Navigation Systems PDF

Navigation Systems / 45 One of the !rst scheduled airline flights in the United States occurred just prior to World War I. The St. Petersburg–Tampa Airboat Lines was established to pro- vide regular passenger service between the two Florida cities. For three months, during the winter of 1914, t...

Navigation Systems / 45 One of the !rst scheduled airline flights in the United States occurred just prior to World War I. The St. Petersburg–Tampa Airboat Lines was established to pro- vide regular passenger service between the two Florida cities. For three months, during the winter of 1914, the airline flourished. But when spring arrived, the tourists departed north, and with the lack of passengers, the airline folded. No other serious attempts at starting airline service were made during World War I. At the conclusion of the war, the federal government disposed of many of its military aircraft, selling them to private individuals as surplus property. This enormous influx of inexpensive aircraft helped establish the aviation industry in the United States. Some airline companies were formed after the war using these surplus aircraft, but they proved to be as short-lived as the St. Petersburg– Tampa line. The available war surplus aircraft were expensive to operate and maintain, forcing the airlines to charge passengers high fares. Only the wealthy could afford to fly at these high prices, and they were accustomed to travel- ing in luxury, not in war surplus aircraft. Trying to lure passengers using these aircraft thus proved to be nearly impossible, and most of the fledgling airline companies folded. In 1916, in the midst of World War I, Congress had authorized the Post Of!ce Department to institute the nation’s !rst of!cial airmail service. The war delayed the implementation of this policy until 1918. The !rst flight, from New York City to Washington, D.C., was !nally conducted on May 15 of that year, using U.S. Army aircraft. Airmail service soon proved to be commercially suc- cessful, and within three months the Post Of!ce Department began to transport the mail using its own aircraft and pilots. Additional routes were soon added, and the Post Of!ce Department eventually came to provide airmail service from coast to coast. Within a few years, in an attempt to stabilize the fledgling airline industry, the Post Of!ce Department began to contract airmail routes to the few remain- ing airline companies still struggling to survive. Airmail contracts proved to be a lifesaver to these airlines, since they could now transport mail while conducting passenger flights and use the airmail payments as a subsidy to reduce fares and attract more passengers. The resultant increase in revenue permitted the airlines to dispose of their war surplus aircraft and invest in larger and more luxurious aircraft speci!cally designed to carry passengers. But this merging of passenger and airmail service complicated airline scheduling and operations. When car- rying only airmail, airlines could delay flights because of poor weather con- ditions or darkness. But delays were unacceptable when carrying fare-paying passengers. Passengers demanded that the airlines fly consistent schedules with as few delays as possible. If the airlines hoped to lure passengers away from their main competitor, the railroads, they would have to offer fast, timely flights with few or no delays. Methods that would permit flying during poor weather or at night would have to be developed if the airlines were to survive and prosper. 46 / CHAPTER 2 Visual Navigation Initially, because they lacked flight instruments or navigation systems, airline pilots were limited to daylight flying during good weather conditions. The pilots were forced to use outside visual references to control their aircraft’s attitude, relying on the natural horizon as a reference. They would note any changes in the flight attitude of their aircraft and make the necessary control adjustments that would keep their aircraft in level flight. Pilotage Pilots navigated from airport to airport using either pilotage or deduced reck- oning (commonly called dead reckoning). Pilotage required that the pilot use a map of the surrounding area as a reference. The pilot would draw a line on the map, extending from the departure to the destination airport, and note any prominent landmarks that would be passed while in flight. As the aircraft passed these landmarks, the pilot would note any deviation from the planned flight path and adjust the aircraft’s heading to return to the preplanned course. Since the winds at the aircraft’s cruising altitude usually caused the air- craft to drift either left or right of course, the pilot was forced to constantly alter the aircraft’s heading to counteract these crosswinds. This change in head- ing is known as the crosswind correction angle or wind correction angle. The resultant path in which the aircraft flies over the ground is known as the ground track or the course. Aeronautical The maps used by pilots in the early 1920s were common road maps available Charts at automobile service stations. These maps were unsuitable for aerial naviga- tion since they lacked the necessary landmark information needed to accurately navigate from one airport to the next. It soon became apparent that pilots needed a specialized chart expressly designed for use in aeronautical naviga- tion. The U.S. government then developed and began to print such air naviga- tion charts, known as sectional charts. Sectional charts are aeronautical charts scaled 1:500,000 or about 8 statute miles to the inch. Sectional charts are still used today and depict the relevant information needed by pilots to navigate accurately and safely. This information includes cities, highways, railroads, airport loca- tions, terrain features, and distinctive objects (see Figure 2–1). Sectional charts also depict navigation aids, federal airways, and air traf!c control facilities. With very little change over the years, sectional charts are still being printed by the National Ocean Service (part of the U.S. Department of Commerce) and are primarily used by pilots flying under VFR rules (see Figure 2–2). In addition, pilots flying IFR usually carry appropriate sectional charts in case of navigational equipment failure. If IFR pilots should encounter any electronic navigation problems during flight, they may be able to continue under VFR conditions using sectional charts for visual navigation.  Navigation Systems / 47 Figure 2–1. An example of a legend for a sectional chart. 48 / CHAPTER 2 Figure 2–2. Sample sectional chart.  Navigation Systems / 49 Some pilots carry world aeronautical charts (WACs) instead of section- als during IFR flights (see Figure 2–3). WACs are similar to sectionals but are scaled 1:1,000,000 or about 16 miles to the inch. They present less-detailed information to the pilot but cover a larger area than a sectional chart. Dead When flying using VFR rules, most pilots use dead reckoning, in combination Reckoning with pilotage, to navigate to their destination. With dead reckoning, the pilot uses the forecast winds at the planned cruising altitude and applies trigonom- etry to deduce the proper heading that the aircraft should fly to counteract the crosswind. Properly calculated, this method of navigation is very accurate; however, it is hampered by the fact that the winds-aloft information is a fore- cast not a reflection of the actual winds. To verify that dead reckoning has calculated the proper heading, the pilot must still visually check the accuracy of the deduced heading by using a sectional chart. Flight The !rst step in planning a flight using both dead reckoning and pilotage is to Planning determine the true course that will lead the aircraft to the destination airport. This is accomplished by drawing a line from the departure airport to the desti- nation on the sectional chart. The pilot then determines the angle of this course in reference to true north, using a device called a plotter. The pilot obtains the forecast wind speed and direction at the chosen cruising altitude and, using either a mechanical or an electronic computer, calculates the true heading that the aircraft must fly. The deduced true heading is the direction that the aircraft must be aimed in order to track to the desired destination. If there is not wind at the aircraft’s cruising altitude, the true heading and the true course will be exactly the same. However if the pilot encounters a crosswind, he or she must angle the aircraft into the wind to remain on course. The angular difference between the aircraft’s heading and the true course is the crosswind correction or wind correction angle. Aircraft Instrumentation Magnetic Aeronautical charts cannot be properly used by pilots unless they have accurate Compass aircraft heading information. All of these charts are oriented with respect to true north. Unfortunately, the only instrument aboard most aircraft that actu- ally indicates heading is a magnetic compass, which usually points toward mag- netic north (see Figure 2–4). The angular difference between true north and magnetic north is known as variation (see Figure 2–5). The variation depends on the aircraft’s current location. In different areas of the United States, the variation may range from 0° to as much as 20°. To properly use the magnetic compass when navigating, the pilot must add the variation to or subtract it from the aircraft’s true heading 50 / CHAPTER 2 Figure 2–3. Sample world aeronautical chart.  Navigation Systems / 51 Beech Aircraft Corporation Figure 2–4. The magnetic compass. Easterly variation Westerly variation 20∞ 15∞ 10∞ 5∞ 0∞5∞10∞ 15∞ 20∞ 20∞ 20∞ 15∞ 10∞ 15∞ 5∞ Agonic line 10∞ 5∞ 0∞ Figure 2–5. Variation chart. to determine the magnetic heading that must be flown. The pilot may then fly this heading using the aircraft’s magnetic compass. Although the magnetic compass is a relatively reliable instrument, it is subject to various inaccuracies. One of these inaccuracies is known as devia- tion. Deviation is caused by the stray magnetic !elds of electrical equipment or metallic structures within the aircraft. Since all aircraft contain some stray mag- netic !elds, every plane is required to be equipped with a compass deviation 52 / CHAPTER 2 card that lists the inaccuracies and the correction that must be applied when interpreting the magnetic compass. A few other conditions can cause the magnetic compass to indicate inaccu- rately. During changes in airspeed or while the aircraft is turning, the magnetic compass will not indicate correctly. These particular inaccuracies are known as acceleration and turning errors. In general, the only time that the magnetic compass can be accurately interpreted is when the aircraft is in straight and level, unaccelerated flight. In addition, the placement of a metal or magnetized object (such as a flashlight, clipboard, or screwdriver) near the compass will alter the local magnetic !eld and cause magnetic compass errors. Heading Many of the problems inherent in the magnetic compass can be alleviated by Indicator using a heading indicator (see Figure 2–6). Because the heading indicator is a gyroscopic instrument, it is not subject to the same problems that affect the magnetic compass. The heading indicator is initially set by the pilot while on the ground. When properly set, it accurately reflects the aircraft’s magnetic heading during flight. As the aircraft turns, the heading indicator rotates, con- stantly displaying the correct heading. The heading indicator is not subject to acceleration or turning errors, and it is immune to stray magnetic !elds. It has, however, a few inherent problems. Since it is unable to sense magnetic !elds, it must be properly adjusted by the pilot before being used. If the pilot sets the indicator incorrectly, it will not accurately reflect the aircraft’s magnetic heading. In addition, since the heading indicator is subject to internal bearing friction and will slowly drift and begin to indicate inaccurately, the pilot must constantly check its accuracy and reset it as necessary during the flight. It is also possible, though highly unlikely, that the heading indicator will fail mechanically, not indicating the proper heading even when properly set. The heading indicator is also subject to precession and should be periodically reset during the flight. Beech Aircraft Corporation Figure 2–6. A heading indicator.  Navigation Systems / 53 VFR Navigation In theory, using dead reckoning alone, the pilot should be able to fly the com- puted heading and arrive over the airport at the calculated time. But in real- ity, because of imprecise winds-aloft forecasts, most pilots use a combination of pilotage and dead reckoning. The proper heading and time must still be deduced and used, but en route navigation checkpoints are established and marked on the appropriate sectional charts. As the pilot flies toward the des- tination, he or she makes periodic checks to determine whether the aircraft is still on course. If it has deviated from the planned route, the pilot will adjust the aircraft’s heading to return to and remain on the desired flight path. As archaic and old fashioned as this may seem, it is still the primary method of navigation for most VFR pilots today. Although visual navigation works quite well during daylight hours, at night or in marginal weather conditions it is almost impossible for pilots to see objects on the ground and make an accurate determination of their air- craft’s position. Sparsely populated areas of the country may not offer suf!cient ground references to permit the pilot to determine the aircraft’s location. If and when the pilot !nally arrives at the destination airport, he or she may !nd it dif!cult to actually locate the runway in the dark and land the aircraft. The solution, of course, is to have both airport and airway lighting. In the 1920s, airports were illuminated through the use of airport bound- ary lighting, which consisted of steady-burning 40-watt lights on wooden stakes every 300 feet around the perimeter of the airport. Eventually, these lights were equipped with lenses to concentrate the light beam and were mounted on orange-colored steel cones so that they could also be clearly seen during daylight hours. With the outline of the airport now quite visible, the pilots were able to safely land and take off at night. As noted in Chapter 1, the !rst airway lighting was also instituted in the 1920s. At equal intervals along the airway, rotating beacon lights were installed that delineated the airway’s center line (see Figure 2–7). These rotating beacons were installed on steel towers and consisted of 1,000-watt electric lamps that produced a white light of approximately 1,000,000 candlepower. Each lamp was housed in a rotating drum assembly equipped with 36-inch-diameter lenses at each end. One lens was clear while the other lens was colored. The beacon rotated at a speed of about six revolutions per minute. These rotating beacons were installed along the airway at 15-mile intervals. As the pilots flew along the airway, the beacons would appear as flashes of light visible from a distance of over 40 miles. To visually navigate along the airway, all the pilot needed to do was to fly from one beacon to the next. Each rotating beacon was equipped with a colored lens that uniquely identi!ed that particular beacon and enabled pilots to accurately determine their position. Each airport along the airway was also equipped with a rotating beacon having one clear and one green lens. These beacons were designed to help pilots determine the airport’s exact location. The green and white rotating 54 / CHAPTER 2 Michael Nolan Figure 2–7. A rotating beacon. beacons are still used at civilian airports today. Other color combinations are used to differentiate other types of airports. The assigned colors for rotating beacons are as follows: White and green Land airport Green and green* Land airport White and yellow Water airport Yellow and yellow* Water airport Green, white, and white Military airport Green, yellow, and white Lighted heliport * Green or yellow rotating beacons are used to prevent confusion when another airport with a similarly colored rotating beacon is located nearby.