STAGE OF WARNING WARNING CAUSE OF WARNING SIGNAL CORRECTIVE
OPERATION SYSTEM SIGNAL ACTIVATION ACTION Landing Landing gear Continuous Landing gear is not down and locked Raise flaps ATA 3200 born when flaps are less than full up Advance throttle and throttle is retarded to idle.
In flight Mach warning Clacker Equivalent airspeed or mach Decrease speed ATA 3400 number exceeds limits. of aircraft Takoff Flight contro Intermittent Throttles are advanced and any Correct the aircraft ATA 2700 hom of following conditions exist. to proper takeoff ATA 4900 Aux power 1. Speed brakes are not down 2. Flaps are not in takeoff range conditions.
3. Auxiliary power exhaust door is open
4. Stabilizer is not in the takeoff setting.
Inflight Pressurization Intermittent horn If cabin pressure becomes equal Correct the ATA 2100 to atmospheric pressure at the specitic condition.
altitude (altitude at time of occurrence).
Any stage Fire warning Continuous bel Any overheat condition or 1. Lower the heat ATA 2600 fire in any engine or in the area wherenacelle, or main wheel or nose wheel in the F/W was well, APU engine or any compartment activated.
having firewarning system installed. Also whenever the firewarning system 2. Signal may be si- lenced by pushing is tested. the F/W bell cutout switch or the APU cutout switch Any stage Communications High chime Any time captain's call button is Release button or if ATA 2300 pressed at external power pane. button remains locked forward or rearward cabin attend in, pull button out.
ant's panel Any stage Communications Tow tone hi-low Whenever a signal has been Press reset button on secal system® chime or received by an HF or VHF com- secal system control ATA 2300 single low chime. munication system and decoded by panel.
the secal® decoder.
*NOTE: Secal system is the Selective Calling System: Each aircraft is assigned a particular four tone audio combination for identification purposes. A ground station will key the signal whenever contact with that particular aircraft is desired. The signal will be decoded by the airborne secal decoder and the crew allerted by the secal warning system.
FIGURE 12-78. Aural warning system.
CHAPTER 13
COMMUNICATIONS AND NAVIGATION SYSTEMS
GENERAL Communications and navigation are the two major functions of airborne radio. Communication systems primarily involve voice transmission and reception between aircraft or aircraft and ground stations. Radios are used in aircraft as navigational aids in a number of applications. They range from a simple radio direction finder to navigational systems which use computers and other advanced electronic techniques to automatically solve the navigational problems for an entire flight. Marker beacon receivers, instrument landing systems (involving radio signals for glide slope and direction), distance measuring equipment, radar, area navigation systems, and omnidirectional radio receivers are but a few basic applications of airborne radio navigation systems available for installation and use in aircraft.
Safe aircraft operation is dependent to a large degree upon the satisfactory performance of the airborne communications and navigation systems.
Reliability and performance of the radio and radar system are directly related to the skills of those who perform the maintenance.
Federal Aviation Regulations require an inspection of radio equipment installations at regular intervals. These inspections include a visual examination for security of attachment, condition of wiring, bonding, shock mounts, radio racks and supporting structure. In addition, a functional check is usually performed to determine that the equipment is operating properly and that its operation does not interfere with the operation of other systems, Aircraft mechanics' responsibilities include the installation and inspection of radios, antennas, navigation equipment, and associated wiring. In addition, the FAA has certified radio repair facilities to perform maintenance on radios, antennas, navigation, and radar equipment. Transmitting equipment is calibrated by persons licensed by the FCC (Federal Communications Commission).
To be in a more favorable position to inspect system installations, a mechanic should possess some basic knowledge and understanding of the principles, purposes, and operation of the radio equipment used in aircraft.
Because of the many different makes and models of equipment and the various systems in use, it is not feasible to describe each in this handbook. The information presented in this chapter is general in nature and provides a broad introduction to radio, its principles, and application to aircraft from a mechanic's viewpoint.
BASIC RADIO PRINCIPLES The principle of radio communication can be illustrated by using a simple transformer. As shown in figure 13-1, closing the switch in the primary circuit causes the lamp in the secondary circuit to be illuminated. Opening the switch extinguishes the light.
Alternating electromagnetic field Primary circuit Gen. - Lamp Switch Secondary circuit.
Iron core FICURE 13-1. A simple transformer circuit.
There is no direct connection between the primary and secondary circuits. The energy that illuminates the light is transmitted by an alternating electromagnetic field in the core of the transformer.
This is a simple form of wireless control of one circuit (the secondary) by another circuit (the primary).
The basic concept of radio communications involves the transmission and reception of electromagnetic (radio) energy waves through space. Alternating current passing through a conductor creates electromagnetic fields around the conductor. Energy 519
is alternately stored in these fields and returned to the conductor. As the frequency of current alternation increases, less and less of the energy stored in the field returns to the conductor. Instead of returning, the energy is radiated into space in the form of electromagnetic waves. A conductor radiating in this manner is called the transmitting antenna.
For an antenna to radiate efficiently a transmitter must supply it with an alternating current of the selected frequency. The frequency of the radio wave radiated will be equal to the frequency of the applied current. When current flows through a transmitting antenna, radio waves are radiated in all directions in much the same way that waves travel on the surface of a pond into which a rock has been thrown. Radio waves travel at a speed of approximately 186,000 miles per second.
If a radiated electromagnetic field passes through a conductor, some of the energy in the field will set electrons in motion in the conductor. This electron flow constitutes a current that varies with changes in the electromagnetic field. Thus, a variation of the current in a radiating antenna causes a similar varying current in a conductor (receiving antenna) at a distant location. Any intelligence being produced as current in a transmitting antenna will be reproduced as current in a receiving antenna.
Frequency Bands The radio frequency portion of the electromagnetic spectrum extends from approximately 30 kHz (kilohertz) to 30,000 MHz (Megahertz). As a matter of convenience, this part of the spectrum is divided into frequency bands. Each band or frequency range produces different effects in trans- Transmitting antenna Microphone Transmitter mission. The radio frequency bands proven most useful and presently in use are:
Frequency Range Band Low frequency (L/F) - --30 to 300 kHz Medium frequency (M/F) --300 to 3,000 kHz High frequency (H/F) -- -__3,000 kHz to 30 MHz Very high frequency (VHF) _30 to 300 MHz Ultra high frequency (UHF) -300 to 3,000 MHz Superhigh frequency (SHF) - 3,000 to 30,000 MHz In practice, radio equipment usually covers only a portion of the designated band, e.g., civil VHF equipment normally operates at frequencies between
108.0 MHz and 135.95 MHz.
BASIC EQUIPMENT COMPONENTS The basic components (figure 13-2) of a communication system are: microphone, transmitter, transmitting antenna, receiving antenna, receiver, and a headset or loudspeaker.
Transmitters A transmitter may be considered as a generator which changes electrical power into radio waves. A transmitter must perform these functions: (1) Generate a RF (radio frequency) signal, (2) amplify the RF signal, and (3) provide a means of placing intelligence on the signal.
The transmitter contains an oscillator circuit to generate the RF signal (or a subharmonic of the transmitter frequency, if frequency doublers or multipliers are used) and amplifier circuits to increase the output of the oscillator to the power level required for proper operation.
The voice (audio) intelligence is added to the RF Receiving antenna Headset Receiver Loudspeaker FIGURE 13-2. Basic communication equipment.
signal by a special circuit called the modulator. The modulator uses the audio signal to vary the amplitude or frequency of the RF signal. If the amplitude is varied, the process is called amplitude modulation or AM. If the frequency is varied, the process is known as frequency modulation or FM.
Transmitters take many forms, have varying degrees of complexity and develop various levels of power. The amount of power generated by a transmitter affects the strength of the electromagnetic field radiating from the antenna. Thus, it follows that the higher the power output from a transmitter, the greater the distance its signal may be received.
VHF transmitters used in single engine and light twin-engine aircraft vary in power output from 1 watt to 30 watts, depending on the particular model radio. However, radios having 3 to 5 watt ratings are used most frequently. Executive and large transport aircraft are usually equipped with VHF transmitters having a power output of 20 to 30 watts.
Aviation communication transmitters are crystalcontrolled in order to meet the frequency tolerance requirements of the FCC. Most transmitters are selectable for more than one frequency. The frequency of the channel selected is determined by a crystal. Transmitters may have from one to 680 channels.
Receivers The communications receiver must select radio frequency signals and convert the intelligence contained on these signals into a usable form; either audible signals for communication and audible or visual signal for navigation.
Radio waves of many frequencies are present in the air. A receiver must be able to select the desired frequency from all those present and amplify the small a.c. signal voltage.
The receiver contains a demcdulator circuit to remove the intelligence. If the demodulator circuit is sensitive to amplitude changes, it is used in AM sets and called a detector. A demodulator circuit that is sensitive to frequency changes is used for IM reception and is known as a discriminator.
Amplifying circuits within the receiver increases the audio signal to a power level which will operate the headset or loudspeaker properly.
Antenna An antenna is a special type of electrical circuit designed to radiate and receive electromagnetic energy. As mentioned previously, a transmitting antenna is a conductor which radiates electromagnetic waves when a radio frequency current is passed through it. Antennas vary in shape and design (figure 13-3) depending upon the frequency to be transmitted, and specific purposes they must serve.
In general, communication transmitting stations radiate signals in all directions. However, special antennas are designed that radiate only in certain directions or certain beam patterns.
VHF communication Communication, navigation Marker beacon Distance measuring equipment Glidescope VHF communication FIGURE 13-3. Antennas.
The receiving antenna must intercept the electromagnetic waves that are present in the air. The shape and size of the receiving antenna will also vary according to the specific purpose for which it is intended. In airborne communications the same antenna is normally used for both transmission and reception of signals.
Microphones A microphone is essentially an energy converter that changes acoustical (sound) energy into corresponding electrical energy. When spoken into a microphone, the audio pressure waves generated strike the diaphragm of the microphone causing it to move in and out in accordance with the instantaneous pressure delivered to it. The diaphragm is 521
attached to a device that causes current to flow in proportion to the pressure applied.
For good quality sound, the electrical waves from a microphone must correspond closely in magnitude and frequency to the sound waves that cause them, so that no new frequencies are introduced. A desirable characteristic is the ability of the microphone to favor sounds coming from a nearby source over random sounds coming from a relatively greater distance. When talking into this type of microphone, the lips must be held as close as possible to the diaphragm.
Persons inexperienced in the use of the microphone are usually surprised at the quality of their own transmissions when they are taped and played back. Words quite clear when spoken to another person can be almost unintelligible over the radio.
Readable radio transmissions depend on the following factors: (1) Voice amplitude, (2) rate of speech, and (3) pronunciation and phrasing. Clarity increases with amplitude up to a level just short of shouting. When using a microphone, speak loudly, without exerting extreme effort. Talk slowly enough so that each word is spoken distinctly.
Avoid using unnecessary words.
POWER SUPPLY The power supply is a component that furnishes the correc: voltages and current needed to operate the communication equipment. The power supply can be a separate component or it may be contained within the equipment it supplies. Electromechanical devices used as electronic power supplies include dynamotors and inverters.
The dynamotor performs the dual functions of motor and generator, changing the relatively low voltage of the aircraft electrical system into a much higher value. The multi-vibrator is another type of voltage supply used to obtain a high a.c. or d.c.
voltage from a comparatively low d.c. voltage.
In many aircraft, the primary source of electric power is direct current. An inverter is used to supply the required alternating current. Common air:
craft inverters consist of a d.c. motor driving an
a.c. generator. Static, or solid-state inverters are replacing the electromechanical inverters in many applications. Static inverters have no moving parts, but use semiconductor devices and circuits that periodically pulse d.c. current through the primary of a transformer to obtain an a.c. output from the secondary.
COMMUNICATION SYSTEMS The most common communication system in use today is the VHF system. In addition to VHF equipment, large aircraft are usually equipped with HF communication systems.
Airborne communications systems vary considerably in size, weight, power requirements, quality of operation, and cost, depending upon the desired operation.
Many airborne VHF and HF communication systems use transceivers. A transceiver is a self-contained transmitter and receiver which share common circuits; i.e., power supply, antenna, and tuning. The transmitter and receiver both operate on the 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. Since weight and space are of great importance in aircraft, the transceiver is widely used. Large aircraft may be equipped with transceivers or a communications system that uses separate transmitters and receivers.
The operation of radio equipment is essentially the same whether installed on large aircraft or small aircraft. In some radio installations the controls for frequency selection, volume, and the "on-off" switch are integral with the radio main chassis. In other installations, the controls are mounted on a panel located in the cockpit and the radio equipment is located in racks in another part of the aircraft.
Because of the many different types and models of radios in use, it is not possible to discuss the specific techniques for operating each in this manual. However, there are various practices of a nonspecific nature which apply to all radios. These general practices will be described.
VHF (Very High Frequency) Communications VHF airborne communication sets operate in the frequency range from 108.0 MHz to 135.95 MHz.
VHF receivers are manufactured that cover only the communications frequencies, or both communications and navigation frequencies. In general, the VHF radio waves follow approximately straight lines. Theoretically, the range of contact is the distance to the horizon and this distance is determined by the heights of the transmitting and receiving antennas. However, communication is sometimes possible many hundreds of miles beyond the assumed horizon range.
Many VHF radios have the transmitter, receiver, power supply, and operating controls built into a 522
communication system mounted on the instrument VHF comm antenna panel and the remainder remotely installed in a radio or baggage compartment.
To perform an operational check of a VHF com- Power source - munication system, a source of electric power must be available. After turning the radio control switch "on", allow sufficient time for the equipment to warm up before beginning the operational checks.
Using the frequency selector, select the frequency of the ground station to be contacted. Adjust the vol- VHF transceiver ume control to the desired level.
Microphone Speaker Earphones FIGURE 13-4. VHF system diagram.
single unit. This unit is frequently installed in a cutout in the instrument panel. A system diagram of a typical panel-mounted VHF transceiver is shown in figure 13-4. Others have certain portions of the With the microphone held close to the mouth, press the microphone button and speak directly into the microphone to transmit; when through talking, release the button. This action will return the communication receiver to operation. When the ground station acknowledges the initial transmission, request that an operational check be made on all frequencies or channels. Prior to transmitting, make certain that a station license is displayed in the aircraft. In addition, the person operating the transmitter must hold a current Restricted Radiotelephone Operator's permit. Both the station license and the operator's permit are issued by the FCC.
Speaker Earphones Antenna output Power source Microphone Antenna coupler HF SSB transceiver Control unit FIGURE 13-5. HF system diagram.
HF (High Frequency) Communications A high frequency communication system (figure 13-5) is used for long-range communication. HF systems operate essentially the same as a VHF system, but operate in the frequency range from 3 MHz to 30 MHz. Communications over long distances are possible with HF radio because of the longer transmission range. HF transmitters have higher power outputs than VHF transmitters.
The design of antennas used with HF communication systems vary with the size and shape of the aircraft. Aircraft which cruise below 300 m.p.h.
generally use a long wire antenna. Higher speed aircraft have specially designed antenna probes installed in the vertical stabilizer. Regardless of the type antenna, a tuner is used to match the impedance of the transceiver to the antenna.
An operational check of an HF radio consists of turning the control switch to "on," adjusting the RF gain and volume controls, selecting the desired channel and transmitting the appropriate message to the called station. Best adjustment of the gain control can be obtained with the volume control set at half range. The gain control is used to provide the strongest signal with the least amount of noise.
The volume control is used to set sound level and affects only the loudness of the signal.
AIRBORNE NAVIGATION EQUIPMENT "Airborne navigation equipment" is a phrase embracing many systems and instruments. These systems include VHF omnirange (VOR), instrument landing systems, distance-measuring equipment, automatic direction finders, doppler systems, and inertial navigation systems.
When applied to navigation, the radio receivers and transmitters handle signals which are used to determine bearing and in some cases distance, from geographical points or radio stations.
VHF OMNIRANGE SYSTEM The VHF VOR (omnidirectional range) is an electronic navigation system. As the name implies, the omnidirectional or all-directional range station provides the pilot with courses from any point within its service range. It produces 360 usable radials or courses, any one of which is a radio path connected to the station. The radials can be considered as lines that extend from the transmitter antenna like spokes of a wheel. Operation is in the VHF portion of the radio spectrum (frequency range of 108.0 MHz - 117.95 MHz) with the result that interference from atmospheric and precipitation static is negligible. The navigational information is visually displayed on an instrument in the cockpit.
The typical airborne VOR receiving system (figure 13-6) consists of a receiver, visual indicator, antennas, and a power supply. In addition, a unit frequency selector is required and in some cases located on the receiver unit front panel. Some manufacturers design a remote control frequency selector so the equipment may be installed in some other area of the aircraft. This frequency selector is used to tune the receiver to a selected VOR ground station.
• Power source
- Audio output VOR/LOC receiver Indicator Frequency selector FIGURE 13-6. VOR system diagram.
The VOR receiver, in addition to course naviga tion, functions as a localizer receiver during ILS (instrument landing system) operation. Also, some VOR receivers include a glide slope receiver in a single case. Regardless of how individual manufacturers may design the VOR equipment, the intelligence from the VOR receiver is displayed on the CDI (course deviation indicator).
The CDI, figure 13-7, performs several functions.
During VOR operation the vertical needle is used as the course indicator. The vertical needle also indicates when the aircraft deviates from the course and the direction the aircraft must be turned to attain the desired course. The "TO-FROM" indicator presents the direction to or from the station along the omniradial. The course deviation indicator also contains a "VOR-LOC" flag alarm. Normally this is a small arm which extends into view 524
Course pointer
- Horizontal pointer Vertical 1'33 pointer *30 Glide slope "flag"
15.
18, Course selector
OFF/TO-FROM Reciprocal course indicator pointer FIGURE 13-7. Course deviation indicator.
only in the case of a receiver malfunction or the loss of a transmitted signal.
When localizer signals are selected on the receiver, the indicator shows the position of the localizer beam relative to the aircraft and the direction the aircraft must be turned to intercept the localizer.
During VOR operation the VOR radial to be used is selected by rotating the OBS (omnibearing selector). The OBS is generally located on the CDI;
however, in some installations it is a part of the navigation receiver. The OBS is graduated in degrees from zero to 360. Each degree is a VOR course to be flown in reference to a ground station.
The following steps are typical of those performed during a ground operational check. When checking a VOR system, follow the specific proce.
dures recommended by the equipment manufacturer. The operational check can be performed using appropriate test equipment, VOT (Very High Frequency Omnirange Test) or the terminal VOR facility.
(1) Place the on/off switch in the "ON" position.
(2) Adjust the frequency selector to the desired station.
(3) Allow sufficient time for equipment to warm up.
(4) The VOR flag will disappear when the VOR station signal is received.
(5) Adjust the volume control to the desired level; assure that the selected VOR station identification is clear and correct.
(6) Check the CDI for vertical needle deflection.
(7) Center the vertical needle by rotating the OBS.
(8) Check "TO-FROM" indicator for "TO" indication.
(9) Rotate the OBS to read 10° higher than the setting at which the vertical needle was centered. The vertical needle should move left and cover the last dot which corresponds to 10° course displacement.
(10) Return the OBS to the original position.
The vertical needle should return to the center position.
(11) Rotate the OBS to read 10° lower than the original setting. The pertical needle should move right and cover the last dot which corresponds to 10° course displacement.
(12) Assure that the vertical needle moves an equal distance in both directions. This total course width or course sensitivity should be 20°.
NOTE: When "TO-FROM" indicator reads "FROM", the vertical needle will deflect in the direction opposite that stated in the above procedures.
If the operational check is unsatisfactory, it will be necessary to remove the VOR receiver and associated instruments from the aircraft and have them calibrated.
INSTRUMENT LANDING SYSTEM The ILS (instrument landing system), one of the facilities of the Federal airways, operates in the VHF portion of the electromagnetic spectrum. The ILS can be visualized as a slide made of radio signals on which the aircraft can be brought safely to the runway.
The entire system consists of a runway localizer, a glide slope signal, and marker beacons for position location. The localizer equipment produces a radio course aligned with the center of an airport runway. The on-course signals result from equal reception of two signals; one containing 90 Hz modulation and the other containing 150 Hz modulation. On one side of the runway center line the radio receiver develops an output in which the 150 Hz tone predominates. This area is called the blue 525
sector. On the other side of the centerline the 90 Hz output is greater. This area is the yellow sector. VOR/LOC ANTENNA The localizer facility operates in the frequency range of 108.0 MHz to 112.0 MHz on the "odd tenths" of the megahertz steps. The VOR receiver also operates in this ferquency range on the even tenths" of the megahertz steps. The airborne VOR VOR/LOC receiver receiver functions as the localizer receiver during ILS operation.
The glide slope is a radio beam which provides Audio output vertical guidance to the pilot, assisting him in making the correct angle of descent to the runway.
Glide slope signals are radiated from two antennas Indicator located adjacent to the touchdown point of the runway. Each glide slope facility operates in the UHF frequency range from 329.3 MHz to 335.0 MHz. Frequency selector The glide slope and VOR/localizer receivers may be separate receivers or combined in a single case.
The glide slope receiver is paired to the localizer and one frequency selector is used to tune both Glideslope antenna receivers. A component diagram of an ILS is shown in figure 13-8.
The information from both localizer and glide slope receivers is presented on the CDI; the vertical needle displays localizer information and the hori- • Power source zontal needle displays glide slope information (figure 13-9). When both needles are centered, the Glideslope receiver aircraft is on course and descending at the proper FIcURE 13-8. Component diagram of an ILS.
Runway Full-scale yellow, Full-scale blue, "fly to right" "fly to left" Localizer pointer Glide slope pointer Fly down Fly Up FIGURE 13-9. Glide slope information.
rate. In addition the CDI contains a red warning flag for each system which comes into view when the receiver fails or the loss of a transmitted signal occurs.
Two antennas are usually required for ILS operation. One for the localizer receiver, also used for VOR navigation, and one for the glide slope. Some of the small aircraft use a single multi-element antenna for both glide slope and VOR/LOC operation. The VOR/localizer antenna is normally installed on the top of the aircraft fuselage or flush mounted in the vertical stabilizer. The glide slope antenna is, in most cases, installed on the nose of the aircraft. On aircraft equipped with radomes, the glide slope antenna is installed under the radome.
Marker Beacons Marker beacons are used in connection with the instrument landing system. The markers are signals which indicate the position of the aircraft along the approach to the runway. Two markers are used in each installation. The location of each marker is identified by both an aural tone and a signal lamp.
The marker beacon transmitters, operating on a fixed 75 MHz frequency, are placed at specific locations along the approach pattern of an ILS facility.
The antenna radiation pattern is beamed straight пр.
A marker receiver (figure 13-10) installed in the aircraft receives the antenna signals and converts them into power to illuminate a signal lamp and produce an audible tone in a headset. The outer Power source marker marks the beginning of the approach path.
The outer marker signal is modulated by a 400-Hz signal which produces a tone keyed in long dashes.
In addition to providing aural identification, the signal lights a purple lamp in the cockpit. The middle marker is usually about 3,500 ft. from the end of the runway and is modulated at 1,300 Hz which produces a higher-pitched tone keyed with alternate dots and dashes. An amber lamp flashes to indicate that the aircraft is passing over the middle marker.
Marker beacon receivers vary in design from simple receivers that have no operating controls and no aural output to more sophisticated receivers that produce an aural tone and have an on/off switch and a volume control to adjust the sound level of the identification code.
Where three lights are used, a white light indicates the aircraft positions at various points along the airways. In addition to the light, a rapid series of tones (six dots per second) of 3,000 Hz is received in the headset. Distance-measuring equipment is rapidly replacing the "along-route" marker system. A 3,000-Hz tone and white light marker are also being used for inner markers (missed approach point) on some Category II, ILS-equipped runways.
The ILS system cannot be ground tested fully without using test equipment simulating localizer and glide slope signals.
If an aircraft is located at an airport which has an ILS-equipped runway, it may be possible to determine if the receiver is functioning by performing Marker antenna Hi-Lo switch Marker lights Marker receiver Earphones FIcURE 13-10. Marker receiver system diagram.
the following. Place the on/off switch (if so equipped) in the "on" position and adjust the frequency selector to the proper ILS channel for the airport where the aircraft is located. Allow sufficient time for the equipment to warm up. In a strong signal area, both the localizer and glide slope warning flags will either start to move or go completely out of view. Observe that both cross pointers are deflected to their maximum displacement.
Some of the more sophisticated solid-state ILS equipment contains self-monitoring circuits. These circuits can be used for performing an operational test using the procedures in the aircraft or equipment manufacturer's service manuals.
DISTANCE-MEASURING EQUIPMENT The purpose of DME (distance-measuring equipment) is to provide a constant visual indication of the distance the aircraft is from a ground station. A DME reading is not a true indication of point-topoint distance as measured over the ground. DME indicates the slant range between the aircraft and the ground station. Slant-range error increases as the aircraft approaches the station. At a distance of 30 to 60 nautical miles the slant range error is negligible.
DME operates in the UHF range of the radio frequency spectrum. The transmitting frequencies are in two groups, between 962 MHz to 1,024 MHz and 1,151 MHz to 1,212 MHz; the receiving frequencies are between 1,025 MHz to 1,149 MHz.
Transmitting and receiving frequencies are given a channel number which is paired with a VOR channel. In some aircraft installations the DME channel selector is ganged with the VOR channel selector to simplify the radio operation. A typical DME control panel is shown in figure 13-11.
The aircraft is equipped with a DME transceiver which is tuned to a selected DME ground station.
Usually DME ground stations are colocated with a
COMM NAV
116.00 108.00 OFF VOR facility (called VORTAC). The airborne transceiver transmits a pair of spaced pulses to the ground station. The pulse spacing serves to identify the signal as a valid DME interrogation. After reception of the challenging pulses, the ground station responds with a pulse transmission on a separate frequency to send a reply to the aircraft. Upon reception of the signal by the airborne transceiver, the elapsed time between the challenges and the reply is measured This time interval is a measure of the distance separating the aircraft and the ground station. This distance is indicated in nautical miles by a cockpit instrument similar to the one shown in figure 13-12.
3 5-•MILES FIGURE 13-12. DME digital indicator.
A typical DME antenna is shown in figure 13-13.
Most DME antennas have a cover installed to protect them from damage. The DME antenna is usually a short, stub type mounted on the lower surface of the aircraft. To prevent an interruption in DME operation, the antenna must be located in a position that will not be blanked by the wing when the aircraft is banked.
FIGURE 13-11. Typical navigation DME control. FICURE 13-13. Typical DME antenna.
To determine if the DME operates, turn the on/ off switch to the "on" position and select the appropriate channel. Allow sufficient time for the equipment to warm up. During this period, the distance indicator, both digital or pointer, will travel from minimum to maximum readings (sweep or search).
When the DME has locked on a station, the indicator will stop searching and the red warning flag (if the indicator is equipped with one) will disappear.
In most installations, no functional check can be made on the ground without a DME test set.
AUTOMATIC DIRECTION FINDERS ADF (automatic direction finders) are radio receivers equipped with directional antennas which are used to determine the direction from which signals are received. Most ADF receivers provide controls for manual operation in addition to automatic direction finding. When an aircraft is within reception range of a radio station, the ADF equipment provides a means of fixing the position with reasonable accuracy. The ADF operates in the low and medium frequency spectrum from 190 kHz through 1,750 kHz. The direction to the station is displayed, on an indicator located in the cockpit, as a relative bearing to the station.
The airborne equipment (figure 13-14), consists of a receiver, loop antenna, sense or nondirectional antenna, indicator, and control unit. Most ADF receivers used in general aviation aircraft are panel mounted. Their operating controls appear on the front of the radio case.
Sense antenna Loop antenna In one type ADF system, the loop antenna (figure 13-15) rotates through 360° and receives maximum signal strength when in a parallel position with the direction of the transmitted signal. As the loop is rotated from this position, the signal becomes weaker and reaches a minimum when the plane of the loop is perpendicular to the direction of the transmitted signal. This position of the loop is called the null position. The null position of the loop is used for direction finding. When the loop is rotated to a null position the radio station is being received on a line perpendicular to the plane of the loop. However, the direction of the radio station from the aircraft may be either of two directions 180° apart. The inability of the loop antenna to determine from which of the two directions the transmitted signal is being received necessitates the installation of a sense antenna.
The loop and sense antenna are both connected to the ADF receiver. When the signal strength of the sense antenna is superimposed on the signal received from the loop antenna, it results in only Internal loop 200 TOO 200 160 20 M03 700 ADF REC OFF ADF receiver and control Enclosed loop FICURE 13-14. Typical ADF installation. FICURE 13-15. Typical ADF antennas.