Sensing elements Command elements Output elements Pressure source Directional gyro indicator он OтF Flight Aileron controller servo actuator Turn-and-bank indicator gyro Rudder servo actuator Computer Attitude indicator Elevator Navigation signals servo actuator Altitude control Heading selector Trim actuator FIGURE 12-75. Typical autopilot system components.
majority of the servos in use today are either electric motors or electro/pneumatic servos.
An sircraft may have from one to three servos to operate the primary flight controls. One servo operates the ailerons, a second operates the rudder, and a third operates the clovators. Each servo drives ita 514
OFI FIGURE 12-76. Typical autopilot controller.
associated control surface to follow the directions of the particular automatic pilot channel to which the servo is connected.
Two types of electric motor-operated servos are in general use. In one, a motor is connected to the servo output shaft through reduction gears. The motor starts, stops, and reverses direction in reponse to the commands of the gyros or controller.
The other type of electric servo uses a constantly running motor geared to the output shaft through two magnetic clutches. The clutches are arranged so that energizing one clutch transmits motor torque to turn the output shaft in one direction; energizing the other clutch turns the shaft in the opposite direction.
The electro/pneumatic servos are controlled by electrical signals from the autopilot amplifier and actuated by an appropriate air pressure source. The source may be a vacuum system pump or turbine engine bleed air. Each servo consists of an electro/ magnetic valve assembly and an output linkage assembly.
FLIGHT DIRECTOR SYSTEMS A Hight director system is an instrument system consisting of electronic components that will compute and indicate the aircraft attitude required to attain and maintain a preselected flight condition.
"Command" indicators on the instrument indicate how much and in what direction the attitude of the aircraft must be changed to achieve the desired result. The computed command indications relieve the operator of many of the mental calculations required for instrument fights, such as interception angles, wind drift correction, and rates of climb and descent.
A flight director system has several components;
the principal ones are the gyroscope, computer, and the cockpit presentaton.The gyro detects deviations from a preselected aircraft attitude. Any force exerted against the gyroscope is electrically transmitted to the computer, which in turn, sends a computed signal to the flight indicator, telling the operator what must be done with the controls. When using a flight director system, the operator is, in a sense, acting as a servo, following orders given by the command indicators.
The computers used in the various types of flight director systems are basically the same; however, the numbers and types of functions available will vary between systems because of the mission of a particular aircraft, the limited aircraft space available for installation, and the excessive cost of functions not absolutely required.
The instrument panel presentations and operating methods vary considerably between different sys.
tems. Command indications may be presented by several different symbols, such as bar-type command indicators with different types of movements, a phantom aircraft symbol, or two-element crossbar indicators.
Many flight director systems are equipped with an "altitude-hold" function, which permits selection of a desired altitude; the flight director computes the pitch attitude necessary to maintain this particular altitude.
A flight director greatly simplifies problems of aerial navigation. Selection of the VOR function electronically links the computer to the omnirange receiver. After selection of the desired omnicourse, the flight director will direct the bank attitude necessary to intercept and maintain this course.
Flight director systems are designed to offer the greatest assistance during the instrument approach phase of flight. ILS localizer and glide slope signals are transmitted through the receivers to the computer, and are presented as command indications.
With the altitude-hold function, level flight can be maintained during the maneuvering and procedure turn phase of the approach. Once inbound on the localizer, the command signals of the flight director are maintained in a centered or zero condition.
Compensation for wind drift is automatic. Interception of the glide slope will cause a downward indication of the command pitch indicator. Any de- 515
viation from the proper glide slope path will cause a fly-up or fly-down indication on the flight director pitch command symbol. When altitude-hold is being used, it automatically disengages when the glide slope has been intercepted.
A flight director system not only shows the present situation, but also predicts the future consequences of this situation. For example, a momentary change in attitude is detected by the computer, and command symbol movement is made to correct this condition possibly before an altitude error can result. Thus, greater precision is achieved with less mental effort on the part of the aircraft operator.
AUTOPILOT SYSTEM MAINTENANCE The information in this section does not apply to any particular autopilot system, but gives general information which relates to all autopilot systems.
Maintenance of an autopilot system consists of visual inspection, replacement of components, cleaning, lubrication, and an operational checkout of the system.
With the autopilot disengaged, the flight controls should function smoothly. The resistance offered by the autopilot servos should not affect the control of the aircraft. The interconnecting mechanism between the autopilot system and the flight control system should be correctly aligned and smooth in operation. When applicable, the operating cables should be checked for tension.
An operational check is important to assure that every circuit is functioning properly. An autopilot operational check should be performed on new installations, after replacement of an autopilot compo- a to the nent, or whenever a malfunction in the autopilot system is suspected.
After the aircraft's main power switch has been turned on, allow the gyros to come up to speed and the amplifier to warm up before engaging the autopilot. Some systems are designed with safeguards that prevent premature autopilot engagement.
While holding the control column in the normal flight position, engage the system using the engaging control (switch, handle).
After the system is engaged, perform the operational checks specified for the particular aircraft. In general, the checks consist of:
(1) Rotate the turn knob to the left; the left rudder pedal should move forward, and the control column wheel should move to and the control column wheel should move slightly aft.
(2) Rotate the turn knob to the right; the right rudder pedal should move forward, and the control column wheel should move to the right. The control column should move slightly aft. Return the turn knob to the center position; the flight controls should return to the level-fight position.
(3) Rotate the pitch-trim knob forward; the control column should move forward.
(4) Rotate the pitch-trim knob aft; the control If the aircraft has a pitch-trim system installed, it should function to add downtrim as the control column moves forward, and add uptrim as the column moves aft. Many pitch-trim systems have an automatic and a manual mode of operation. The above action will occur only in the automatic mode.
Check to see if it is possible to manually override or overpower the autopilot system in all control positions. Center all the controls when the operational checks have been completed.
Disengage the autopilot system and check for freedom of the control surfaces by moving the control columns and rudder pedals. Then re-engage the system and check the emergency disconnect release circuit. The autopilot should disengae each time the release button is actuated.
When performing maintenance and operational checks on a specific autopilot system, always follow the procedure recommended by the aircraft or equipment manufacturer.
Annunciator System Instruments are installed for two purposes, one to display current conditions, the other to notify of unsatisfactory conditions. Colored scales are used; usually green for satisfactory; yellow for caution or borderline conditions; red, for unsatisfactory conditions. As aircraft have become more complex with many systems to be monitored, the need for a centralized warning system became apparent. The necessity to coordinate engine and flight controls emphasized this need. What evolved is an annunciator or master warning system (figure 12-77).
Certain system failures are immediately indicated on an annunciator panel on the main instru ment panel. A master caution light and a light indicating the faulting system flash on. The master light may be reset to "Off, but the indicating light will remain "On" until the fault is corrected or the equipment concerned is shut down. By resetting.
the master caution light is ready to warn of a sub- 516
SYSTEM ATA NUMBER INDICATION uses bimetallic strips set at predetermined temperatures. The second group responds to pressure Aircraft Fuel Fuel Pressure Low changes and uses a flexible chamber that moves Engine Fuel Fuel Pressure Low when pressurized. The third group consists of Electrical Inverter Out mechanically operated switches and/or contacts on Generator Generator Generator Overheated Generator Out a relay.
Starting Starter Engaged An annunciator system may include any or all Oil Pressure Low of the following indications or others as applicable.
Brake Pressure Low Not Locked Down Aural Warning System Anti-Skid Out Cabin Pressure High Aircraft with retractable landing. gear use an una Cabin Pressure Low aural warning system to alert the crew to an unsafe Speed Brake Extended condition. A bell will sound if the throttle is re- Not Set for Takeoff Thrust Reversal locked condition (figure 12-78). tracted and the landing gear is not in a down and Aural warning systems range in complexity Doors ked from the simple one just described to that system necessary for safe operation of the most complex Doors ked ater transport aircraft.
Out A typical transport aircraft has an aural warnpen ing system which will alert the pilot with audio signals to: An abnormal takeoff condition, landing Low condition, pressurization condition, mach-speed con- Firewarning dition, an engine or wheel well fire, calls from the crew call system, and calls from the secal system.
Shown in figure 12-78 are some of the problems FIGURE 12-77. Warning in annunciator system. which trigger warning signals in the aural warning system. For example: a continuous horn sounding sequent fault even before correction of the initial during landing would indicate the landing gear is fault. A press to test light is available for testing not down and locked when flaps are less than full the circuits in this system. up and the throttle is retarded. The corrective vices divided into groups, according to their method One late model business jet has the sensing de- throttle. action would be to raise the flaps and advance the of operation. The fast group responds to heat and (See Agure 12-78 on next page) 517
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