10 Things Pilots Should Know About Datalink Weather

by HPA · June 13, 2017

2017.06.14 04.13 Flyhpa 5940b80229999 800x53410 Things Pilots Should Know About Datalink Weather

by Paul Volk

This article is about one of my favorite topics: weather in the cockpit. I have been involved with this technology since 1999 when I signed on with a group in Hampton, VA to help develop and market a product idea they had patented a decade earlier – a cockpit device for advising pilots of weather. The invention of datalink weather revolutionized GA flying in a way few other technologies have. For those that have already used it, imagine giving it up. For those of you that haven’t, it will change the way you fly.

Commercial versions of these systems have catchy names like XM WX and InFlight. The FAA uses an acronym, FIS-B, for Flight Information Services-Broadcast. (It’s pronounced like a character from a Dickens novel – “Fizby”). Enjoy the following list of FIS-B facts and figures.

1. A Very Brief History

The first experimental version of cockpit weather was used for the exact opposite purpose that pilots use it for now… to fly INTO thunderstorms. In the late 1980s, the NASA Storms Hazards Program (in the name of protecting aircraft from lightning) flew a specially-equipped F-106 into thunderstorms to intentionally get hit by lightning! Rather than use ground-based radar to vector the fighter into storms, NASA developed a system to datalink the radar picture to the cockpit, allowing the pilot to steer into the heart of the storm. A former NASA engineer that I know filed a patent based on this system, but he (wisely) suggested that the best use of this invention might be to help pilots AVOID hazardous weather.

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2. Know Your Sources, Part One: Weather Radar

The National Weather Service (NWS) operates the NEXRAD (NEXt-generation RADar) network, consisting of 159 Doppler weather radars, to collect various kinds of atmospheric data. NWS turns that data into the mosaic-style maps of precipitation we are familiar with. NEXRAD coverage for the lower-48 of the continental United States (CONUS) is the source data that is ultimately distributed by FIS-B.

For pilots using NEXRAD data, probably the most important thing to know is that the mosaic takes time to collect and assemble. When it is finally distributed via FIS-B, the NEXRAD data is at LEAST five minutes old, and could be much older. More on this later.

Environment Canada operates a similar weather radar network, and this data is available from commercial FIS-B services for a subscription fee.

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3. Know Your Sources, Part Two: Weather Service Providers

Raw NEXRAD data includes undesirable artifacts like ground clutter, false echoes, etc. that are removed by basic post-processing. There are commercial enterprises that sell more sophisticated post-processed NEXRAD data to weather-reporting outlets. In the early 2000s, these enterprises turned their attention to the aviation market. The value-added enhancements for aviation are what’s “under the hood,” and include features like echo tops, storm tracking indicators and enhanced radar color palettes with snow, ice and mixed precipitation colors.

The major players in this business are Baron (XM WX, ForeFlight) and The Weather Company, (ADS-B In, WSI InFlight). Of course, when you purchase avionics, or a pilot app with weather, you don’t really get a choice of service providers (sort of like Lycoming or Continental engines when you buy an aircraft). But if you notice a difference between various weather apps, this might be why.

2017.05.31 07.30 Flyhpa 592f19c96d37c 800x3004. We’re the FAA, and We’re Here to Help

When commercial FIS-B services burst on the scene in the early 2000s, there were no standards to govern their implementation, or to enforce consistency in their human interfaces. To some extent, that situation still exists, but the FAA has stepped in with an advisory circular that recommends a path to airworthiness certification for FIS-B systems. Compliance with the AC is not mandatory, but it points to two solid standards documents, one for commercial datalink systems and one for the UAT link used by ADS-B, that further the cause of consistency.

Still, some potentially confusing differences exist between systems. The illustrations below show a typical G1000 Weather Data Link legend (left), and a similar illustration from ForeFlight documentation (right). Note the difference in colors used. When switching from one system to another, a pilot could easily be confused about whether an airport is reporting VFR or MVFR, IFR or LIFR weather. When in doubt, refer to the textual product that is also part of the uplinked information.

2017.05.31 07.29 Flyhpa 592f19add80095. Learning Your dBZs: The Radar Color Palette

There is a fascinating history behind the evolution of the color palette used to display storm intensity on digital displays. It goes back to research done in 1965, before color displays existed and everything on radars was shades of green. The basic green-yellow-red palette of a radar mosaic intuitively suggests “go-caution-stop,” right? Sort of, but let’s look at the research and some numbers.

The basic idea is to correlate the radar reflectivity of precipitation (measured in units called dBZ), and its associated turbulence level, with a specific color in the NEXRAD image. Usually (if it’s not freezing), it is the turbulence associated with precipitation that pilots want to avoid, especially severe and extreme turbulence. It has been said that in severe turbulence, you think you’re about to die. In extreme turbulence, you know you are.

While light to moderate turbulence may occur between 20-40 dBZ of reflectivity, the real concerns begin at 40 dBZ, where there is a small chance (3.5%) of encountering severe turbulence. At 50 dBZ, the chance of momentary loss of control due to turbulence is unacceptably high (37%). How does the color palette in cockpit systems communicate this to pilots? Not consistently, which is why users need to be vigilant. Look at some sample color palettes pictured below. Note that one palette goes from yellow to orange at 40 dBZ; another extends yellow to 45 dBZ. Both palettes would advise caution about flying in “yellow” precip, but one might certainly get you a wetter and bumpier ride than the other.

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6. NEXRAD Composite vs. Base Reflectivity: Tale of the Tilts

NEXRAD radars emit beams at different tilt angles to completely map the sky. The image from the lowest beam is called base reflectivity. Composite reflectivity combines the strongest return from all angles into one image. Base reflectivity shows rain that is falling from the bottom of a cloud, but not what is falling at higher flight levels. While composite reflectivity may make areas of precipitation look larger than they are at lower altitudes, consider the example shown in the images below. There is clearly more (light) rain falling at higher altitude, but is it evaporating or being held up by strong updrafts from the storm to the west? That additional shield of light precipitation on the composite image might be in the “anvil head” at high altitude in front of the storm’s movement. This is an instance where it might be a good idea to visually compare the composite image in the cockpit with what you are seeing out the window, because…

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7. Your Eyes are Still Your Most Important Sensor in Weather Flying

Thirty-five years of weather flying in GA aircraft has taught me this…if you can stay visual, do it. I think this gives you the most options, and by that, I mean escape routes if things go sideways. The NEXRAD image is an excellent tool, but not perfect. Cumulus build-ups may not yet have precipitation in them, making them “invisible” on NEXRAD, and most are turbulent inside. What look like gaps between cells on NEXRAD may have already filled in, or may be filling in quickly. Remember, NEXRAD is not real-time. Your eyes are the best tool for determining this—trust them. Since I’ve flown with cockpit weather, I’m almost NEVER in IMC when I’m flying near strong storms. There’s no percentage in it; I’d rather request a deviation, stay visual and stay dry.

Cockpit weather still plays an important role here. Instead of clogging the frequency with constant requests to deviate, use the power of the NEXRAD image overlaid with your flight plan to strategically plan your weather deviation, and collaborate early with ATC on a re-route.

8. Collaborating with ATC

ATC sees the same weather picture that you see in the cockpit, right? No, but that’s not necessarily a bad thing. It can affect coordinating a weather deviation, but some ATC weather radars are a nice complement to the FIS-B system in the cockpit.

For instance, approach controllers aren’t looking at NEXRAD imagery at all. They use a real-time radar system to detect traffic and weather in terminal airspace. Several generations of this system are still in use, and there are even older systems that paint rain, but not its intensity. (If a controller says, “Intensity unknown” in a precip warning, that’s why.) Still, if you’re negotiating a weather deviation in congested terminal airspace, it’s best to listen to what ATC suggests; their picture is more up-to-date than yours.

Center controllers use a different system that overlays NEXRAD imagery on their displays. Since it is processed differently than FIS-B, ATC’s image doesn’t show light precipitation, so don’t expect any warnings from Center until the precipitation reaches moderate levels. As with any NEXRAD-based system, the data the controller sees is not real-time; it can be up to six minutes old. When requesting weather deviations from Center, FIS-B gives the pilot a reasonably common picture, which can make this process smoother.

If ATC asks if you are radar-equipped, you can’t answer “yes” if all you have is FIS-B. Seems like a partial truth, but a former controller friend of mine points out that by answering “no”, you’ll get more weather advisories than you would if ATC thinks you are already seeing everything (and you may not be).

9. Key Differences Between ADS-B In and Commercial FIS-B Systems

There are two major differences between ADS-B and FIS-B: signal coverage and resolution.

Commercial, satellite-based FIS-B customers are used to seeing weather as soon as they power their systems on. ADS-B line-of-sight reception generally requires altitude. The FAA provides an ADS-B interactive coverage map tool on their website that allows you to see regional coverage at different altitudes. The higher you fly, the better your chances of receiving ADS-B, so check the coverage for where you operate. You can bridge this gap on the ground with internet service (like a cell network), but as you climb out, there may be altitudes where you are not receiving weather data.

As for resolution, commercial systems have more bandwidth available to them, and they transmit high-resolution (2km) radar information for the entire CONUS. ADS-B ground stations conserve their lower-bandwidth link by transmitting two separate images; a low-resolution (11km) NATIONAL image and a higher resolution REGIONAL one that covers the 250nm surrounding the station. Since ground station coverage overlaps, you should always have access to the REGIONAL image where you are flying. When you select a viewing range beyond the REGIONAL coverage, the app fills in with the lower res NATIONAL image, which is still detailed enough for strategic weather planning.

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10. Must Read: NTSB Safety Alert SA-017

Every pilot who flies with FIS-B should read this NTSB alert, sub-titled “Actual Age of NEXRAD Data Can Differ Significantly from Age Indicated on Display.” I think this is the definitive document on the subject, complete with cautionary tales of pilots who may not have appreciated this aspect of in-cockpit weather. The fatal accidents that resulted were due to “operating near quickly developing and fast-moving convective weather,” despite having “access to NEXRAD mosaic imagery.”

Most FIS-B systems indicate the age of the radar image being shown, but this is the age of the mosaic created by the service provider. The inherent delays in delivering the raw NEXRAD data to the service provider, added to transmission delays, etc. can add up to significant additional time. The alert says it best, “Weather conditions depicted on the mosaic image will ALWAYS be older than the age indicated on the display.” In the worst case, 15-20 minutes older, which is more than enough time for a fast-moving thunderstorm to fly over your intended route.

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Paul Volk is a pilot, engineer and aviation enthusiast with over 4200 hours of flying time. His experience in avionics development and avionics-related research has been at the vanguard of technologies such as ADS-B, cockpit-based decision aids and FAA NextGen capabilities that will affect the future of general aviation.

How it Started

One of my best friends in high school, (Doug Gray) was a private pilot. He offered to take me up for a flight in a 1967 Cessna 150, N6228S. We took off from Calhoun, Georgia, and he took me on a scenic tour of the area, I was hooked. I later found out that my English teacher, (Jan Haluska) was also a flight instructor and the school was offering a ground school course the next year, which he taught, along with flight training with the goal of becoming a private pilot. I managed to talk my dad into funding the training, at the time the total cost was right around $500. Cessna 150 rental rates where $12 an hour, and the 172 we used for cross country was $15 an hour including fuel.

Training Begins

It started August of 1977. The fall semester rolls around, and I am enrolled in ground school, and if memory serves me, we met twice a week. First came the paperwork for my student pilot certificate, which at the time was included with the 3rd class medical. I loved ground school, especially learning navigation, plotting courses on the sectional, again this was before iPads and GPS, so we did a lot of dead reckoning and VOR navigation for cross countries. At the end of the course, I made an 83% on the Private Pilot written exam, remember this was before we had the question-and-answer books that on future test I would consume before taking a written.

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Flying

The fun really began on August 22, 1977, it was my first flight. To my surprise we used the same Cessna 150, N6228S that my friend Doug Gray had taken me for a ride in. The Cessna 150 had a standard VFR instrument panel, with one NAV/COM, and one VOR CDI. No intercom, so no headset, at the time I didn’t know what I was missing. I didn’t get my first headset until I started my instrument training in 2003. I was a big guy, I was 6’ 4” and weighed 245 lbs. but I do not remember being uncomfortable in the 150 even with the instructor in the right seat.

First flight lasted .8 hours, and we accomplished orentation, shallow turns, stability, and effects of flight controls. Over the next few months we added steep 720’s, slow flight, stalls, turns around a point, S-turns, emergencies, landings, (short field, soft field and normal).

Learning

At this point I want to talk about a training experience that still stands out. We were close to solo and were practicing takeoffs and landings. Turning base to final Jan got very upset at the way I was cross controlling the aircraft. Cross control is when you are in, say a left turn, and use opposite rudder to line the nose up with the runway. So, he had me depart the pattern and head east to the practice area, and climbing up to 5500 feet MSL, he had me slow down to just above stalling speed, start a left shallow turn and add right rudder. As the airplane stalled I had the strangest sensation, no roller coaster has ever come close, instead of blue sky in the windscreen I was looking at brown ground, and as far as I could tell we were upside down, through the terror of the moment Jan talked me out of the spin, controls neutral, oposite rudder, pull slowly out of the dive. As we leveled off he asked me what altitude we were at, as I remember it was around 2,800 feet MSL. Then he asked me what would happen if I experenced this on base to final in the traffic pattern. The answer was obvious, I would be a pile of wreckage off the end of the runway with a very short-lived aviation career. Needless to say this cured my cross-control tendencies.

Another training event the sticks out in my mind was the first time we did takeoffs and landings at the High School runway. The runway was 1,500 feet long, not sure of the width, but it seems like we had about 5 feet on each side of the wheels when on the center line of the runway. Landing from the south you also had to go between a cutout in the trees to be able to stop in time on the runway. After getting confortable landing here every runway since seemed huge. I remember landing at Chattanooga (KCHA) on a night cross country and commenting to Jan, that I felt like I could of landed sideways on the runway, it felt that large.

Very soon after this I started wearing old shirts to all my flight lessons, the reason for this occurred on February 8, 1978. The lesson that day was stalls, takeoffs and Landing. As we were taxing back in Jan told me it was time for my first solo. I was very excited, and after some last minute advice from Jan, including watch for floating on landing the plane will be light with him not in the right seat, complete 3 takeoffs and landings to a full stop. It was a blast, and at the end of my shirt was shorter in the back because Jan cut the tail out of it signed and dated my solo [an aviation tradition]. Total flight time accumulated on the day of my solo was 18.1 hours. I was now officially a pilot with solo priveleges.

In my next article I start cross country training, when Jan decided that it would be best accomplished this in the 172. Checking out in N5970R was like moving into a 747, this started a love affair with one of my faviriote planes to date. Since this time I have flown over 900 hours in many different models of the 172 and feel like I am stepping into an old friend every time.

More about Randy here: https://www.flyhpa.com/team/randy-delong/

 

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