Instrument Flying PDF

Navigation Systems / 55 Instrument Flying Lighting the airports and airways proved to be a tremendous advance in night- time navigation, but it still required that...

Navigation Systems / 55 Instrument Flying Lighting the airports and airways proved to be a tremendous advance in night- time navigation, but it still required that pilots !y in weather conditions that would permit them to see the rotating beacons. If a pilot !ew in or above a cloud layer, or if the !ight visibility diminished to less than 15 miles, he or she would be unable to see the rotating beacons and navigate to the destination airport. As advances were made in aircraft design and instrumentation, it soon became possible for pilots to control their aircraft using just cockpit instrumen- tation, !ying their aircraft without visual reference to the natural horizon. The new cockpit instruments were based on gyroscopic principles and included the artificial horizon (now called the attitude indicator), the heading indicator, and the turn and bank indicator (now called the turn coordinator). The attitude indicator (Figure 2–8) mimics the movements of the natural horizon, providing the pilot with accurate aircraft attitude information. Using the attitude indicator, pilots can determine whether their aircraft is banked and whether its nose is pointed up or down. This allows them to adjust their aircraft’s !ight attitude and keep the aircraft upright and under control. The heading indicator, as described previously in this chapter, provides a more stable and accurate indication of the aircraft’s !ight direction than does the magnetic compass. The turn coordinator (Figure 2–9) is used by the pilot to indicate the direction and the rate of turn. These instruments, used in conjunction with the already existing altimeter (Figure 2–10) and airspeed indicator (Figure 2–11), made it possible for pilots to accurately control their aircraft without reference to the natural horizon. Unfortunately, the federal airway system had not kept pace with these instrumentation developments and, until the late 1920s, was still based on sec- tional charts and rotating beacons, which required that pilots have at least 15 to 20 miles of visibility to navigate at night. There was no provision for naviga- tion when the visibility dropped below these values. Beech Aircraft Corporation Figure 2–8. The attitude Figure 2–9. The turn Figure 2–10. The Figure 2–11. The indicator. coordinator. altimeter. airspeed indicator. 56 / CHAPTER 2 Electronic Navigation Four-Course In an early attempt to remedy this situation, the federal government began to Radio Range install the four-course radio range in the late 1920s. This new radio device was placed at intervals along each federal airway and permitted the pilot to navi- gate without using outside visual references. Four-course radio ranges soon became the U.S. and international standard for aviation navigation and were widely used in the United States until the 1950s. The four-course radio range used a 1,500-watt transmitter that oper- ated on a frequency between 190 and 565 kHz. The transmitting antenna con- sisted of two single-wire vertical loops strung out on five wooden masts. These wires were attached to the masts to form two figure-eight patterns (see Figure 2–12). This arrangement produced two separate radio transmission patterns g O le n- e co rs ur ou s c e n- le O g N Transmitter A A N g O le n- e co urs ur co se n- le O g Figure 2–12. Field pattern of four-course radio range.  Navigation Systems / 57 that overlapped slightly. One loop constantly transmitted the Morse code for the letter A (dot-dash); the other transmitted the Morse code for the letter N (dash-dot). Any pilot wishing to use a four-course range simply tuned the receiver on the aircraft to the proper frequency and listened to the transmitted signal through earphones. If the aircraft was located somewhere within the A sector, the pilot would hear Morse code for the letter A (dot-dash) constantly being repeated. If the aircraft was in the N sector, the pilot would hear the Morse code for the letter N (dash-dot). If the aircraft was located where the two transmissions overlapped (the on-course line of position), the dot-dash and the dash-dot would be of equal strength and would produce a constant tone in the pilot’s headset. When navigating to a four-course radio range, all the pilot needed to do was to head the aircraft toward one of these on-course “legs” and then proceed along it to the radio range. If the aircraft drifted off course, the pilot would begin to hear the individual A or N Morse code becoming dominant, requir- ing an adjustment in the aircraft’s heading until he or she could again hear the constant on-course tone. The wire loop antennas of the four-course range could be constructed in such a manner as to “aim” the on-course legs toward other radio ranges. Although it permitted navigation during periods of low visibility, the four-course (or A–N) range still had a number of deficiencies that limited its usefulness. For example, disoriented pilots found it very difficult to accurately determine their position using the A–N range. Although pilots could easily determine whether their aircraft was located on one of the on-course legs, they often found it impossible to determine which of the four legs they were on. Through trial and error, a pilot could eventually determine which head- ing would keep the aircraft on course and lead to the station. If the pilot was between on-course legs (totally within either an A or an N sector), it was time consuming and difficult to pinpoint the aircraft’s location and determine the proper heading that would lead to an on-course leg. In addition, since the A–N ranges operated in the 190 to 565 kHz band (just below the present AM radio band), the transmitted signal could easily be distorted by obstructions or dis- rupted by lightning-induced static. In mountainous areas, it was possible for the radio transmission of the four-course range to bounce off nearby terrain and produce false on-course signals. During thunderstorms, when pilots desperately needed the course guidance of the radio range, lightning-induced static could overwhelm the relatively weak signal transmitted by the radio range, leaving the pilot with only static emanating from the receiver. Certainly, the A–N range was a tre- mendous advancement in instrument navigation, but these deficiencies limited its overall use. Introduction The A–N range provided the pilot with only bearing and course information. of Marker It did not provide any information concerning distance to the station. To mini- Beacons mize this problem, the CAA began installing marker beacons along the on- course legs. These low-powered radio beacons were designed to transmit a 58 / CHAPTER 2 distinctive tone and code that could be received by the aircraft as it passed directly overhead. The pilots could use the code to identify which beacon was crossed and use this information to accurately determine their aircraft’s posi- tion along the on-course leg. But whenever the aircraft was between marker beacons or no longer on one of the on-course legs, the marker beacons were useless in helping determine the aircraft’s location. Nondirec- While the CAA was developing and installing A–N radio ranges, the nondirec- tional tional radio beacon (NDB) was also being developed. The NDB transmits a Beacons uniform signal omnidirectionally from the transmitter, using the low- and medium-frequency band (190–540 kHz). The receiver on the aircraft (known as a direction finder or DF) was originally equipped with a looptype antenna that the pilot rotated manually. When the antenna was rotated so that the plane of the loop was perpendicular to the transmitted signal, the “null” position was reached, and the pilot would be able to hear the transmitted signal. Using the magnetic compass and the NDB receiver, the pilot could then determine the aircraft’s bearing from the nondirectional beacon. This bearing could be plot- ted on a chart as line of position. Plotting lines of position from two NDBs permitted the pilot to pinpoint the aircraft’s exact location. If the pilot wished to !y toward the NDB, he or she would turn until the NDB station was located directly ahead of the aircraft. If the winds aloft caused the aircraft to drift off course, the pilot would readjust the aircraft’s heading, keeping the NDB directly ahead of the aircraft. This method of navigation is called homing. Automatic Trying to manually manipulate the DF antenna while !ying the aircraft proved Direction to be a cumbersome method of navigation and usually provided the pilot with Finder relatively inaccurate position information. As advances were made in aircraft electronics, the manually operated NDB receiver was soon replaced by the automatic direction finder (ADF), which could electronically determine the bearing to the NDB and display this information to the pilot (see Figure 2–13). Using ADF equipment in conjunction with the aircraft’s heading indicator, the Beech Aircraft Corporation Figure 2–13. ADF receiver and indicator.  Navigation Systems / 59 pilot could easily determine the aircraft’s relative bearing from the station and use this information to determine the proper heading that would lead to the beacon. The development of the ADF hastened pilot acceptance of the NDB as a navigation aid. The first NDB was installed in the United States in 1924. By 1964, 272 high-powered NDBs had been installed throughout the country. A series of federal airways using NDBs for en route navigation were soon developed. Because these airways were designated by a color and a number (for example, RED-64 or GREEN-32), they were soon referred to simply as colored airways. Compass In addition to their role as en route navigation aids, NDBs were located at Locators airports or along instrument-approach paths to assist pilots who were conduct- ing such approaches. NDBs along the final approach are known as compass locators. In 1965, the federal government began to decommission the high- powered NDBs used for en route navigation. Due to their extremely low cost and ease of installation, however, low-power units continued to serve smaller airports as instrument-approach aids. Visual Aural In 1937, the Radio Development Section of the Bureau of Air Commerce dem- Range onstrated an improved radio range at its research center in Indianapolis. This new radio range, called the visual aural range (VAR), was an improvement over the old A–N range in two major areas. The VAR was designed to operate in the very high frequency (VHF) band located around 63 mHz. This frequency band was chosen since transmitters operating on VHF frequencies are rarely affected by static caused by lightning. VHF transmissions are also line of sight, which means that they do not follow the curvature of the earth. One significant advan- tage of using VHF frequencies is that although they can easily be blocked by terrain and obstructions, they are seldom re!ected by them. The use of VHF frequencies would thus minimize the re!ection problem that plagued the A–N ranges. The VAR also solved the orientation problem inherent with the A–N range by transmitting four radio signals instead of two. While retaining the Morse-coded A and N signals, the VAR also transmitted overlapping “blue” and “yellow” signals perpendicular to the A and N signals (see Figure 2–14). An instrument on board the aircraft would indicate whether it was in the blue or the yellow sector. The pilot was still required to listen to the VAR to determine whether the aircraft was within the A or the N sector, however. The addition of the overlapping color signals gave each sector a unique identification that enabled pilots to accurately determine their aircraft’s location. The first operational VAR was installed at Matawan, New Jersey, in 1944. By 1948 a total of sixty-eight VARs had been commissioned by the CAA and were located along federal airways. The VAR never gained wide acceptance, however, since it was soon replaced by an improved radio range that emitted an infinite number of courses instead of just four. This new navigation aid was 60 / CHAPTER 2 g O le n- se co ur urs co e n- le O g N Yellow Transmitter sector A A N Blue sector g O le n- se c ou r ou rs c e n- le O g Figure 2–14. VAR operation. called the VHF omnidirectional range (VOR). In the early 1950s, as VORs were being installed around the country, the CAA began to decommission the VARs, with the last being retired from service in 1960. VHF Omni- Research on a radio range that would offer pilots more than four courses and directional transmit in the static-free VHF radio spectrum had started in 1937. The Range (VOR) Washington Institute of Technology delivered the first operable VHF omnidi- rectional range (VOR) to the CAA in 1944. This experimental VOR operated on a frequency of 125 mHz. After extensive testing and development by the CAA, three prototype VORs were installed at Patuxent River, Maryland; Philipsburg, Pennsylvania; and Ogden, Utah. After operational testing at these three sites, the CAA adopted the VOR as the national civil navigation standard in 1946. The  Navigation Systems / 61 Wadsworth, Inc., photo by Paul Bowen Photography, Inc. Figure 2–15. VOR ground station. VOR was also selected as the international civil navigation standard in 1949 by the International Civil Aviation Organization (see Figure 2–15). VOR Operation The VOR offered a number of improvements over the old A–N and VAR methods. The VOR transmits an infinite number of navigation courses, selectable by the pilot, instead of just four. The VOR is also relatively immune to the re!ections and static inherent in the operation of the A–N ranges. Each VOR is assigned a frequency between 108.10 and 117.90 mHz. The VOR transmission is modulated with two signals: a reference-phase signal that is constant in all directions and a variable-phase signal whose phase varies with azimuth. The variable-phase signal is modulated so that at magnetic north the reference and variable signals are precisely in phase with each other. In any other direction, the VOR is designed so that the two signals are no longer in phase. The VOR receiver on board the aircraft measures the phase difference between the two signals to determine the azimuth angle of the aircraft in relation 62 / CHAPTER 2 R V R 360∞ R Radial 315∞ 45∞ V Radial V Radial R R VOR 270∞ 90∞ Radial Radial V V R R 225∞ 135∞ Radial Radial 180∞ Radial V V V R = Reference signal V = Variable signal Figure 2–16. VOR operation. to the VOR transmitter. When the aircraft is directly east of the VOR, the vari- able signal will lag the reference signal by 90°. An aircraft located directly east of the VOR is said to be on the 90° radial of the VOR (see Figure 2–16). An aircraft directly south of the VOR will receive the variable signal lagging the reference signal by 180° and will be on the 180° radial. An aircraft located on the 359° radial (north of the VOR) will receive the variable signal lagging the reference signal by 359°. The radial to be !own by the pilot is selected on the aircraft’s VOR indica- tor (see Figure 2–17) using the omni bearing selector (OBS). After selecting the appropriate VOR frequency, the indicator in the cockpit will inform the pilot whether the selected course will lead to the station or away from it (known as the To–From !ag). The VOR indicator will also display any lateral deviation from the selected course, using a vertical pointer known as the course deviation  Navigation Systems / 63 Bendix/King Division of Allied Signal Aerospace Company Figure 2–17. VOR receiver and indicator. indicator (CDI). If the aircraft is to the right of the selected course, the CDI will be to the left of center, advising the pilot to alter the aircraft’s course to the left in order to return to the selected radial. If the aircraft is left of course, the CDI will be right of center. If the aircraft is precisely located on the radial selected by the pilot, the CDI will be centered. VORs used for en route navigation have an output power of 200 watts and are assigned a frequency between 112.00 and 117.90 mHz. This signal permits en route VOR reception up to a distance of 200 miles. Terminal VORs (used solely for instrument approaches) have an effective radiated power of 50 watts and are assigned a frequency between 108.10 and 111.80 mHz. Terminal VORs can be received up to a distance of about 25 nautical miles. Since VOR transmissions are line of sight, these reception distances vary depending on the receiving aircraft’s altitude (see Figure 2–18). VOR Categories A number of difficulties were encountered as soon as the CAA began to install VORs along the federal airways. Since VHF transmissions are line of sight, low-!ying aircraft were unable to receive the VOR signal if they were “below the horizon.” This limitation forced the CAA to place the VORs no farther than 80 miles from each other to ensure adequate recep- tion for aircraft operating at low altitudes. Because only a limited number 64 / CHAPTER 2 A and B received A received No B received signal VOR A VOR B Approximate coverage Maximum distance in Altitude of statute miles from VOR aircraft 45 1,000 100 5,000 150 10,000 200 20,000 Figure 2–18. VOR reception distances. of frequencies can be assigned to VORs, some would have to be assigned the same operating frequency, which could cause interference problems for aircraft operating at very high altitudes, as they might receive the signals being broad- cast from two or more VORs operating on the same frequency. The resulting interference would render the navigation signal unusable. The CAA responded by designating every VOR as a terminal, low-, or high-altitude VOR. Terminal VORs (TVORs) are low powered and are usable up to a distance of 2.5 nautical miles. TVORs are not to be used for en route navigation but are reserved for local navigation and instrument approaches. Low-altitude VORs guarantee interference-free reception to aircraft operating up to 40 nautical miles away. This interference-free zone is guaranteed only at or below 18,000 feet. Low-altitude VORs cannot be used by aircraft operating above 18,000 feet or farther than 40 miles away, as there is no guarantee that another VOR operating on the same frequency will not cause interference. High- altitude VORs are used by aircraft operating between 18,000 and 60,000 feet, at ranges up to 200 nautical miles. These limitations imposed upon VORs are known as service volumes (see Figures 2–19 and 2–20).  Navigation Systems / 65 100 n mi 60,000 ft. 40 n mi 130 n mi 18,000 ft. 45,000 ft. 18,000 ft. 1,000 ft. 14,500 ft. Low-altitude 40 n mi 1,000 ft. VOR High-altitude 25 n mi VOR 12,000 ft. 1,000 ft. TVOR Figure 2–19. VOR service volumes. Unusable Radials Testing of the VOR found that a clear zone of several thou- sand square feet around the VOR was necessary for proper operation. Any obstruction within this area could blank out or re!ect some of the signal from the VOR and cause incorrect course information to be transmitted to the air- craft. Tall buildings located thousands of feet from the VOR transmitter could even distort the transmitted signal. In an attempt to solve this problem, the CAA developed the Doppler VOR (DVOR). Although the operating principles of this VOR differ radically from a conventional VOR, the information available to the pilot is exactly the same. The VOR receiver on board the aircraft is unable to differentiate between Doppler or conventional VOR transmissions. Doppler VOR is less sensitive to re!ections from buildings or terrain. In locations unsuitable for conventional VOR installation, a DVOR might be necessary (see Figure 2–21). 66 / CHAPTER 2 Figure 2–20. VOR service volume chart. If the DVOR fails to correct the re!ections or blanking, the affected radi- als must be listed as unusable, which means that although the pilot may be able to receive these radials, they are not accurate and should not be used. Unusable VOR radials are published in the Airport Facility Directory (see Figure 2–22). Airway The CAA faced an enormous task when trying to determine where VORs Altitudes should be located. CAA planners had to consider potential obstructions, ter- rain, and the position of other VORs operating on the same frequency to deter- mine that a suitable interference-free signal could be received by any aircraft operating along a VOR-equipped airway. The airways must be constantly !ight checked. After these checks, a minimum en route altitude (MEA) is designated for each airway. Aircraft operating at or above the MEA are guaranteed clear- ance above any obstruction located along or near the airway.