Using radar systems to identify and avoid adverse weather.
By David Ison
Professor, Graduate School Northcentral University
When flying IMC, you can’t know there’s a thunderstorm simply by staring out the window. You learn about it from the stresses of managing your weather avoidance avionics, ATC, and everything else while dissecting the nearby clouds. The ideal then, of course, would be to have a high level of both experience and knowledge.
To keep out of trouble when the weather starts to rear its ugly thunderhead, you’ll need a balance of both skill and know-how. Airborne radar systems should be used to determine the danger that a storm may present to an aircraft.
Almost assuredly, if you evaluate weather you intend to penetrate in the following 4 ways – contouring, attenuation, sensitivity, and tilt – you will not get yourself into trouble with nearby storms.
Step one in the evaluation process is to look for contouring. Contouring has 2 attributes. One is the presence of 2 or more colors (intensity levels) being displayed for a cell. The second is how close the colors appear – also referred to as “steep gradients.” Therefore, if all you see is green on your radar scope, the storm is considered non-contoured and pretty weak.
If you see green with an area of yellow inside, this would be considered a contoured cell. If the green ring surrounding the yellow is narrow, this would be of greater concern, as it is a potential sign of a building convective cell. If the green area is large and the yellow coloring is not solid (ie, it has green patches within it), this could mean some light rain with patches of heavier precipitation within. Another consideration when it comes to contouring is your location and the time of the year.
Lots of contoured cells in Florida during the summer months means you’re dealing with thunderstorms, while cells with some contouring in the winter in the Pacific Northwest point to your typical juicy moderate-to-heavy non-convective flows that are pretty normal that time of year. While all cells should be evaluated using the 4-step process, contoured cells are the ones that will need a comprehensive evaluation by the remaining 3 tools.
Attenuation is arguably the most telling of all tools available to determine if you can fly safely through or into an area of weather, especially if it is near or over your intended departure or arrival airport. Attenuation is the absorption or blockage of radar energy by a solid or near-solid object.
Two objects that significantly attenuate radar beams are mountains and extreme storms. You won’t see any returns (or at most a reduced amount) beyond both types of object. Savvy pilots know all about attenuation, and use it to their advantage. If you can paint the ground behind a radar return, then there is little or no attenuation present.
Once a storm is attenuating your radar beam, watch out! You cannot know what is within the area of attenuation or beyond it. Therefore, this type of storm should be avoided at all costs, and you should not take off or land near such events.
This means that it’s critical to confirm that ground returns are still clearly visible beyond the cell before penetrating the area. Or, as I like to say, “If there’s no ground, go around.” There is no logical or sane reason for messing with a cell attenuating your radar.
Operators of onboard radar can adjust the sensitivity of their radars using the gain control feature. Most modern radars have an automated gain setting that is reasonable to use during most of the flight. If this setting does not exist, leave it set to the maximum until otherwise needed.
However, when doing your deep-dive evaluation of a cell, you will want to play with the gain. An old-school recommendation is that, if you turn down gain 50%, avoid whatever weather still shows up on your scope. Another is to turn it down 100% and do the same.
Newer model radars allow users to see how much the gain is being adjusted in decibels (dBZ). The scale of precipitation intensity is shown on the display. You can adjust the gain to eliminate specific intensities from the display, since green equates to around 20–29 dBZ, yellow is 30–39, and red represents ~40 or more.
For example, if you want to cut out green (light precip), turn down the gain by 10 dBZ. Black or no display is usually measured as 0–19 dBZ. Whatever you have left is what you’d probably want to avoid. Turning down the gain by 10 dBZ will usually take all colors down a notch – green to nothing, yellow to green, and red to yellow.
If you’re looking to isolate the worst of the worst on display, turn gain down 20 dBZ. Whatever remains yellow is the nasty news areas of weather.
Tilt is probably the most useful set of all, and takes the most study and practice to master. Tilt refers to the ability to control the angle of the radar antenna upwards or downwards. Without going into too much detail, the primary use of tilt is to make sure the radar is aiming in the right place, evaluate areas for attenuation, and calculate the top of an area of precipitation.
To ensure that the radar is aimed in the right place, the simplest method for cruise flight is to set the range to 80 nm and tilt down until the ground clutter shows up at the edge of the screen. When taking the runway for takeoff, you will want the radar tilted up 5–10 degrees.
Where exactly is the ideal spot of a storm to aim at? According to a wide range of meteorological research, the worst part of a typical storm hangs out around 15,000 ft, plus or minus a few thousand feet. So aiming the radar beam to this spot will get a picture of the most severe part of a cell.
To be sure to capture adjacent regions of strong precipitation, paint from the ground to 24,000 ft as a starting tilt setting from which to work. Digging deeper, consider 2 useful formulas to help ensure the radar antenna is pointed in the right direction.
The first is what I call the magic number distance (MND), which positions the tilt to include the altitude range in which one can expect the worst of a cell. MND is measured in nautical miles. MND = 240 ÷ angular width of the radar beam. The MND is the range at which you should paint ground clutter on your scope.
If you’re flying a big jet with a radar beam that is 3º wide, MND (240 ÷ 3) = 80 nm. The second formula is height of radar beam, which equals radar width angle x range at which ground clutter appears (in nautical miles) x 100. This is simply a rearranged MND formula.
So, if we set our 3º radar to paint the ground at 80 nm, it would be 3 x 80 x 100 = 24,000 ft. Thus, at 80 nm, we are painting from the ground up to 24,000 ft, ensuring we include the cell’s severe core. Another critical function of tilt is to tilt down to ensure ground clutter appears behind suspicious cells.
This attenuation check is part of the overall evaluation process of any cell that may be close to or in the way of the route of flight. Cells showing signs of attenuation, such as crescent shapes with ground clutter shown beyond it, are indications of a storm to avoid by giving it a large berth.
Datalink weather radar
There is little doubt that datalink weather has been a game-changer for operators who do not have access to sophisticated onboard radar, particularly general aviation users. In fact, some of the capabilities of datalink users rival those of the airlines.
While datalink weather radar information is a fantastic enhancement to safety, it is not perfect. Pilots must be informed about the limitations to what is being shown (or not shown) on their datalink display. First, it is critical to know that there are differences in what products are available and how they are updated, based on the datalink provider used, whether satellite, such as SiriusXM (SXM), or via ADS, like FIS-B.
For instance, SXM provides high-resolution composite and base reflectivity radar imagery in the continental US. In contrast, FIS only provides high-resolution composite imagery to 250 nm away from the aircraft. Beyond this range, it provides low-resolution imagery.
Furthermore, FIS does not offer base reflectivity. Another advantage to SXM over FIS is a faster update frequency. Using datalink radar information is essentially the same as viewing Nexrad imagery. That’s what you’re probably using already during your preflight planning process.
Moreover, composite images generally paint a more conservative picture, as they will show weather as being more intense than one would probably see displayed by airborne radar. This is due to the cumulative effects of stitching together a composite image from multiple local sites, and may encourage users to stay clear of or further away from areas of strong returns.
There are a few important caveats about datalink of which pilots should be aware before taking off. One is that datalink may not be received on the ground or until you reach an altitude high enough to be in the line of sight of a transmitter, so you could be in the blind, particularly in remote or mountainous areas, until you are a couple of thousand feet up.
This would be disadvantageous if there was a thunderstorm in the area. Along similar lines, depending on the local terrain and the distance of the weather from a Nexrad ground antenna, some weather may not be detectable by the radar itself.
This is especially an issue around mountain ranges in the western US. Datalink can’t show you weather the radar cannot detect, so be sure you’re aware of Nexrad coverage in the areas in which you intend to fly so that you’re not lulled into believing that a blank screen means there is no precipitation with which to contend.
There are 2 critical things to know about datalink. First, it is not intended to be used for close-in maneuvering around severe weather or its penetration. Second, what is displayed in the cockpit is not a live image. Nexrad stations take a few minutes to generate images from what is detected, especially when processing and collating the range of imagery needed to create a composite image.
It also takes some time to transmit the final Nexrad image to the service provider, which then broadcasts it to its subscribers. Datalink imagery comes with a time stamp that indicates how long ago the image was transmitted from the data provider – not how old the data used to create the image may be.
There is a delay between when a Nexrad station detects precipitation and when a pilot receives such data in the cockpit. This delay can be as much as 20 minutes. If a severe storm is moving fast, as they often do, the location of a cell shown on a datalink display could, in reality, be up to 20 nm downwind.
Not surprisingly, a few users have been caught by this trap. One such example is a medevac helicopter that flew into a severe storm that the pilots believed was still miles away from their location. The result was fatal for all occupants. Because of the frequency of such occurrences, particularly in the early years of datalink, NTSB issued its Safety Alert SA-017, which warned of data latencies and ways to deal with them.
NTSB stated that datalink radar imagery should be used as a means of knowing where the weather was, not where the weather is. Moreover, pilots should assume that the cell shown could be 20 nm or more downwind of the location shown by datalink.
Not surprisingly, NTSB also recommended deviating upwind and being clear of any detected weather. The key is to be well educated about what your datalink system can and cannot do, and any idiosyncrasies for the system installed in your aircraft.
It should be evident how crucial it is for radar users, regardless of system type, to learn the intricacies of their avionics in order to create a foundation for the experience that comes from actually using the device on live flights, because knowledge is only part of what is needed to be a proficient radar user.
I always recommend trying to exercise one’s radar muscles on days when their proper use is not critical to safety, so be sure to play with the various settings and features whenever possible. For instance, one can practice using the formulas mentioned above, and manipulating tilt to see attenuation from terrain, or just check out ground clutter appearances in various locations.
With datalink systems, get to know the various products, settings, and available displays. The beauty of most datalink systems is that they can be used to view weather somewhere, even if there isn’t any to check out locally. By becoming educated on the systems installed in the aircraft you fly, in addition to getting a little low-stress practice using such avionics, you will become a balanced and savvy user of one of the most important systems aboard modern aircraft.
David Ison, PhD, has 35 years of experience flying aircraft ranging from light singles to widebody jets. He is a professor in the graduate school at Northcentral University. His book Navigating Weather was recently released by ASA Publications.