Aircraft

1.2 Aircraft

An aircraft is a machine that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. It counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil or in a few cases the downward thrust from jet engines. In another word aircraft is weight carrying structure that can travel through the air, supported either by its own buoyancy or by the dynamic action of air against its surfaces. The human activity that surrounds aircraft is called aviation. Crewed aircraft are flown by an onboard pilot, but unmanned aerial vehicles may be remotely controlled or self-controlled by onboard computers. Aircraft may be classified by different criteria, such as lift type, propulsion, usage, and others.

1.3 Airport

An airport is a location where aircraft such as fixed-wing aircraft, helicopters, and blimps take off and land. Aircraft may be stored or maintained at an airport. An airport consists of at least one surface such as a runway for a plane to take off and land, a helipad, or water for takeoffs and landings, and often includes buildings such as control towers,  hangars  and  terminal buildings. Larger airports may have fixed base operator services, seaplane docks and ramps, air traffic control, passenger facilities such as restaurants and lounges, and emergency services. A military airport is known as an airbase or air station. A water airport is a water aerodrome (an area of open water used regularly by seaplanes or amphibious aircraft for landing and taking off), usually with passenger facilities on adjacent land, which acts as an airport.

1.4 Airspace

Airspace means the portion of the atmosphere controlled by a country above its territory, including its territorial waters or, more generally, any specific three-dimensional portion of the atmosphere. Airspace may be further subdivided into a variety of areas and zones, including those where there are either restrictions on flying activities or complete prohibition of flying activities. By international law, the notion of a country's sovereign airspace corresponds with the maritime definition of territorial waters as being 12 nautical miles (22.2 km) out from a nation's coastline.

1.5 Air traffic Controller

Air traffic control (ATC) involves communication with aircraft to help maintain separation that is they ensure that aircraft are sufficiently far enough apart horizontally or vertically for no risk of collision. Controllers may co-ordinate position reports provided by pilots, or in high traffic areas (such as the United States) they may use radar to see aircraft positions.

ATC is especially important for aircraft flying under Instrument flight rules (IFR), where they may be in weather conditions that do not allow the pilots to see other aircraft. However, in very high-traffic areas, especially near major airports, aircraft flying under Visual flight rules (VFR) are also required to follow instructions from ATC.

There are generally four different types of ATC:

·         Center controllers, who control aircraft en route between airports

·         Control towers (including tower, ground control, clearance delivery, and other services), which control aircraft within a small distance (typically 10–15 km horizontal, and 1,000 m vertical) of an airport.

·         Oceanic controllers, who control aircraft over international waters between continents, generally without radar service.

·         Terminal controllers, who control aircraft in a wider area (typically 50–80 km) around busy airports.

1.5 Air Traffic Management

ATM is the regulation of air traffic in order to avoid exceeding airport or air traffic control capacity in handling traffic, and to ensure that available capacity is used efficiently. Because only one aircraft can land or depart from a runway at the same time, and because aircraft must be separated by certain time to avoid collisions, every airport has a finite capacity; it can only safely handle so many aircraft per hour. This capacity depends on many factors, such as the number of runways available, layout of taxi tracks, availability of air traffic control, and current or anticipated weather. Especially the weather can cause large variations in capacity because strong winds may limit the number of runways available, and poor visibility may necessitate increases in separation between aircraft. Air traffic control can also be limiting, there are only so many aircraft an air traffic control unit can safely handle. Staff shortages, radar maintenance or equipment faults can lower the capacity of a unit. This can affect both airport air traffic control as well as en-route air traffic control center.

2. AERONAUTICAL COMMUNICATION

2.1. Aviation Band

Aviation Band or Airband or Aircraft band is the name for a group of frequencies in the VHF radio spectrum allocated to radio communication in civil aviation, sometimes also referred to as VHF, or phonetically as "Victor". Different sections of the band are used forradionavigational aids and air traffic control.

The VHF airband uses the frequencies between 108 and 137 MHz The lowest 10 MHz of the band, from 108–117.95 MHz, is split into 200 narrow-band channels of 50 kHz. These are reserved for navigational aids such as VOR beacons, and precision approach systems such as ILS localizers.

As of 2012, most countries divide the upper 19 MHz into 760 channels for amplitude modulation voice transmissions, on frequencies from 118–136.975 MHz, in steps of 25 kHz. In Europe, it is becoming common to further divide those channels into three (8.33 kHz channel spacing), potentially permitting 2,280 channels. Some channels between 123.100 and 135.950 are available in the US to other users such as government agencies, commercial company advisory, search and rescue, military aircraft, glider and ballooning air-to-ground, flight test and national aviation authority use. A typical transmission range of an aircraft flying at cruise altitude (35,000 ft), is about 200 miles in good weather conditions.

Aeronautical voice communication is also conducted in other frequency bands, including satellite voice on Inmarsat and high frequency voice in the North Atlantic and remote areas. Military aircraft also use a dedicated UHF-AM band from 220.0–399.95 MHz for air-to-air and air-to-ground, including air traffic control communication. This band has a designated emergency and guard channel of 243.0 MHz.

Some types of navaids, such as Non-directional beacons and Distance Measuring Equipment, do not operate on these frequencies; in the case of NDBs the Low frequency and Medium frequency bands are used between 190–415 kHz and 510–535 kHz. The ILS glide path operates in the UHF frequency range of 329.3–335.0 MHz, and DME also uses UHF from 962–1150 MHz

3. AERONAUTICAL NAVIGATION

3.1 INDRODUCTION

3.1.1. Introduction to Navigation System

Finding the way from one place to another is called NAVIGATION. Moving of an aircraft from one point to another is the most important part for any kind of mission. Plotting on the paper or on the map a course towards a specific area of the earth , in the past, used to be a task assigned to a specialized member of the aircraft's crew such a navigator. Such a task was quite complicated and not always accurate. Since it depended on the observation, using simple maps and geometrical instruments for calculations. Today, aerial navigation has become an art which nears to perfection. Both external Navaids (Navigational Aids) and on-board systems help navigate any aircraft over thousands of miles with such accuracy that could only be imagined a few decades ago.

3.1.2. Piloting

Piloting is the most common method of air navigation. This method, the pilot keeps on course by following a series of landmarks on the ground. Usually before take-off, pilot will making pre-flight planning, the pilot will draw a line on the aeronautical map to indicate the desired course. Pilot will notes various landmarks, such as highways, railroad tracks, rivers, bridges. As the pilot flies over each of landmark, pilot will checks it off on the chart or map. If the plane does not pass directly over the landmark, the pilot will know that he has to correct the course.

3.1.3. Dead Reckoning

Dead reckoning is the primary navigation method used in the early days of flying. It is the method on which Lindberg relied on his first trans-Atlantic flight. A pilot used this method when flying over large bodies of water, forest, deserts. It demands more skill and experience than pilot age does. It is based on time, distance, and direction only. The pilot must know the distance from one point to the next, the magnetic heading to be flown. Pilot works on the pre-flight plan chart, pilot plan a route in advance. Pilot calculates the time to know exactly to reach the destination while flying at constant speed. In the air, the pilot uses compass to keep the plane heading in the right direction. Dead reckoning is not always a successful method of navigation because of changing wind direction. It is the fundamental of VFR flight

3.1.4. Radio Navigation

Radio navigation is used by almost all pilots. Pilots can find out from an aeronautical chart what radio station they should tune to in a particular area. They can then tune their radio navigation equipment to a signal from this station. A needle on the navigation equipment tells the pilot where they are flying to or from station, on course or not.

3.2.6 Advantages of NDB

Although there are now several more accurate navigational systems available on other radio frequency bands, the NDB is still used in every country in the world, and will continue to do so for many more years to come. The reasons are obvious which can be outlined as follows:

# Very simple air-borne and ground equipment

# Inexpensive to install and maintain

# Omni-directional information

#  Any number of aircraft can use the same radio beacon

# Responsibility of accuracy mainly depends on airborne receiver

# Multi-purpose uses

3.2.7 Limitation of NDB

1. Night effect

Radio waves take two paths to the radio compass receiver. The first and normal path is along the earth’s surface. If only these waves were received, the compass would point directly to the NDB. The second path is via one or more wave refracting layers above the earth (the ionosphere) returning to earth to mix with directs waves. Complete changes in the nature of the waves take place on this path and produce errors in direction.

                                          

The ratio of the intensity of indirect to direct waves in the total received signal determines the liability of error of the radio compass. As the strength of the indirect waves is far greater at night, errors then are more common and of greater magnitude: this is called ‘night effect’. Often this effect is more pronounced within an hour of sunrise or sunset, when the changes in the state of ionisation of the upper atmosphere are particularly violent.

The night time range of an NDB is only dependable over distances where the ground wave transmission predominates, which is approximately 60 miles over land and 100 miles over sea under reasonable propagating conditions. As the distance increases the ratio of indirect to direct waves will increase and bearing indications will become erratic. Treat with caution NDB reception beyond these ranges.

3.2.10.2 Radiation resistance:

The base radiation resistance is another important characteristic. It is a characteristic, which has a direct relationship to the radiated power and consequently to effective range of the NDB. Because the NDB antennas are electrically very short (less than 30°), the current distribution along the antenna is linear and radiation resistance may be calculated to reasonably close approximation by the formula:

R = q² /328 , where q is the electric length of the antenna in degree.  l = 360°

With the above formula it is evident that by increasing the length of the antenna its radiation resistance increases, and hence the efficiency increases. See following table.

Antenna Height (ft)	Radiation resistance in Ohms
	200KHz	300KHz	450KHz
50 75 100 150 200 250	0.041 0.092 0.163 0.367 0.657 1.020	0.092 0.207 0.367 0.825 1.467 2.295	0.206 0.466 0.825 1.857 3.300 5.164

3.2.10.3 Antenna Q :

Antenna system Q is the ratio of the reactance of the antenna capacitance to the antenna total system resistance. It is always preferable to keep the Q as low as possible to reduce losses in the antenna system.

Since    Q = Xc/R ,

Q can be reduced by increasing capacitance of the aerial. I.e. by addition of top loading or by increasing the height. NDB transmitters are usually required to have a bandwidth of at least 2X1020 Hz.  1020 Hz being the max ident frequency.

Bandwidth =  f/Q  

Therefore, at 300 KHz   Q = 300,000/2040 = 147

Which means a Q of 147 at 300 KHz NDB station will insure that the ident modulation will be radiated without any distortion. If bandwidth of the antenna is low ( Q is high) then instead of 1020 Hz ident modulation of 400Hz should be used.

3.2.10.4 Expected range 

NDB antenna should be designed in such a way that it should radiate reliable signal up to the required coverage area. ICAO has specified that in the coverage areas the field strength should not be less than 70mV per meter. Between the latitudes 30°N and 30°S field strength of 120mV may be required.

Conclusion : To increase the efficiency and to improve the performance of an NDB antenna its capacitance should be as high as possible and should be more than 500pF at the lower frequencies. This can be achieved either by increasing the height of the antenna or by providing additional top loading.

3.2.11 Transmitting Equipment

The NDB transmitter is relatively very simple equipment. The RF carrier is amplitude modulated either by 400Hz or by 1020 Hz tone, which is coded with two to three letters station identification in Morse Code. A simplified block diagram is shown in Fig. below:

                                                                                   Antenna

Monitor equipment monitors the performance of the radiating signal. Radiation is done in A0/A2 mode. Depending upon the use an NDB could be classifies as one of the following:

High Power:  usable range extends up to 400 NM. Radio beacons of this type are considered as en-route or homing radio navigational aids. The transmitter output is normally 100W to 5KW.

Low Power : usable range extends from 10 NM to 25 NM. Radio beacons of this type are called locators and are normally used for approach or holding purposes. The transmitter output power is kept below 100W.

3.2.12 Monitoring and Calibration

Normally the NDB beacon has two transmitters and two monitors, i.e. dual equipment system. Monitor analyzes the radiated signal and checks the following:

# Gives alarm if the transmitted carrier power is reduced more than 3dB. i.e. 50%

# Gives alarm if the identification signal is removed or continuous by any reason.

# Gives alarm if the monitor itself becomes faulty.

3.3.4 VOR Transmission Techniques

A 9960 Hz

 9960 Hz generator produces the sub-carrier frequency, which is the basis for the REF signal. The 9960 Hz is fed to a 30Hz frequency modulator in order to produce the frequency deviation of ± 480 Hz. This again is amplitude modulated with 30% on the VHF transmitter, normally 50 to 200 Watts output. The REF signal is fed to an omnidirectional antenna, which normally is a loop antenna, called Alford loop antenna. The REF signal is also in parallel fed to a modulation eliminator, which removes the modulation, and the signal output from this block is then a clean continuous wave (CW) signal with the same carrier frequency. This signal is fed to the horizontally polarized dipole antenna to obtain the figure of eight pattern.Since the frequency of these two signals are the same, they will combine together to form a cardioid. The goniometer rotates the figure of eight at 1800 rpm, which will also cause the cardioid to rotate at the same rate.

A cardioid has maximum and minimum radiation pattern. While rotating, when the maximum pattern is towards the receiver it will receive maximum signal and for minimum pattern the signal received will be minimum. Therefore, if the cardioid is made to rotate at 1800 times per minute (30 times per second), the receiver will get the signal as 30 Hz AM. The following Fig. 2.2.6 explains the rotation of the cardioid pattern and the resulting AM signals received by the airborne VOR receivers at north, east, south and west directions. Form these figures it is evident that the variable phase of the amplitude modulation (space modulation) is dependent of azimuth degree by degree.

VOR is a phase comparison system.  This  means  simply  that  the  phase  of one  signal  is  compared  with  the  phase  of  another.  However,  a  problem  arises  in that  this  type  of  comparison  is  possible  only  between  signals  whose  frequencies  are identical,  and  one  also  needs  to  be  able  to  identify  the  source  of  each  signal.  Some-times  this  identification  can  be  accomplished  by  time  multiplexing  the  signals  and storing  the  phase  information  in  the  receiver  circuitry  for  later  comparison.  But in  the  VOR  system both  signals  are  transmitted  simultaneously,  and  there  needs  to be  a  way  to  prevent  the  two  from  producing  a  resultant  as  they  pass  through  space. This  is  accomplished  by  transmitting  one  signal  as  amplitude  modulation  and  the other  as  frequency  modulation.  Detection  in  the  receiver  produces  two  separate audio  signals  of  exactly  the  same  frequency  but  with  measurable  phase  difference.

The two signals being compared are both 30 Hz signals.  One  is  transmitted in  a  manner  so  as  to produce  a  circular  radiation  pattern.  Thus  all  aircraft  at  the  same  instant  in  time,  and  at  the  same  distance  from  the  transmitter,  receive  the same  exact  phase  of  this  30  Hz  signal.  Consequently, this signal is called the reference signal.  The  other  signal  has  a  radiation  pattern  shaped  like  a  cardioid, and  this  pattern  is  caused  to  rotate  about  the  station  30  times  per  second.  The  airborne  equipment  which  receives  this  cardioid signal  detects  a  signal  strength  which  depends  on  which  part  of  the  cardioid  it  is receiving  at  any  particular  instant  of  time.  By  the  rotation  of  this  pattern  amplitude modulation  is  created,  and  the  received  signal  strength  from  the  variable  signal undergoes  a  cyclic  change  which  is  repeated  30  times  per  second.

Since  both  signals  occur  at  the  same  30  Hz  rate  there  is  a  repetitive  synchronization  between the  two,  and  all  that  is  needed  is  an  index  point  so  that  they may  be  compared  for  phase  difference.  This  index  point  is  established  on  the  reference  signal,  and  the  phase  of  the  variable  signal  is  initially  adjusted  so  that  the phase  difference  between  the  two  is  0^o  at  magnetic  north.  The  rotation  of  the  variable  signal  from  north  azimuth  creates  a  phase  difference  of  one  electrical  degree for  each  rotational  degree;  therefore,  an  observer  can  determine  his  geographical azimuth  by  simply  measuring  the  phase  difference  between  the  phase  of  the  reference signal  and  the  phase  of  the  variable  signal. A  receiver  anywhere  within  the  coverage  area  of  a  facility  will  receive  two  30  Hz  signals-one  from  the  reference  field  and  one  from  the  variable  field  cardioid  that  is  passing by.  The  phase  of  the  signal  from  the  variable  field  will  lag  that  of  the  reference field  by  the  exact  number  of  degrees  the  receiver  bears  from  magnetic  north.

3.3.6. VOR Errors

1) Multi-path errors: The major bearing errors in the VOR system are caused by multi-path reception. Signals reaching the aircraft receiver may include those that arrive after reflections from natural or man-made objects as well as those arriving by a direct path. The multi-path signals will add and subtract as the phases of direct and reflected signals vary while the aircraft flies along the course.

2) Ground station errors: results from misphasing of the 30 Hz reference and variable phases, misalignment of the north and other calibration errors at the VOR station. The major ground station error is due to spurious vertical polarization generated by the antennas resulting in undesirable vertically polarized 30 Hz azimuth dependent component. This spurious 30Hz component will not be in phase with the actual 30 Hz horizontally polarized variable phase. The aircraft antenna, although horizontally polarized, will pick up some of this vertically polarized signal when the aircraft will tilt. These factors may cause an additional error to the tune of  ±1°.

3) Aircraft receiver error: It is a function of the cost and age of the aircraft receiver. The older generation aviation receivers tend to have errors, which in new equipment have been essentially eliminated. The modern aircraft receivers have performance better than ±2°.

4) Pilotage or flight technical error: It is a function of many parameters, which are all difficult to measure. Studies of flight technical errors show the error to be higher when the aircraft makes a turn than on a straight-line route.

Instrument Landing System

The Instrument Landing System, abbreviated as ILS, is a system of electronic equipment, which assists the landing aircraft to make straight in approach by using cockpit indications at any non-visual meteorological conditions. ILS is a standard aid, adopted by the members of ICAO since its development in 1940's. There are hundreds of ILS's in operation at all modern airports throughout the world. It is still considered to be the most reliable, most utilized and most implemented precision approach system in the world.

The system comprises of a Localizer, a Glide Slope, and two to three Marker Beacons. The landing path is determined by the intersections of two planes, as shown in Fig.1-A, and could be explained as follows:

Ø  A vertical plane containing the runway centerline, is defined by a VHF Transmitter, called Localizer (LLZ).

Ø  A horizontal plane of 2º-4º vertical angle containing the runway centerline, is defined by an UHF transmitter, called Glide Slope (GS).

Ø   Vertically radiated VHF Markers (IM,MM & OM) transmitters provide fixed distance information

Ø   

                                                           (Radiation pattern of an ILS)

All these stations form a system that provides an electronic passage, exactly at an approach angle that is required for a safe landing.

The ILS helps to bring the aircraft safely down to a pre-defined height, called the Decision Height, from where the pilot has to make his own decision whether to land or to make a missed approach. The missed approach is an aviation terminology for unsuccessful landing. In this case, the aircraft has to make a turn and try to land once again. In category- IIIC, visibility is not needed and a blind landing can be made using electronic equipment. Therefore, based on decision height and the visibility of the runway, three categories of ILS are defined by the International Civil Aviation Organization (ICAO) which is tabulated below.

                                                                                                Table -T1

ILS CATEGORIES	DECISION HEIGHT (M)	VISIBILITY (M)
CAT - I CAT - II CAT - IIIA CAT - IIIB CAT - IIIC	60                  30                    0                    0                    0	800                 400                 200                   50                     0

Since the pilots fully rely on ILS guidance for landing, the signals radiated by an ILS should be very accurate and authentic. ICAO Annex-10 specifies the necessary technical tolerances that have to be maintained for the above three categories of ILS's.

In many occasions, where the geographical conditions do not permit installation of Marker beacons at predefined distances, Distance Measuring Equipment (DME) is co-located with ILS. The DME can be co-located with an ILS in the following three ways

-  DME with Glide Slope

-  DME with Localizer

-  DME installed independently.          

Thus, while landing on ILS, a pilot determines his position from the runway end by DME. In some locations, an NDB is installed on the centerline of the runway in stead of a Marker. Such an NDB is then called a compass locator.

Coverage of an ILS

Localizer:  The horizontal localizer coverage sector is extended from the center of the localizer antenna array to the distances of:

5.3.4 Comparison with Primary Radar

Primary Radar depends on the reflection of radar signal and operates on one frequency. As a consequence reflections from ground, buildings, precipitations, etc., will be fed to the display along with aircraft echoes. This can cause the display to be cluttered with unwanted signals. Also, the power output of a primary radar must be sufficiently high to allow for the two-way path and for the losses on reflection. Nevertheless, primary radar is self-contained and does not require equipment to be installed in the aircraft.

Secondary radar is independent of aircraft echoeing area. In fact, because the airborne equipment has its own transmitter, much less power is needed than in primary radar for the same operational range. Also, as the airborne responses of the secondary radar is on a different frequency to the ground transmission, ground clutter and other unwanted echoes are not accepted for presentation. With suitable coding, secondary radar can provide aircraft indentification without aircraft maneuvres and thus with much less use of other communication system. The coding system may also be used to send printed information from aircraft to air-traffic controllers on ground. Unfortunately, the coding system relies on pulses being either “present” or “not present” and therefore weak signals cannot be tolerated as they may generate false information. Secondary radar also suffers from the disadvantage that it requires each aircraft to carry a transponder.

5.3.5   The Coding System

The method used in SSR for communicating information consists of the transmission and reception of pulses. The ground interrogator first transmits a pair of pulses with definite width and spacing to the aircraft. Depending upon various modes used in radar for various purposes, the pulse spacing could vary from each other. In secondary radar it is necessary to design the system to reduce the chances of transponder operation as a result of receiving spurious pulses. For this reason, two pulses with a known spacing are transmitted from the interrogator. The ground based interrogation pulses are produced differently depending upon “mode of operation” of the secondary radar.

 

                                                       0.8 ± 0.1ms

Mode-A

     8 ± 0.2ms

Mode-B                                      

                                                     17 ± 0.2ms

Mode-C                                      

       21 ± 0.2ms

Mode-D                                      

                                                               25 ± 0.2ms

                   (Fig: Various modes of SR and their pulse spacing)

After receiving the interrogation pair of pulses from the ground interrogator, the aircraft transponder transmits back a reply to ground. In the process of replying, the aircraft transponder does not reply to any other signals at least for the duration of the reply pulses train. It is known as receiver dead time of radar transponder. This dead time does not last more than 125 microseconds after transmission of the last reply pulses. The aircraft transponder recognizes the interrogation mode by the time spacing between two pulses. The transponder reply, as illustrated below, consists of a train of pulses each of which is 0.45 ms wide and 1ms apart.

   

            0.45ms

    F1                     6 or 12 information pulses             F2                        Ident

                               

    1.45ms

   

                                  20.3ms                                                   4.35ms

                                    (Fig. 5.14 Reply Pulse Train from transponder)

The train of pulses is built up as follows:

Two framing pulses 20.3ms apart (F1 and F2). The first frame pulse is always the first pulse in the train. The information pulses lie between two frame pulses and composed of either 6 or 12 pulses. The information signal is formed in binary codes, which means either presence or absence of pulses. If six  pulse positions are used then the number of codes available is 2⁶ = 64. For 12 positions the number of codes available is 2¹² = 4096. The identity pulse is transmitted only when the switch is activated in the aircraft at the request of the air trafic controller. The identity pulses are then transmitted automatically for a period of twenty seconds.

5.4 Monopusle SSR and Mode-S

5.4.1 Monopulse SSR

The new Mode S system was intended to operate with just a single reply from an aircraft, a system known as monopulse. The accompanying diagram shows a conventional main or "sum" beam of an SSR antenna to which has been added a "difference" beam. To produce the sum beam the signal is distributed horizontally across the antenna aperture. This feed system is divided into two equal halves and the two parts summed again to produce the original sum beam. However the two halves are also subtracted to produce a difference output. A signal arriving exactly normal, or boresight, to the antenna will produce a maximum output in the sum beam but a zero signal in the difference beam. Away from boresight the signal in the sum beam will be less but there will be a non-zero signal in the difference beam. The angle of arrival of the signal can be determined by measuring the ratio of the signals between the sum and difference beams. The ambiguity about boresight can be resolved as there is a 180° phase change in the difference signal either side of boresight.

7.2 Radio direction finder (Radio compass)

7.2.1 Operation

Radio Direction Finding works by comparing the signal strength of a directional antenna pointing in different directions. At first, this system was used by land and marine-based radio operators, using a simple rotatable loop antenna linked to a degree indicator. This system was later adopted for both ships and aircraft, and was widely used in the 1930s and 1940s. On pre-World War II aircraft, RDF antennas are easy to identify as the circular loops mounted above or below the fuselage. Later loop antenna designs were enclosed in an aerodynamic, teardrop-shaped fairing. In ships and small boats, RDF receivers first employed large metal loop antennas, similar to aircraft, but usually mounted atop a portable battery-powered receiver.

        Fig: radio compass

In use, the RDF operator would first tune the receiver to the correct frequency, then manually turn the loop, either listening or watching an S meter to determine the direction of the null (the direction at which a given signal is weakest) of a long wave (LW) or medium wave (AM) broadcast beacon or station (listening for the null is easier than listening for a peak signal, and normally produces a more accurate result). This null was symmetrical, and thus identified both the correct degree heading marked on the radio's compass rose as well as its 180-degree opposite. While this information provided a baseline from the station to the ship or aircraft, the navigator still needed to know beforehand if he was to the east or west of the station in order to avoid plotting a course 180-degrees in the wrong direction. By taking bearings to two or more broadcast stations and plotting the intersecting bearings, the navigator could locate the relative position of his ship or aircraft.

Later, RDF sets were equipped with rotatable ferrite loopstick antennas, which made the sets more portable and less bulky. Some were later partially automated by means of a motorized antenna (ADF). A key breakthrough was the introduction of a secondary vertical whip or 'sense' antenna that substantiated the correct bearing and allowed the navigator to avoid plotting a bearings 180 degrees opposite the actual heading. After World War II, there many small and large firms making direction finding equipment for mariners, including Apelco, Aqua Guide, Bendix, Gladding (and its marine division, Pearce-Simpson), Ray Jefferson, Raytheon, and Sperry. By the 1960s, many of these radios were actually made by Japanese electronics manufacturers, such as Panasonic, Fuji Onkyo, and Koden Electronics Co., Ltd. In aircraft equipment, Bendix and Sperry-Rand were two of the larger manufacturers of RDF radios and navigation instruments.

7.6.1SSR Transponder Modes

Mode A interrogations are sent to request the specified aircraft identification code. Mode C is used to request altitude reporting together with identification. The coded reply signal is composed of a series of pulses. In Mode A operation, the number of pulses generated in a reply signal is determined by setting the four octal (0 to 7) digit code switches on the Transponder to the assigned identification code. This allowes for 4'096 different identification codes. Certain transponder codes are reserved for special applications to activate an alert on the controller's console:

7500 indicates that a hijack is in process

• 7600 reports a communication radio failure
• 7700 indicates an emergency condition

Replies to Mode C interrogations generate pulses that correspond to the aircraft's altitude. The received altitude information is then displayed directly on the controller's console. Altitude information is not selected by the code switches of the transponder, but is obtained directly from an encoding altimeter.

7.7 Flight Data Recorder (FDR)

Flight recorders comprise two systems, a flight data recorder (FDR) and a cockpit voice recorder (CVR). Sometimes, both FDR and CVR functions are combined into a single unit (ICAO Definition: Combination recorders). Combination recorders need to meet the flight recorder equipage requirements as specifically indicated in ICAO Annex 6 - Operation of Aircraft.

7.7.1 ICAO Requirements

According to the provisions in ICAO Annex 6 - Operation of Aircraft, Vol 1 and Vol. III, a Type I FDR shall record a number of parameters the parameters required to determine accurately the aeroplane flight path, speed, attitude, engine power, configuration and operation. Types II and IIA FDRs shall record the parameters required to determine accurately the aeroplane flight path, speed, attitude, engine power and configuration of lift and drag devices.

7.7.2 Objectives

The recorder is installed in the most crash survivable part of the aircraft, usually the tail section. The data collected in the FDR system can help investigators determine whether an accident was caused by pilot error, by an external event (such as windshear), or by an airplane system problem. Furthermore, these data have contributed to airplane system design improvements and the ability to predict potential difficulties as airplanes age. An example of the latter is using FDR data to monitor the condition of a high-hours engine. Evaluating the data could be useful in making a decision to replace the engine before a failure occurs.

7.7.3 Principles of Operation

The FDR onboard the aircraft records many different operating conditions of the flight. By regulation, newly manufactured aircraft must monitor at least eighty-eight important parameters such as time, altitude, airspeed, heading, and aircraft attitude. In addition, some FDRs can record the status of more than 1,000 other in-flight characteristics that can aid in the investigation. The items monitored can be anything from flap position to auto-pilot mode or even smoke alarms. It is required by regulations that on an annual basis an FDR verification check (readout) is performed in order to verify that all mandatory parameters are recorded.

§  Magnetic Tape - The introduction of the CVR in the late 1960s and DFDRs in the early 1970s made magnetic tape the recording medium of choice until the introduction of solid-state flight recorders in the late 1980s. There were a variety of tapes and tape transports used by the various recorder manufacturers. The most widely used tapes were mylar, kapton, and metallic. The tape transports were even more varied, using designs such as coplaner reel to reel, coaxial reel-to-reel, endless loop reel packs and endless loop random storage. Tape CVRs record four channels of audio for 30 minutes, and the DFDR records 25 hours of data. CVRs and FDRs record over the oldest data with the newest data in an endless loop-recording recording pattern.

§  Digital Recording - Most DFDRs require a flight data acquisition unit (FDAU) to provide an interface between the various sensors and the DFDR. The FDAU converts analog signals from the sensors to digital signals that are then multiplexed into a serial data stream suitable for recording by the DFDR. Industry standards dictated the format of the data stream, which for the vast majority of tape-based DFDRs is 64 12-bit data words per second. The recording capacity of the tape DFDR is limited by the length of tape that can be crash-protected and the data frame format. The capacity of the tape DFDRs was adequate for the first generation of wide-body transports, but was quickly exceeded when aircraft like the Boeing 767 and Airbus A320 with digital avionics were introduced.

§  Solid State Technology - The introduction of solid-state flight recorders in the late 1980s marked the most significant advance in evolution of flight recorder technology. The use of solid-state memory devices in flight recorders has expanded recording capacity, enhanced crash/fire survivability, and improved recorder reliability. It is now possible to have 2-hour audio CVRs and DFDRs that can record up to 256 12-bit data words per second, or 4 times the capacity of magnetic tape DFDRs.