Knowing the ins and outs of your radar will keep you out of danger.
By David Ison
Professor, Graduate School Northcentral University
A Part 135 operation scheduled a flight using a King Air C90 from ZPH (Zephyrhills FL) to BFM (Intl, Mobile AL). Even though it was just before 9:00 am, there was plenty of convective activity along the latter parts of the flight – some with tops to 45,000 ft.
The IFR flight departed without a hitch and was almost immediately being cleared up to the filed cruising altitude of FL180.
Once switched over from Tampa Departure to Jacksonville Center, things started to get more interesting. Soon after checking in with Jacksonville, the King Air was notified that there were large areas of heavy precipitation from south of Panama City all the way to the Seminole VORTAC near Tallahassee.
The pilot responded that they were okay with proceeding without deviation, based on what was displayed on their radar. As the aircraft closed in on its destination, it was switched over to Pensacola Approach and cleared down to 11,000 ft.
The pilot stated they wanted to deviate right for weather, which was approved. There were no more communications from the aircraft, which ended up in pieces, having come apart amid a Level 5 thunderstorm.
Are you a good radar operator?
Every pilot who has radar at his/her fingertips should know how to use it properly. There are many philosophies regarding the correct use of radar, but a combination of common sense, distancing oneself adequately from convective activity (20 nm or more), and simple memory tools can keep you safe from mother nature’s violence. All you need to remember is to make a proper CAST, where C is for colors, A is for attenuation, S is for sensitivity (also referred to as gain), and T is for tilt.
The order of the CAST mnemonic does not reflect the importance of each sub-aspect. Instead, all parts should be used collectively to make decisions about navigating dangerous weather. If you use CAST effectively, you should not only stay safe in stormy skies – you will also be more likely to provide your passengers with the smoothest rides.
Colors observed on a radar screen are critical to inflight decision-making in the face of poor weather. The most basic rule of thumb is to think about radar colors like a traffic light – green means go, yellow is caution, and red (magenta) is a no-go.
Colors also refer to how the different hues are distributed on the radar screen. If there are sections of green, yellow, and red stacked together closely, it is indicative of convective activity, which should be avoided. This type of distribution is also referred to as contouring.
Steeply contoured cells – those where green, yellow, and red are closely spaced and have clear borders between colors – are worthy of deviation.
On the contrary, if there is a large swath of green with a smattering of yellow and no red, this is likely to be non-convective, and therefore reasonable to consider traversing. (See Figure 1.)
The distribution of colors sometimes paints cells of a particular shape. Several storm shapes are associated with severe weather, such as those that look like a hook, are bowed or curved, are rotating, or form a line of highly contoured cells in front of another line of weather (also known as a squall line). In rare cases, a hail core may appear as a line extending from the core (the worst part) of the storm and extending downwind (see Figure 2). Any storms mimicking these shapes should be given a wide berth.
Colors must also be put into a geographical context. For example, the appearance of red in Florida pretty much any time of year signals convective activity that should be avoided. But if you see speckling of red within large areas of yellow and green in the Pacific Northwest (PNW) in December, this is almost assuredly just heavy rain.
You will also want to pay attention to what happens to the coloring of radar returns as you get closer to them. Due to some of the radar energy being absorbed by the precipitation itself, cells appear to get stronger as you get closer. This is particularly so in the PNW type of example, in which you have wide area coverage of moderate to heavy rain that is neither convective nor dangerous.
Among the pieces of CAST, the one that can almost guarantee the avoidance of weather that can kill you is the ability to identify and avoid attenuation. Attenuation is when the energy of the radar beam is reduced through absorption, scattering, or blockage.
Any time radar energy goes through a raindrop, it loses some oomph. The denser the raindrops (ie, the heavier the precipitation), the less energy that will make it back to the radar. Avionics designers know this, so they arm modern radars with a software enhancement that will adjust the appearance of radar returns to help compensate for attenuation.
However, there is a limit to this adjustment. If significant attenuation is occurring, the radar returns beyond such a point will be understated. In extreme cases, there may be no returns at all. You do not want to go anywhere near a storm that is strong enough to gobble up a lot or all of the available radar energy. One easy way to avoid becoming a victim of attenuation is to ensure that you can paint the ground behind all areas of weather through which you plan to fly. (See Figure 3.)
Sensitivity refers to an adjustable feature of radar systems called gain, which can be altered manually via a rotating knob or other interface. When a pilot reduces the gain, the way the weather is depicted will change so that weaker returns will disappear and stronger storms will remain on screen.
Gain can be used as an evaluation or validation tool. For example, when approaching an area of suspicious weather, a simple trick is to turn the gain down 50%. Whatever remains depicted is an area to avoid. If one further reduces gain and a cell remains on the screen, you can be assured that it’s quite endangering.
Gain can be particularly useful in evaluating “soft spots” in the weather as well as paths for the best rides through precipitation cells. Turning down the gain reveals the weakest portions of the weather. When enveloped in a large weather system, gain can be reduced to show where the precipitation is the lightest, thereby providing hints as to which path to take. (See Figure 4.)
Last but not least is tilt. Tilt management is an essential skill for all airborne weather radar users. To understand how tilt works, imagine that you are walking at night with a flashlight. That flashlight and its beam of light are your radar. If you point your beam in the wrong place, you can’t see what is in front of you.
The most rudimentary way to use tilt is to simply point the beam down until you can start to paint ground returns (where the bottom of the beam touches the ground) on the far end of the screen.
From there, you will want to adjust the beam up and down to evaluate any cells. If you can “see” the ground beyond a return, this means that attenuation is not a major issue and thus potentially provides a safe path (upon further investigation).
You may be thinking to yourself that you don’t need to know how to work the tilt because your radar has an “auto tilt” function. Think again. Most manufacturers state that manual evaluation of weather is the only way to navigate areas of weather safely. So be sure you’re proficient in tilt management. And if you do, you can pinpoint what part of the sky you’re investigating.
According to meteorological research, the worst part of a thunderstorm hangs out around 18,000 to 25,000 ft. Thus, in order to get the best picture of a cell, we want to target this range. There is a simple formula for figuring out how much of the sky is being painted by radar, which is the width of the radar beam (degrees) x the distance at which ground returns are painted by tilting down.
This gives the number of feet of sky being scanned by the radar beam (after adding 2 zeros). Let’s say you tilt the radar down and start to paint. Then, plug the numbers into the aforementioned formula: 3° radar beam width x 80 nm = 240. Add 2 zeros, and it’s 24,000 ft. Therefore, the radar is painting from the ground to 24,000 ft at 80 nm.
Fine-tuning your radar view
We can then fine-tune our view at any time. If you are painting a cell along with ground clutter at 80 nm, you may notice that the cell will start to look weaker or even disappear as you get closer to it.
This is because the radar beam stays fixed, thus eventually pointing beyond the heart of the cell. At 60 nm, you tilt down to see ground clutter there. So now the radar beam is showing the sky from the ground to 18,000 ft (3º x 60 nm = 180, or 18,000 ft).
To view the core of the storm, simply tilt up 1 degree from this point. At 60 nm, tilting up 1 degree moves both the bottom and the top of the beam up 6000 ft (1° x 60 nm = 60, or 6000 ft). The beam is now pointing between 6000 ft above the ground to 24,000 ft – right in the sweet spot.
Using the same formula, one can determine the top of precipitation (not the top of the cloud). Using the same numbers as above, with a cell at 60 nm, and painting the aforementioned 6000 ft to 24,000 ft of sky, keep tilting the radar up 1 degree at a time.
Recall that each time you do so, you are adding 6000 ft to the floor and top of the radar beam. Hypothetically, if the cell disappears as you reach 6º up from where you started, the bottom of the beam is at 36,000 ft, meaning the top of precipitation is somewhere close to that altitude. This is no place for an aircraft to be.
While this only scratches the surface of the skills and knowledge you need in order to operate airborne weather radar properly, these easy-to-use tactics can and will keep you out of lethal conditions. The key is to keep things simple with a proper CAST so that, when time is short, you can set up the radar quickly to tell you what you need to know to stay safe.
Don’t run any red lights and keep your eyes peeled for steeply contoured cells. Ensuring you can always paint ground clutter on the other side of a storm, you can confidently steer clear of anything that can take out an aircraft.
Lastly, proper tilt management will back up your investigations for attenuation and provide you with critical information about any storms you may encounter.
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.