineer (or other parameters in helicopters) - track 4
Channel 2 - First officer - track 1
Channel 3 - Captain - track 2
Channel 4 - Area microphone - track 3
What does the SELCAL shorting plug establish?
Four four-bit codes corresponding to one of the 16 audio tones used in SELCAL calling
In ACARS, what is OOOI?
- Out of gate
- Off the ground
- On the ground
- In the gate
What is the maximum message length for ACARS and what happens when it is exceeded?
Maximum message block length is 220 characters of text
Messages longer than one block are divided into a number of blocks which do not exceed the 220-character limit
What are the advantages of the Integrated Modular Avionics system?
- The platform is a standard item, it is the applications which decide its function
- There is a standard that all IMA platforms should conform to
- A modification of a system is normally a software upgrade
- Ideally platforms are interchangeable so less spares need to be kept in stock
What ARINC protocol deals with Application Programming Interface (API)?
ARINC 653
What is a virtual link?
There is a set period of time for data transmission, which is a form of timeshare of the available physical link
What is the ARINC protocol for Automatic Full Duplex Exchange (AFDX)?
ARINC 664 Pt 7
What are the transmission rates of ARINC 664?
100 Mb/s for normal operation
10 Mb/s for safety critical systems
What is the maximum size of the AFDX data payload?
The size of the AFDX packet will vary depending on the amount of payload information sent
Overall, the size can range from 1 to 1471 bytes
The minimum payload size is 17 bytes, but if the it is less than this, then padding bytes are used
What is the purpose of the sequence number in AFDX?
The SN describes the frames position in a sequence of frames
When the frames reach the destination by different routes, they enter the end system on separate inputs
Here they are error checked by the redundancy manager and the one with the most up-to-date SN is then passed to the application
Which is bigger in size, implicit or explicit?
Explicit due to the extra information within this message, a larger bandwidth for this package is required
What are the three information areas for OIS?
Flight, maintenance and cabin
What is the Network Server System (NSS)?
Central point through which terminals are able to communicate with avionics systems, access data and applications stored in the NSS mass memory storage
What does a typical NSS include?
- File server
- Data processing
- Mass storage
- Human-machine interface (HMI)
What are the three NSS domains?
- Avionics
- Flight ops
- Communication and cabin
What is a class 3 Electronic Flight Bag?
Permanently installed and integrated into the aircraft systems
They are subjected to airworthiness requirements such as crash safety and emission control
What is the distinct difference between class 1 and class 2 EFBs?
Both can be PEDs but class 2 uses the aircraft supplies
What class of EFB can software class C run on?
Can be used as a multifunction device to control other systems as well as a class EFB function
Due to this functionality, this type of software is subject to airworthiness requirements and must run on a class 3 EFB
How does a leaky line antenna work?
The leaky line antenna is a coaxial cable with regular slots (apertures) cut in the outer conductor
Allows RF energy to be received or transmitted
What is the purpose of the Open World Diodes?
Prevents communication access from the flight and cabin domains to the avionic domain, but allows unidirectional communication from the avionics to the other 2 domains
What are the 3 communications functions controlled by the CIDS?
- Passenger address
- Cabin interphone
- Service interphone
What is the hierarchy of priorities for the PA?
- Cockpit acoustic devices
- Cockpit handset
- Cabin handset
- Pre-recorded announcements
- IFE
Which DEU controls passenger functions?
DEU A
What is the purpose of the service interphone system?
Allows maintenance staff to communicate with the cockpit or cabin crew and between service interphone jack points
What happens to the IFE system during emergency or PA announcements?
Audio and video is inhibited
Which units distribute the signals to the overhead/wall mounted monitors?
Located in ceiling areas, they take the ethernet signals distributed by the Aircraft Data Network, and convert them into suitable signals used to drive overhead/wall mounted display units (DU)
