Magenta Sky: Know Your Radar

Magenta Sky: Know Your Radar

Magenta Sky: Know Your Radar

In addressing thunderstorm flying, the Aeronautical Information Manual (AIM) begins with this gem: “Never regard any thunderstorm lightly, even when radar echoes are of light intensity. Avoiding thunderstorms is the best policy.” It sounds like sage advice. One of those things that are great in theory, but rapidly becomes muddled in the grey shades of the real world. I have lost track of the number of times that I have landed at an airport with the thunderstorm (TS) descriptor amended to the terminal weather. 

I do recall the first time it happened. I was a raw captain on a Beech
1900 approaching Denver international Airport. I could actually see the field, and told the controller that they should get rid of the TS code present in the weather: We were not, I explained, allowed to fly through thunderstorms. The proverbial crickets chirped a few moments before the controller calmly countered that none of the 150 aircraft that had landed in the past hourhad complained. 

Here is the first thing to know about thunderstorms: They can kill you. Here is the second thing: For aviation to be useful as transportation, you will eventually have to fly through an area in which they are present.

Here is the first thing to know about thunderstorms: They can kill you. Here is the second thing: For aviation to be useful as transportation, you will eventually have to fly through an area in which they are present. While there are a number of different resources available to assist in making the penetration of weather safe, few are of higher value than weather radar. 

Know Your Radar
The first bit of information that you should memorize about your radar is the angle of the beam. With X-band radar, the radar beam’s width is inversely related to the size of the antenna dish; the larger the antenna, the narrower the beam will be. A narrow beam gives a radar system more fidelity over distance, as the radar pulse is much more concentrated. Think of it like a flashlight: If you have a wide beam, the light gets dim very quickly; with a narrow beam, it has a much longer range. 

An 8-degree radar beam (a standard size for civil aircraft) ends up covering 40,000 feet worth of sky at 50 nm (you can calculate this via the ancient rule of 60, where 1 degree equals 100 feet per nm). Although 40,000 feet worth of radar sounds impressive, the problem is that this can begin to stretch the limits of physics. At 40,000 feet, the radar’s initial energy pulse is covering just shy of 7 nm worth of sky. This can make it easy to miss magenta danger areas at longer ranges, as radar returns become fruitlessly diffuse. 

The physical limits of the radar beam also make the use tilt extremely important since onboard radar covers a limited slice of the sky (particularly at shorter ranges). At lower altitudes tilt management can be fairly basic: Below 10,000 feet, dangerous weather is largely defined by the amount of precipitation that exists above the aircraft. Start from zero tilt (with half the beam above and half below the aircraft altitude) and tilt up by half of the width of the radar beam. The result will be that the bottom of the radar beam will be at your present altitude, with the radar displaying the precipitation above you. (This assumes zero tilt is properly calibrated; you can use the rule of 60 and a little math to verify this while painting the ground). 

At higher altitudes, hazardous weather is progressively defined by the amount of precipitation below the aircraft. It is important to remember that frozen precipitation is not nearly as reflective as liquid droplets. Since weather radar (in general) assumes liquid precipitation when depicting intensity, precipitation above the freezing level tends to underestimate the hazard ahead. As such, green (low intensity) returns for precipitation above 20,000 feet indicate hazardous weather. Confirming this analysis is easy: Tilt the antenna down to see what is below you; in many cases, once the radar beam is pointed towards the liquid part of the cell it will paint red and magenta returns – the exact sort of thing that you do not want to be flying above. 

Hazardous turbulence can exist many thousands of feet above those red and magenta areas – not to mention dry hail, which is the precipitation form of a stealth fighter (i.e., dangerous and resistant to radar). The goal should be to tilt towards the area of the sky that gives the greatest indication of hazardous weather, and that area almost exclusively exists below 20,000 feet. 

A simple solution above 20,000 feet is to tilt down in order to paint the outer third of the radar display with ground returns. This will keep the radar focused at the lower, liquid portion of storm cells. In combination with this, routinely vary the range that the radar is displaying. To paint the ground on the outer third of the display at shorter ranges, you will have to tilt the antenna down even further. This will ensure that you are not inadvertently overflying a rapidly growing cumulonimbus. (We had a passenger not long ago get seriously injured following an encounter with severe turbulence due to a cell rapidly growing beneath the radar beam as the aircraft approached it. The invariable tilt of the radar resulted in the hazardous weather never being presented to the flight crew). 

One last advantage of painting the outer portion of the display with the ground is that it makes for a foolproof means to identify truly strong weather. A ground shadow behind a cell (i.e., a conspicuous area where no ground is being painted) indicates that all the radar energy is being absorbed by the storm cell itself. This is a cell to avoid by a minimum of 20 nm, every time.

Ground-Based Radar
The size and capabilities of ground-based radar far exceed that found onboard aircraft, though they are not without limitations as it relates to inflight decision making. There are three common ways to access these ground-based resources: on the ground via internet connection, in the air via data-link, and verbally via communication with Air Traffic controllers. All of these methods provide a more complete picture than onboard radar alone is capable of. It is the combination of these radar resources that ensure the safest path of flight for a given situation (sometimes the safest path of flight is to be tied down and chocked). 

Turbulence can be encountered in the clear air around a cell.

Air Traffic Control as a resource varies by location. In the United States, Center and Approach controllers have weather radar data available at their stations. They are required by rule to provide weather alerts, but their primary focus remains the separation of IFR traffic. Controllers are available to provide information, but they do not always
offer it unsolicited. If you fly in less developed countries, be aware that controllers may not have access to weather, and that there may be large gaps in ground radar coverage when using data-link weather. Do not assume a lack of displayed weather means that the area is free of convective weather. 

Be proactive in asking controllers for information. Understand the services that are available in the airspace that you are flying in. If you are unsure whether controllers have weather depiction capabilities along your route, ask them. It is easy to fly yourself into a corner when deviating around closer weather while being unaware of more distant cells that are behind. ATC and data-link services fill the hole nicely, providing hundreds of miles worth of radar information to help generate to plan of attack.

While ground radar is a great supplementary tool, in practice, it possesses some shortcomings as well. You will occasionally find radar echoes observed inflight differ substantially from those produced from the ground. There are many assumptions that radar systems utilize to depict weather, and this can create conflicting information. Properly functioning airborne radar is the most reliable resource for making decisions en route. Not only will it keep you safe but it can (at times) shave off hundreds of miles worth of needless deviations.

Weather radar is a precipitation detector, plain and simple. It can detect other things as well –namely the ground – but its design and purpose is to display the precipitation that normally accompanies convective activity. It is important to recognize that the presence of precipitation in and of itself does not mean that the area cannot safely be navigated. Precipitation echoes can also be associated with broad, non-convective rain showers as well. You want to err on the side of safety, but you do not want to divert due to a little rain.  

Convective buildups come in many different shapes (there is an entire catalog of radar shapes that meteorologists use to identify extreme weather), but most hazardous weather shares this in common: well defined red or magenta areas encircled by a relatively narrow band of yellow. Extremely sharp gradients in intensity represent violent storms that should be given plenty of room. Magenta returns also indicate areas to avoid. 

Nonhazardous rain showers tend to appear splotchy, with gradual and uneven gradients from red, to yellow, to green. If other sources indicate that these areas are benign (such as pilot reports, lightning strike indicators, or tower reported surface winds), they can generally be transited without fuss. 

The decision to proceed, delay or divert should be made as early as possible based on the best information available. As you prepare for an approach at an airport with questionable weather, ask ATC what they are depicting and solicit suggestions for approaches that will keep you clear of significant returns. Outside of 100 nm, onboard radar is of limited use (it does a good job displaying well-defined lines of weather, but small isolated cells – even when there are dozens of them in a concentrated area – are oftentimes not displayed at longer ranges). Controllers in these cases can be a reliable source of information, if only you ask.

The approach phase of flight is high workload, and becoming fixated on the radar can greatly erode situational awareness to other required flying duties (always remember: aviate, navigate and communicate). Assess the information available, commit to a course of action and adjust only if new information arises which is relevant. Second-guessing is a worthless distraction. Focus your attention on flying the airplane instead. Thunderstorm encounters are a much less common cause of accidents than Loss of Control In-flight (LOC-I). 

You should remember throughout the process that diverting to an alternate represents the successful completion of a flight. The only measure that matters in aviation is a safe landing for every takeoff. To the degree that pressure exists to land at your destination, you must purposefully ignore that impulse and focus on making a smart decision. Do not roll the dice with your life or the lives of your passengers. If you have any serious question about the intensity of the weather, either divert or (if you have enough fuel) hold until it clears. 

Enjoy The View   
There is one thing that is better than radar: When in visual conditions, the best way to avoid hazardous thunderstorms is to use your eyes. There is no device on our increasingly sophisticated aircraft that beats biology. 

Radar is supplementary in the case of visual flight, but still important. It is quite difficult to determine the distance to cells by sight – a function that radar excels. It is also under visual conditions that we become increasingly comfortable with interpreting radar images. Since we can actually see the cells, it provides direct feedback in developing a mental picture from the radar display for those times when visibility is restricted.   

For all the platitudes of weather radar, there is nothing safer and more pleasing than remaining in visual conditions to enjoy – from a safe distance – that beautiful, powerful and awe-inspiring cumulonimbus. 

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