Severe turbulence, hail, and icing associated with thunderstorms constitute severe hazards to flight. You must avoid these thunderstorms whenever possible. Airborne weather radar, if operated and interpreted properly, can be an invaluable aid in avoiding thunderstorm areas.
You must be aware of factors and limitations affecting thunderstorm radar returns to get the most out of the radar. Some of these factors are not meteorological and depend on the characteristics of the radar and the way it is used. The same weather target can vary considerably in its appearance from ground mapping mode to weather mode. Navigators must ensure they use the radar as intended for weather avoidance. Primary meteorological factors that affect radar returns are the amount of moisture in the weather target and atmospheric absorption characteristics between the radar antenna and the target.
The predominant weather-induced returns on most radarscopes are caused by precipitation-size water droplets, not by clouds. Intense returns indicate the presence of very large droplets. These large droplets are generally associated with the most hazardous phenomena; those with strong vertical currents that are necessary to maintain these droplets in the cloud. It is possible, however, to encounter such strong turbulence in an echo-free area or even in an adjacent cloud-free area, so avoiding areas giving intense returns does not necessarily guarantee safe flight in the vicinity of thunderstorms. Make careful note of all areas forecast to have the potential for hazardous weather.
Generally, the map mode of the radar with a moderate amount of gain applied is adequate for obtaining a return from hazardous cells. Sometimes, ground returns hamper detection in the area by hiding the storm. This can occur in mountainous areas where ground returns are similar and airmass lifting action breeds the cells. For these reasons, raising the radar tilt or switching to pencil beam, or both, are techniques that aid weather detection.
There are two types of weather avoidance with radar: avoidance of isolated thunderstorms and penetration of a line of thunderstorms. Avoid an isolated return by first identifying it and then circumnavigating it at a safe distance.
After detecting a weather system, determine its extent. Analyze the weather’s layout relative to planned track and decide either to deviate around it or penetrate the line. If the system is complex, remember your deviation could worsen the situation by flying into a sucker hole, where a solid system could surround the aircraft. Sometimes, what seems to be a good heading at short range seems foolish when viewed at long range. Remember that turning around is always an option, and ARTCC can sometimes assist in weather analysis. A simple technique for flying around weather at a preferred distance (say 20 NM) is the flying disc technique. Imagine the aircraft is a disc defined by the 20 NM range mark on the display. The heading marker is the nose of the disc. Draw an imaginary tangent from the disc to the edge of the weather, or use a pencil or plotter. Turn the aircraft the same number of degrees that it would take to get the heading marker to fire parallel to the tangent. After the turn, recheck the heading in the same manner. This technique works best with a scan of more than 180°. [Figure 7-19]
Penetration of a line of thunderstorms is a last resort and presents a different problem. Since the line may extend for hundreds of miles, circumnavigation is not practical or even possible. If no other course of action exists, the main objective is to avoid the more dangerous areas in the line.
Figure 7-20 shows an example of frontal penetration using radar. An iso-echo equipped radar can discriminate between the safe and violent areas. Without it, decreasing the gain works to highlight the worst areas by leaving the densest water cells as the last returns on the display. Upon approaching the line, the navigator determines an area that has weak or no returns and that is large enough to allow avoidance of all intense returns by the recommended distances throughout penetration. The navigator directs the aircraft to that point, making the penetration at right angles to the line so as to remain in the bad weather areas for the shortest possible time. Avoid the dangerous echoes by a safe distance. Penetration of a line of severe thunderstorms is always a potentially dangerous procedure. Attempt it only when you must continue the flight and cannot circumnavigate the line. Always advise ARTCC of your intentions when deviating from your flight-planned route.
Heading Marker Correction
For optimum accuracy, it may sometimes become necessary to correct the bearings taken on the various targets. This necessity arises when the heading marker reading does not agree with the TH of the aircraft when azimuth stabilization is used, or the heading marker reading does not agree with 360° when azimuth stabilization is not used.
For example, if the TH is 125° and the heading marker reads 120°, all of the returns on the scope indicate a bearing that is 5° less than it should be. Therefore, if a target indicates a bearing of 50°, add 5° to the bearing before plotting it. Conversely, if the heading marker reads 45° when the TH is 040°, all of the scope returns indicate a bearing that is 5° more than it should be. Therefore, if a target indicates a bearing of 275°, subtract 5° from the bearing before plotting it. The greater the distance to the target from the aircraft, the more important this heading marker correction becomes.