Garmin G500 Manual
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4-65190-01102-02 Rev. DGarmin G500 Pilot’s Guide ForewordSec 1 System Sec 2 PFD Sec 3 MFD Sec 4 Hazard Avoidance Sec 5 Additional Features Sec 6 Annun. & Alerts Sec 7 Symbols Sec 8 Glossary Appendix A Appendix B Index tion at 15 NM. The curvature of the earth can also be a factor, especially at range settings of 150 NM or more. NM Figure 4-57 Radar Beam in Relation to the Curvature of the Earth 4.9.1.3 Radar Signal Attenuation The phenomena of weather attenuation needs to be kept in mind whenever operating the weather radar. When the radar signal is transmitted, it is progres- sively absorbed and scattered, making the signal weaker. This weakening, or attenuation, is caused by two primary sources, distance and precipitation. Attenuation because of distance is due to the fact that the amount of ra\ dar energy at a distance from the antenna is inversely proportional to the square of the distance. The reflected radar energy from a target 40 miles away that fills the radar beam will be one fourth the energy reflected from an equivalent target 20 miles away. This would appear to the operator that the storm is gain- ing intensity as the aircraft gets closer. Internal circuitry within the GWX 68 system compensates for much of this distance attenuation. Attenuation due to precipitation is not as predictable as distance attenuation. It is also more intense. As the radar signal passes through moisture, a portion of the radar energy is reflected back to the antenna. However, much of the energy is absorbed. If precipitation is very heavy, or covers a large area, the signal may not reach completely through the area of precipitation. The weather radar system cannot distinguish between an attenuated signal and area of no precipitation. If the signal has been fully attenuated, the radar will display a “radar shadow.” This appears as an end to the precipitation when, in fact, the heavy rain may extend much further. A cell containing heavy precipitation may block another cell located behind the first, preventing it from being displayed on the radar. Never fly into these shadowed areas and never assume that all of the heavy precipitation is being displayed unless another cell or a ground target can be seen beyond the heavy cell. The WATCH™ feature of the GWX 68 Weather Radar system can help in identifying these shadowed areas. Areas in question
4-66Garmin G500 Pilot’s Guide190-01102-02 Rev. D Foreword Sec 1 System Sec 2 PFD Sec 3 MFD Sec 4 Hazard Avoidance Sec 5 Additional Features Sec 6 Annun. & Alerts Sec 7 Symbols Sec 8 Glossary Appendix A Appendix B Index will appear as “shadowed” or gray area on the radar display. Proper use of the antenna tilt control can also help detect radar shadows. Attenuation can also be due to poor maintenance or degradation of the radome. Even the smallest amount of wear and tear, pitting, and pinholes on the radome surface can cause damage and system inefficiency. 4.9.2 Radar Signal Reflectivity 4.9.2.1 Precipitation Precipitation or objects more dense than water, such as earth or solid structures, will be detected by the weather radar. The weather radar will not detect clouds, thunderstorms or turbulence directly. It detects precipitation associated with clouds, thunderstorms, and turbulence. The best radar si\ gnal reflectors are raindrops, wet snow or wet hail. The larger the raindrop the better it reflects. The size of the precipitation droplet is the most important factor in radar reflectivity. Because large drops in a small concentrated area are characteristic of a severe thunderstorm, the radar displays the storm as a strong return. Ice, dry snow, and dry hail have low reflective levels and often will not be displayed by the radar. A cloud that contains only small raindrops, such as fog or drizzle, will not reflect enough radar energy to produce a measurable target return. Figure 4-58 Precipitation Type and Reflectivity
4-67190-01102-02 Rev. DGarmin G500 Pilot’s Guide ForewordSec 1 System Sec 2 PFD Sec 3 MFD Sec 4 Hazard Avoidance Sec 5 Additional Features Sec 6 Annun. & Alerts Sec 7 Symbols Sec 8 Glossary Appendix A Appendix B Index 4.9.2.2 Ground Returns The intensity of ground target returns depends upon the angle at which the radar beam strikes the ground target (Angle of Incidence) and the reflective properties of that target. The gain can be adjusted so shorelines, rivers, lakes, and cities are well defined. Increasing gain too much causes the display to fill in between targets, thus obscuring some landmarks. Cities normally provide a strong return signal. While large buildings and structures provide good returns, small buildings can be shadowed from the radar beam by the taller buildings. As the aircraft approaches, and shorter ranges are selected, details become more noticeable as the highly reflective regular lines and edges of the city become more defined. Bodies of water such as lakes, rivers, and oceans are not good reflectors, and normally do not provide good returns. The energy is reflected in a forward scatter angle with inadequate energy being returned. They can appear as dark areas on the display. However, rough or choppy water is a better reflector and will provide stronger returns from the downwind sides of the waves. Mountains also provide strong return signals to the antenna, but also block the areas behind. However, over mountainous terrain, the radar beam can be reflected back and forth in the mountain passes or off canyon walls using up all or most of the radar energy. In this case, no return signal is received from this area causing the display to show a dark spot which could indicate a pass where no pass exists. 4.9.2.3 Angle of Incidence The angle at which the radar beam strikes the target is called the Angle of Incidence. Incident angle (“A”) is illustrated below. This directly affects the detectable range, the area of illumination, and the intensity of the displayed target returns. A large incident angle gives the radar system a smaller detect- able range and lower display intensity due to minimized reflection of the radar energy.
4-68Garmin G500 Pilot’s Guide190-01102-02 Rev. D Foreword Sec 1 System Sec 2 PFD Sec 3 MFD Sec 4 Hazard Avoidance Sec 5 Additional Features Sec 6 Annun. & Alerts Sec 7 Symbols Sec 8 Glossary Appendix A Appendix B Index Figure 4-59 Angle of Incidence A smaller incident angle gives the radar a larger detectable range of opera- tion and the target display will show a higher intensity. Since more radar energy is reflected back to the antenna with a low incident angle, the resulting detectable range is increased for mountainous terrain. 4.9.3 Operating Distance The following information establishes a minimum safe distance from the antenna for personnel near an operating airborne weather radar. The minimum safe distance is based upon the FCC’s exposure limit at 9.3 to 9.5 GHz for gen- eral population/uncontrolled environments which is 1 mW/cm 2. See Advisory Circular 20-68B for more information on safe distance determination. 4.9.3.1 Maximum Permissible Exposure Level (MPEL) (GWX 68) The zone in which the radiation level exceeds the US Government stan- dard of 1 mW/cm 2, is the semicircular area of at least 11 feet from the 12 inch antenna as indicated in the illustration below. All personnel must remain outside of this zone. With a scanning or rotating beam, the averaged power density at the MPEL boundary is significantly reduced. 4.9.3.2 Maximum Permissible Exposure Level (MPEL) (Other Radars) See the appropriate documentation for MPEL.
4-69190-01102-02 Rev. DGarmin G500 Pilot’s Guide ForewordSec 1 System Sec 2 PFD Sec 3 MFD Sec 4 Hazard Avoidance Sec 5 Additional Features Sec 6 Annun. & Alerts Sec 7 Symbols Sec 8 Glossary Appendix A Appendix B Index MPELBou ndary 1 1’ f or 1 2” a ntenna Figure 4-60 MPEL Boundary 4.9.4 Basic Antenna Tilt Setup The following discussion is a simple method for setting up the weather radar antenna tilt for most situations. It is not to be considered an all encompassing setup that will work in all situations, but this method does provide good overall parameters for the monitoring of threats. Ultimately, it is desired to have the antenna tilted so that the bottom of the radar beam is four degrees below parallel with the ground. The following discussion explains one way of achieving this. With the aircraft flying level, adjust the antenna tilt so ground returns are displayed at a distance that equals the aircraft’s current altitude (AGL) divided by 1,000. For example, if the aircraft is at 14,000 feet, adjust the tilt so the front edge of ground returns are displayed at 14 NM. Note this antenna tilt angle setting. Now, raise the antenna tilt 6º above this setting. The bottom of the radar beam is now angled down 4º from parallel with the ground. Practical Application Using the Basic Tilt Setup At this point, when flying at altitudes between 2,000 and 30,000 feet \ AGL, any displayed target return should scrutinized. If the displayed target advances on the screen to 5 NM of the aircraft, avoid it. This may be either weather or ground returns that are 2,000 feet or less below the aircraft. Raising the antenna tilt 4º can help separate ground returns from weather returns in rela- tively flat terrain. This will place the bottom of the radar beam leve\ l with the ground. Return the antenna tilt to the previous setting after a few sweeps.
4-70Garmin G500 Pilot’s Guide190-01102-02 Rev. D Foreword Sec 1 System Sec 2 PFD Sec 3 MFD Sec 4 Hazard Avoidance Sec 5 Additional Features Sec 6 Annun. & Alerts Sec 7 Symbols Sec 8 Glossary Appendix A Appendix B Index If the aircraft is above 29,000 feet, be cautious of any target return that gets to 30 NM or closer. This is likely a thunderstorm that has a top high enough that the aircraft cannot fly over it safely. If the aircraft altitude is 15,000 feet or lower, set the displayed range to 60 NM. Closely monitor anything that enters the display. Also, after setting up the antenna tilt angle as described previously, ground returns can be monitored for possible threats. The relationship between antenna tilt angle, altitude, and distance is one degree of tilt equals 100 feet of altitude for every one nautical mile. Vertical Change of Radar Beam (feet) Change in Antenna Tilt 10 nm 0 1000 2000 3000 4000 1000 2000 3000 4000 -1° 0° -2° -3° -4° +1° +2° +3° +4° Figure 4-61 Vertical Change in Radar Beam per Nautical Mile Therefore, with the antenna tilt set so that the bottom of the beam is four degrees below parallel with the ground, a target return at 10 NM is approximately 4,000 feet below the aircraft; at 20 NM, 8,000 feet; at 50 NM, 20,000 feet. In other words, at this tilt setting, a ground return (such as a mountain peak) being displayed at 10 NM would have a maximum distance below the aircraft of 4,000 feet. If that ground target return moves to 5 NM, maximum distance below the aircraft will be 2,000 feet. This setup will provide a good starting point for practical use of the GWX 68. There are many other factors to consider in order to become proficient at using weather radar in all situations. 4.9.5 Weather Mapping and Interpretation 4.9.5.1 Weather display Interpretation When evaluating various target returns on the weather radar display, the colors denote approximate rainfall intensity and rates as shown in the table below.
4-71190-01102-02 Rev. DGarmin G500 Pilot’s Guide ForewordSec 1 System Sec 2 PFD Sec 3 MFD Sec 4 Hazard Avoidance Sec 5 Additional Features Sec 6 Annun. & Alerts Sec 7 Symbols Sec 8 Glossary Appendix A Appendix B Index Weather Mode Color GWX 68 Radars 3rd Party Radars Approximate Intensity Approximate Rainfall Rate (in/hr) Radar Return Level (see radar documen- tation for details) BLACK < 23 dBZ < .01 0 GREEN 23 dBZ to < 32 dBZ .01 - 0.1 1 YELLOW 32 dBZ to < 41 dBZ 0.1 - 0.5 2 RED 41 dBZ to < 50 dBZ 0.5 - 2 3 MAGENTA 50 dBZ and greater > 24 Table 4-17 Precipitation Intensity Levels 4.9.5.2 Thunderstorms Updrafts and downdrafts in thunderstorms carry water through the cloud. The more severe the drafts, the greater the number and size of the precipitation droplets. With this in mind, the following interpretations can be made from what is displayed on the weather radar. Avoid these areas by an extra wide margin. • In areas where the displayed target intensity is red or magenta (indicating large amounts of precipitation), the turbulence is considered severe. • Areas that show steep color gradients (intense color changes) over thin bands or short distances suggest irregular rainfall rate and strong turbulence. • Areas that show red or magenta are associated with hail or turbulence, as well as heavy precipitation. Vertical scanning and antenna tilt management may be necessary to identify areas of maximum intensity. Along squall lines (multiple cells or clusters of cells in a line), individual cells may be in different stages of development. Areas between closely spaced, intense targets may contain developing clouds not having enough moisture to produce a return. However, these areas could have strong updrafts or downdrafts. Targets showing wide areas of green are generally precipitation without severe turbulence. Irregularities in the target return may also indicate turbulence, appearing as “hooks,” “fingers,” or “scalloped” edges. These irregularities may be present in green areas with no yellow, red, or magenta areas and should be treated as highly dangerous areas. Avoid these areas as if they were red or magenta areas.
4-72Garmin G500 Pilot’s Guide190-01102-02 Rev. D Foreword Sec 1 System Sec 2 PFD Sec 3 MFD Sec 4 Hazard Avoidance Sec 5 Additional Features Sec 6 Annun. & Alerts Sec 7 Symbols Sec 8 Glossary Appendix A Appendix B Index Steep Gradient Squall Line Hook or FingerScalloped Edge Figure 4-62 Cell Irregularities Thunderstorm development is rapid. A course may become blocked within a short time. When displaying shorter ranges, periodically select a longer range to see if problems are developing further out. That can help prevent getting trapped in a “blind alley” or an area that is closed at one end by convective weather. Figure 4-63 The “Blind Alley” Overhead View In areas of multiple heavy cells, use the Vertical Scan feature along with antenna tilt management to examine the areas. Remember to avoid shadowed areas behind targets. Figure 4-64 The “Blind Alley” Vertical Scan
4-73190-01102-02 Rev. DGarmin G500 Pilot’s Guide ForewordSec 1 System Sec 2 PFD Sec 3 MFD Sec 4 Hazard Avoidance Sec 5 Additional Features Sec 6 Annun. & Alerts Sec 7 Symbols Sec 8 Glossary Appendix A Appendix B Index 4.9.5.3 Tornadoes There is no conclusive radar target return characteristics which will identify a tornado, however, tornadoes may be present if the following characteristics are observed: • A narrow, finger-like portion, as shown on the previous page, extends and, in a short time, curls into a hook and closes on itself. • A “hook” which may be in the general shape of the numeral “6,” especially if bright and projecting from the southwest quadrant (northeast quadrant in the southern hemisphere) of a major thunderstorm. • V- shaped notches. • Doughnut shapes. These shapes do not always indicate tornadoes, nor are tornado returns limited to these characteristics. Confirmed radar observations of tornadoes most often have not shown shapes different from those of a normal thunderstorm display. 4.9.5.4 Hail Hail results from updrafts carrying water high enough to freeze. Therefore, the higher the top of a thunderstorm, the greater the probability that it contains hail. Vertically scanning the target return can give the radar top of a thunderstorm that contains hail. Radar top is the top of a storm cell as detected by radar. It is not the actual top, or true top of the storm. The actual top of a storm cell is seen with the eyes in clear air and may be much higher than the radar top. The actual top does not indicate the top of the hazardous area. Hail can fall below the minimum reflectivity threshold for radar detection. It can have a film of water on its surface, making its reflective characteristics similar to a very large water droplet. Because of this film of water, and because hail stones usually are larger than water droplets, thunderstorms with large amounts of wet hail return stronger signals than those with rain. Some hail shafts are extremely narrow (100 yards or less) and make poor radar targets. In the upper regions of a cell where ice particles are “dry” (no liquid coating), target returns are less intense. Hail shafts are associated with the same radar target return characteristics as tornados. U-shaped cloud edges 3 to 7 miles across can also indicate hail.
4-74Garmin G500 Pilot’s Guide190-01102-02 Rev. D Foreword Sec 1 System Sec 2 PFD Sec 3 MFD Sec 4 Hazard Avoidance Sec 5 Additional Features Sec 6 Annun. & Alerts Sec 7 Symbols Sec 8 Glossary Appendix A Appendix B Index These target returns appear quite suddenly along any edge of the cell outline. They also change in intensity and shape in a matter of seconds, making vigilant monitoring essential. 4.9.6 Radar Operation in Weather Mode WARNING: Begin transmitting only when it is safe to do so. When transmitting while the aircraft is on the ground, no personnel or objects should be within 11 feet of the antenna. CAUTION: In Standby mode, the antenna is parked at the center line. It is always a good idea to put the radar in Standby mode before taxiing the aircraft to prevent the antenna from bouncing on the bottom stop and possibly causing damage to the radar assembly. When the weather radar system is in the Weather or Ground Map mode, the system automatically switches to Standby mode on landing. Operating ModeAntenna Stabilization Weather Alert (GWX Only) Precipitation Scale Control Window Bearing Line Figure 4-65 Horizontal Scan Display