Advanced avionics cockpit weather systems provide many of the same weather products available on the ground and have a variety of uses that can enhance awareness of weather that may be encountered during almost any phase of flight. Radar images, satellite weather pictures, Aviation Routine Weather Reports (METARs), terminal weather forecasts (TAFs), significant meteorological information (SIGMETs), Airmen’s Meteorological Information (AIRMETs), and other products are now readily accessible at any time during flight. Weather products provided by cockpit weather systems are typically presented on an MFD. Some installations allow the overlay of this data in the PFD. You must learn the procedures required to show each kind of weather product on the MFD and/or PFD, and how to interpret each type of weather product. Know the limitations of each type of product, and the ways in which cockpit weather systems can be used to gather information and remain clear of weather hazards throughout the flight.
Thunderstorms and Precipitation
Thunderstorms and general areas of precipitation are detected through the use of radar. In the advanced avionics cockpit, radar data can come from one of two sources: an onboard weather radar system or a ground weather surveillance radar system, such as the Next Generation Radar (NEXRAD) system. Ground weather surveillance system data is transmitted to the cockpit via a broadcast (or datalink) weather service. Onboard weather radar and ground weather surveillance radar systems each offer advantages and disadvantages to the pilot. Some aircraft use a combination of both systems.
While onboard radar is real time, many downloaded radar images and other reports are delayed for some time period for various reasons. Given the nature of thunderstorms and other weather hazards, this delay could prove hazardous. You must know the true quality and age of the data.
Most MFDs are capable of presenting radar data together with aircraft position and the programmed route, as shown in Figure 5-11.
Onboard Weather Radar Systems
Onboard weather radar uses an adjustable aircraft mounted radar antenna to detect, in real time, weather phenomena near the aircraft. The coverage of an onboard weather radar system is similar to a flashlight beam, as illustrated in Figure 5-12. You should always remember that the radar displays only areas of water or moisture (rain, sleet, snow, and hail). Radar does not display turbulence or lightning.
Although the tilt of the radar antenna can be adjusted upward and downward, the weather phenomena that the weather radar can detect are limited in both direction and range. The radar system in Figure 5-12 fails to detect the two cells that lie below and beyond the radar beam.
As illustrated in Figure 5-12, you must be careful not to assume that the only cells in the area are the ones shown on the radar display. The two additional cells in Figure 5-12 are present, but not detected by the onboard weather radar system.
When a cell is detected by an onboard weather radar system, that cell often absorbs or reflects all of the radio signals sent out by the radar system. This phenomenon, called attenuation, prevents the radar from detecting any additional cells that might lie behind the first cell. Figure 5-13 illustrates radar attenuation, in which one cell “shadows” another cell.
A simple color-coding scheme, as shown in Figure 5-14, is used to represent the intensity of radar echoes detected by an onboard weather radar system.
Ground Weather Surveillance Radar
Ground weather surveillance integrates weather information from many ground radar stations. The weather information collected from many sources is then used to create a composite picture that covers large volumes of airspace. These composite radar images can then be transmitted to aircraft equipped with weather data receivers.
Except in those areas for which no ground radar coverage is available, the range of ground weather surveillance radar systems is essentially unlimited. Ground radars have the luxury of large antennas; big, heavy power supplies; and powerful transmitters—without the constraints of aerodynamic drag, power, weight, and equipment volume restrictions and concerns.
Unlike onboard weather radar systems, weather data received from a ground weather surveillance radar system is not realtime information. The process of collecting, composing, transmitting, and receiving weather information naturally takes time. Therefore, the radar data reflect recent rather than current weather conditions.
The color-coding scheme used by one ground weather surveillance radar system (NEXRAD) is shown in Figure 5-15. Note that this color-coding scheme is slightly more sophisticated than that for the onboard system in Figure 5-13. It is capable of distinguishing rain, snow, and mixtures of the two.
Limitations of Both Types of Weather Radar Systems
Weather radar does not detect most other kinds of hazardous weather such as fog, icing, and turbulence. The absence of radar return on a radar display does not in any way mean “clear skies.” Skillful users of weather radar are able to recover clues of other weather phenomena, such as hail and turbulence, from radar data.
A second limitation of weather radar is that the earliest (cumulus) stage of a thunderstorm is usually free of precipitation and may not be detected by radar. Convective wind shear, severe turbulence, and icing are characteristic of thunderstorms during the cumulus stage.
The pilot must beware of areas that offer no radar coverage. In many cases, these areas appear blank on a weather display. The absence of weather hazards as shown on a screen does not imply the actual absence of weather hazards.