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As the density of aircraft in sky is increasing day by day, it has become absolutely essential to install different types of equipment in the aircraft to avoid various types of hazards. Presently, these equipments are installed in the aircraft as per the requirements of the state regulations. They are very efficient in operation and constructed using the State of the Art technologies.
These equipments can be divided into two main categories. The first category is of the equipments which can avoid hazards before the aircraft encounters the hazardous situations and the second category is of equipments which help speeding up the crash recovery process. The safety hazards can be classified into three categories viz. possible collision with terrain, possible collision with other aircraft, and weather related hazards. Equipments are available to help the aircraft avoid these hazards.
The safety hazards can be classified into three categories viz. possible collision with terrain, possible collision with other aircraft, and weather related hazards. Equipments are available to help the aircraft avoid these hazards. In the analysis of worldwide accidents in aviation, it has been found that CFIT (Controlled Flight Into Terrain) accidents result in more number of fatalities than any other type of accidents. It is seen that out of 2500 worldwide airline accidents between 1988 and 1995, around 2219 are CFIT type of accidents. CFIT is an accident where the aircraft is flown into ground, water or any man made obstacle while the aircraft and crew are completely airworthy.
I.I Enhanced Ground Proximity Warning System
All the modern aircraft are equipped with EGPWS (Enhanced Ground Proximity Warning System) to avoid CFIT type of accidents. Its purpose is to provide the flight crew with situational awareness about the terrain and predictive alerts for flights flying dangerously close to terrain. The system monitors the aircraft's flight path and generates visual and aural warnings / alerts when the aircraft comes under one of the defined hazardous situations due to terrain.
The classic GPWS (Ground Proximity Warning Systems), an old generation system, used information from LRRA (Low Range Radio Altimeter) as the primary input and calculated the height of the aircraft above the ground level continuously to keep track of dangerous conditions. But this system had a major disadvantage that it could not detect the presence of steeply raising terrain well in advance and this sometimes resulted in availability of very less time for the flight crew to take some evasive step.
The modern EGPWS augments the classic GPWS modes of operation by including in its computer a model of the earth's terrain and manmade objects including the airport locations and runway details worldwide. With the digital terrain elevation and periodically updated airport database inside the computer can continuously compare the aircraft's position to a virtual three dimensional map of the real world, thus predicting an evolving hazardous situation much in advance. In addition to retaining classic GPWS alerting functions, the EGPWS also offers some new features such as Wind Shear alerting systems, detailed terrain display and distinct synthetic voices in the cockpit. There are many new features which are likely to be added in future to protect the aircraft from terrain related threats. The figure below illustrates the advantage of modern EGPWS over conventional GPWS.
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Advantages of EGPWS over GPWS
I.II Traffic alert and Collision Avoidance System
Another important hazard avoidance system installed in the aircraft is TCAS (Traffic alert and Collision Avoidance System). The purpose of this equipment is to avoid collisions between aircrafts flying or approaching the same altitude. It detects any aircraft flying in the vicinity with a working Mode-S transponder. The T-CAS processor on board an aircraft interrogates all the aircrafts (intruders) equipped with a functional Mode-S transponder in the vicinity inside its working range. From the replies, the T-CAS processor computes a factor called tau (Γ). Tau is the measure of the closure rate i.e. the time left for the collision to take place. Tau is calculated by the formula
Γ = range/(range/rate)
The T-CAS processor makes use of a directional antenna located at the top and bottom of the fuselage to determine the direction of the received reply thus it determines the bearing of the intruder aircraft. By combining the bearing information with the computed tau value the, T-CAS gives appropriate warnings and advisories to the flight crew to evade the threat.
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Functional Block Diagram of T-CAS
The development of an effective airborne collision avoidance system started in 1950 itself. But today's T-CAS has a lot of advanced features. In future there are plans to integrate the T-CAS with GPS (Global Positioning System) to make the system more precise and accurate. TCAS operates within a range of 20 nautical miles in front and at the altitude envelope of + 8700 feet from the own aircraft. TCAS evaluates the threat potential well in advance and presents two types of warnings viz. Traffic advisory (TA) and Resolution advisory (RA). The Traffic advisory alerts the crew with traffic symbols and synthetic voice. Resolution advisory presents the flight crew with visual escape commands and synthetic voice. If all the aircraft in the sky are fitted with TCAS processors and a Mode-S transponder, then a coordinated maneuvering is possible to avoid potential conflicts. The illustration below shows the T-CAS range and tau values for issuing TA and RA.
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T-CAS range diagram showing ranges and tau values for TA and RA
I.III Weather Radar System
Bad weather is one of the major threats to any aircraft. Bad weather conditions like heavy rain, thunderstorm, windshear etc jeopardize the safety of the aircraft by exerting immense amount of external force on the airframe or by deposition of frost which lead to loss of aerodynamicity of the aircraft structure. This may result in total loss of control over the aircraft by the flight crew or in worst case scenario the disintegration of the airframe. Thus bad weather should be avoided at all costs and the flight crew should be warned about bad weather well in advance.
This feat is accomplished with the use of weather radar system. Weather radar is a pulse Doppler radar which operates in the X-band (8-12 GHz) of microwave frequency range. The radar transmits high intensity pulse into the space in front of the aircraft and waits for the echo signal. After receiving the echo it compares its intensity with the intensity of the transmitted signal in order to determine the intensity of precipitation. Since the radar incorporates the technique of Doppler shift, it also determines the distance to the area of precipitation with respect to own aircraft.
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Functional blocks of a Weather Radar system
The detailed functionalities of an on-board weather radar system are as follows:-
Detection and localization of the atmospheric disturbances in the area defined by the antenna scanning: + 90 deg. of aircraft centerline and up to 320nmi in front of the aircraft.
Detection of turbulence areas caused by the presence of precipitations up to a distance of 40nmi.
Presentation of terrain mapping information by the combination of the orientation of the radar beam and of the receiver gain.
Modern weather radar systems have an additional feature of predictive windshear function. If the predictive windshear function is active then the weather radar detects microburst windshear events in the area defined by the antenna scanning + 60 deg. The weather radar is coupled with the IRS (Inertial Reference System) in order to obtain stabilization signal for the antenna. This helps keep the antenna beam pointed towards the front of the aircraft irrespective of the aircraft attitude. Weather radar antenna is a flat plate antenna which is placed inside the nose radome of the aircraft.
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Location of Weather Radar Antenna
The reason for using flat plate antenna is that it provides a narrow beam which facilitates high directivity and range. Thus areas far off from the aircraft can also be scanned for bad weather.
The weather radar computer alternates the intensity of the transmitted pulse between high and low for accurate determination of water content in clouds which are located both far and close to the aircraft. The weather information from the radar is presented to the flight crew either in a dedicated display or on the Navigation Display. For displaying weather information five different colours are used each depicting a different intensity of precipitation. Windshear is indicated using an icon consisting of alternate red and black arcs.
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Weather Radar display in the cockpit
I.IV Emergency Locator Transmitter
In an event of a crash, time is most critical. It is of utmost importance to start the Search And Rescue (SAR) operation as soon as possible to help the survivors and to retrieve the black box (Cockpit Voice Recorder + Flight Data Recorder) quickly. From the analysis of crashes worldwide, it has been found that a lot of human lives could be saved when the search and rescue teams had reached the crash site immediately after crash.
Emergency Locator Transmitter (ELT) is one of the many instruments fitted in an aircraft which helps in crash recovery by helping the SAR teams to locate the crash site. ELT is located at the rear part of the fuselage (empennage) and is connected with a G-switch. In the event of a crash when the G-switch experiences a force of + 2.5 G, it activates the ELT.
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Location of ELT Antenna
The ELT transmits signals on the international distress frequencies of 121.5 MHz and 406.025 MHz . The 121.5 MHz signal can be received by any VHF transceiver and is used to home in on the point of crash. The 406.025 MHz signal sent to the satellites. The 406.025MHz transmission turns on and off every 50 seconds. This frequency transmits the standard or specialized message formats with additional information. During this transmission, the 121.5 and 243 MHz transmitters are inhibited. The 406.025 MHz signal is transmitted only for a period of twenty four hours after the ELT has been activated. The 121.5 MHz signal is transmitted till the battery lasts which is typically seventy two hours.
The message format used during 406.025 MHz transmission is of two types- short message format and long message format. The standard form of transmission is short message format which transmits the signal for duration of 40 milliseconds. The long message format transmits signal for 520 milliseconds. During this transmission, a digitally encoded message about the aircraft and systems in a specified format is transmitted. The information contained in the transmitted signal is:-
Serial number of the ELT transmitter.
Identification code of the airline.
Once the ELT is activated and the satellite detects the 406.0 MHz signal, then the position of the crash site is calculated. The 121.05 MHz transmissions are used to home in to the crash site. However, due to the availability of limited information, position information calculated by the satellite is not so accurate.
The ELT has to operate on a standalone battery unit, as it is not possible to get power from aircraft electrical power sources after crash. Normally, ELT is provided with Lithium Manganese Dioxide Cells, which are not re-chargeable. The design of ELT antenna is very crucial. The single ELT antenna systems should operate in all the tree frequencies as per the duty cycle, with the standing wave ratio of 2:1. The antenna pattern is omni-directional and vertically polarised. This antenna pattern ensures that the signal is radiated in all directions and covers a large amount of distance. Furthermore, the antenna should be able to withstand airspeed rating of close to one Mach as they are mounted outside the fuselage and the antenna should be mounted as close to the unit as possible so as to avoid detaching of antenna during a crash.
The ELT must be housed in a high impact, fire resistance case. During crash, there is a chance for high impact and extreme temperature near the unit and thus the circuits inside should not get damaged. The normal power of radiation of ELT in the 121.5 MHz is about 100 mW during the first 50 hours of transmission under normal temperature and operating conditions. This is a requirement set by the aviation regulatory authorities. The approximate power radiated for 406.025 MHz transmission is about 5 watts and the transmitter is turned on every 47.5 to 52.5 seconds.
Apart from the ELT fixed with the aircraft body, commercial airliners also carry portable hand held ELTs. These ELTs can be activated by the survivors just by putting the ELT in contact with any sort of saline liquid. These ELTs transmit only the 121.5 MHz signal and thus help the SAR party to home in on to the survivors.