Aircraft Avionics and Autopilot Systems PDF

Aircraft Avionics And Autopilot Systems (AVIONICS-AS) Aircraft Communication System Radio Theory  Radio theory emphasizes the transmission and detection of communication signals consisting of electromagnetic waves that travel through the air in a straight line or by reflec...

Aircraft Avionics And Autopilot Systems (AVIONICS-AS) Aircraft Communication System Radio Theory  Radio theory emphasizes the transmission and detection of communication signals consisting of electromagnetic waves that travel through the air in a straight line or by reflection from the ionosphere or from a communications satellite. Wave  A progressive disturbance in a medium.  Moves outward, possesses energy.  Energy is lost due to friction. Electromagnetic Wave  Electromagnetic waves are formed when an electric field comes in contact with a magnetic field. They are hence known as ‘electromagnetic’ waves. The electric field and magnetic field of an electromagnetic wave are perpendicular (at right angles) to each other. Radio wave  Radio wave is a type of Electromagnetic Wave.  Radio wave is from the portion of the electromagnetic spectrum at lower frequencies than microwaves. They are used in standard broadcast radio and television, shortwave radio, navigation and air-traffic control, cellular telephony, and even remote-controlled toys.  The SI unit for wave frequency is the hertz (Hz), where 1 hertz equals 1 wave passing a fixed point in 1 second. A higher- frequency wave has more energy than a lower-frequency wave with the same amplitude. Radio wave Low Frequency  Long Wavelength  Low Energy  Poor Quality High Frequency  Short Wavelength  High Energy  Good Quality Factors affecting radio waves Diffraction  Occurs in low frequency radio waves, it is the bending of a radio wave. Reflection  Occurs in high frequency radio waves, it is the bouncing back of a radio wave. Refraction  Occurs in high and low frequency radio waves, it is the changing in direction of a radio wave caused by a disturbance. (E.g. Rain) Attenuation  It is a reduction of signal strength during transmission, it is the loss of energy of a radio wave, friction may cause attenuation. Radio Wave Propagation  A radio wave is transmitted via omni directional, directional or pulse & echo method. From the antenna it moves out along three paths, depending primarily upon its frequency. These paths are surface waves, sky waves, and space waves. Radio Wave Propagation Omni directional  Radio wave is propagated in all directions. Directional  Radio wave is propagated in one direction. Pulse & echo  Bouncing back of radio waves. Types of Radio Waves Ground waves / Surface waves  Follows the curvature of the Earth, this wave is restricted by gravity.  VLF, LF, and MF Sky wave  This type of radio wave is stronger than the ground waves and it can overcome gravity, but it cannot penetrate the ionosphere.  HF Space wave  This type of radio wave can overcome gravity and can penetrate the ionosphere, its limitation is line of sight.  VHF, UHF, SHF, EHF Frequency band limits Radiotelephony  Pilots, engineers and air traffic controllers communicate by using the radiotelephony phraseology that consists of a set of standardized words and phrases approved for the radiotelephony communications by ICAO in all routine aircraft situations.  The communication is made possible due to their common and work-related topics. According to the ICAO Doc 9432 Manual of Radiotelephony the following transmitting techniques will assist in ensuring that transmitted speech is clearly and satisfactorily received: 1. Before transmitting, listen out on the frequency to be used to ensure that there will be no interference with a transmission from another station. 2. Be familiar with good microphone operating techniques. 3. Use a normal conversational tone, speak clearly and distinctly. 4. Maintain an even rate of speech not exceeding 100 words per minute. When it is known that elements of the message will be written down by recipient, speak at a slightly slower rate. 5. Maintain the speaking volume at a constant level. According to the ICAO Doc 9432 Manual of Radiotelephony the following transmitting techniques will assist in ensuring that transmitted speech is clearly and satisfactorily received: 6. A slight pause before and after numbers will assist in making them easier to understand. 7. Avoid using hesitation sounds such as "er". 8. Depress the transmit switch fully before speaking and do not release it until the message is completed. This will ensure that the entire message is transmitted. 9. An irritating and potentially dangerous situation in radiotelephony is a "stuck" microphone button. Operators should always ensure that the button is released after a transmission and the microphone placed in an appropriate place that will ensure that it will not inadvertently be switched on. HF and VHF Communication  VHF and HF communication systems use transceivers.  A transceivers is a self contained transmitter and receiver.  The transmitter and receiver both operate at same frequency, and the microphone button determines when there is an output from the transmitter.  In the absence of transmission the receiver is sensitive to incoming signals.  This combination of transmitters and receivers saves a lot of space and weight and are hence largely used. VHF Communication  Very High Frequency is a term used to describe the 30MHz to 300MHz portion of the radio spectrum.  This range of frequencies will provide short-range LOS (line of site) communications.  The range for VHF communications depends on equipment used, antenna height, and terrain(typically 2 to 20 miles). HF Communication  The frequency range is 3-30 MHz  This is used for long range communications because of its longer transmission range.  This is the basic band for long-range communications, mainly because its transmissions are reflected from the ionosphere.  HF transmitters have higher power outputs than VHF transmitters.  A Tuner is used to match the impedance of the transceiver to the antenna.  Use and installation of the HF communication system is same as that of the VHF communication system.  The advantage of using HF communication system over VHF communication system is that it can transmit information over long distance as the output power of the HF transmitter is higher than that of the VHF transmitter. Service Interphone System  The service interphone system provides ground crew personnel and all aircraft crewmembers with facilities that enable two-way voice communications during aircraft servicing or during normal flight operations.  Handset jacks are located throughout the aircraft, internally and externally, for use by ground crew personnel to facilitate aircraft servicing and maintenance. The service interphone switch on pilots' overhead panel P5 is used to disconnect these jacks from the system when they are not in use. Passenger Address System  The passenger address (PA) system provides means of communicating with the passengers and entertaining them with recorded music played over the speakers in the passenger compartment.  It also provides chime signals for the Passenger Call Systems, Flight Crew Call System, and Passenger Signs to attract the attention of flight attendants as well as passengers.  Announcements may be made using any of the three hand microphones or using the microphones and audio selector panels in the flight interphone system.  The passenger address system consists of a PA amplifier, loudspeakers, microphones, a tape reproducer, and interconnecting wiring. SELCAL Systems  SELCAL is a signaling method which can alert an individual aircraft that a ground station wishes to communicate with it. Two identical SELCAL (selective calling) systems are installed in the aircraft.  A four-tone alert signal from a ground station, is sent to an aircraft VHF or HF system and is detected by the SELCAL system.  The SELCAL system alerts the crewmembers by means of a light and a chime whenever a message is to be received on one of the communication systems.  The SELCAL systems are composed of a decoder, chime, and SELCAL control panel.  The chime is enclosed within the aural warning unit on the forward right side of the control stand. The SELCAL control panel is installed on the pilots' aft overhead panel.