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Cockpit Weather Systems (Part Two)

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Lightning


Most MFDs are also capable of depicting electrical activity that is indicative of lightning. Like radar data, lightning data can come from two sources: onboard and broadcast weather systems. Both systems have strengths and limitations and work together to present a more complete weather picture. Lightning data is an excellent complement to radar data for detecting the presence of thunderstorms.

An onboard lightning detection system consists of a simple antenna and processing unit that senses electrical discharges in the atmosphere and attempts to determine which electromagnetic signals have the “signature” of lightning strikes. Lightning detectors or spherics receivers, such as Stormscope® and Strikefinder ®, have been known to indicate areas of static consistent with turbulence even where there was no rain associated with the turbulence. The MFD in Figure 5-16 depicts lightning strikes detected by a lightning detection system. Onboard lightning detection systems provide real-time information about electrical discharges. Estimates of the direction (or azimuth) of electrical discharges provided by an onboard lightning system are usually quite accurate. Estimates of the range (or distance) of electrical discharges tend to be less accurate.

Figure 5-16. Lightning strikes shown on an MFD.

Figure 5-16. Lightning strikes shown on an MFD.

Broadcast (or data link) weather services are also capable of transmitting lightning data to the cockpit. The symbology used to present lightning data derived from these sources is similar to that used by onboard lightning detection systems. The lightning data provided by ground weather surveillance systems is also a delayed weather product. Since the lightning data provided by a broadcast service is derived from multiple sensors, estimates of the range of electrical discharges are more accurate than those provided by onboard systems.

Clouds

Weather products that describe cloud coverage are generally available only from broadcast weather services. One popular broadcast weather service offers graphical displays of visible cloud cover along with the cloud top altitude as determined from satellite imagery. Figure 5-17 shows an MFD that depicts cloud cover and cloud tops.

Figure 5-17. Cloud cover and cloud tops shown on an MFD.

Figure 5-17. Cloud cover and cloud tops shown on an MFD.

Other Weather Products

Broadcast weather services offer many of the other weather products that can be obtained during a pre-flight briefing on the ground. Broadcast weather services can also provide graphical wind data, SIGMETs and AIRMETs, freezing levels, temporary flight restrictions, surface analyses, and hurricane tracks. The MFD in Figure 5-18 shows METAR and TAF data.

Figure 5-18. METAR and TAF data shown on an MFD.

Figure 5-18. METAR and TAF data shown on an MFD. [click image to enlarge]

Using Advanced Weather Data Systems

The increased availability of weather information is changing the way pilots think about weather briefing and the weather decision-making process. You are no longer limited to obtaining weather forecast products prior to a flight, only to discover different actual flight conditions in the air. Now more than ever, weather briefing is an activity that begins prior to departure and continues until the flight is completed.

Preflight Overview

A broadcast weather service allows you to see these products in the cockpit. Most systems offer a movable cursor that allows scrolling through the display to center on any location along the route. This capability, combined with the MFD’s range control, allows you to look for significant weather anywhere along the planned route of flight prior to departure.

Track Progress of Significant Weather En Route

The same scroll and range control features allow you to look ahead and check for weather conditions along upcoming portions of the flight route. Weather forecasts, such as TAFs, SIGMETs, and AIRMETs, issued after departure can be easily checked en route.


Investigate Weather Phenomena Reported by Radio

You can use cockpit weather systems to further investigate advisories received from HIWAS and other radio broadcasts. Another practical use is to check the METAR for a destination airport before flying in range of the airport’s ATIS broadcast. When you suspect that changing weather conditions have made continuation to the destination airport inadvisable, the radar and satellite features can be used to search for alternate airports. Since not all weather products can be viewed at once, a key pilot skill is the ability to determine which weather products to display at what times.

Broadcast Weather Products Versus Onboard Weather Sensors

Onboard weather sensor systems and broadcast weather services contribute to the weather decision-making process in slightly different ways. Broadcast weather services provide delayed information over a wider coverage area. Broadcast weather services are useful for making strategic decisions about which areas to fly into and which areas to avoid. Using a broadcast weather product to attempt to find a hole in a line of thunderstorms is inappropriate, since you cannot know if the current location of the thunderstorm cells is the same as when the broadcast weather product was generated. Onboard weather sensor systems provide real-time information about weather phenomena in the immediate proximity of the aircraft. Onboard weather sensor systems are useful when making immediate, close-range decisions about flying in the vicinity of potentially hazardous weather phenomena. You must keep in mind the limitations of onboard weather systems.

Common Error: Skipping the Preflight Weather Briefing

The easy availability of weather information in the cockpit can lure you to skip the preflight weather briefing. Time pressure adds further incentive to simply jump in and go. Keep in mind that flight services stations (FSS)/automated flight service stations (AFSS) offer many advantages over an advanced weather data system, so do not use advanced avionics weather data systems as a substitute for a pre-flight weather briefing. As a simple example, when talking to an FSS/AFSS weather briefer, it is possible to get a better overall picture of the weather system and pilot reports not yet entered into the system. The FSS/AFSS briefer can also supply more Notice to Airmen (NOTAM) and other detailed information for your particular route of flight; without such briefing, the pilot might expend many precious moments searching for a critical bit of information, instead of managing the flight. Often, it is much easier to get a thorough briefing on the ground than attempting to read small reports on a bouncing MFD in a small airplane in turbulent conditions.

 

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