Thunderstorm flying in the age of datalink weather

Datalink radar or onboard radar? XM or ADS-B? Panel mount display or iPad? The options for receiving and viewing in-flight weather have never been greater, with a proliferation of affordable and capable avionics coming on the market over the past decade. This is a great problem to have–airline pilots of 50 years ago would have killed for any of these options–but it’s still a problem. Which one is best? And what’s the right way to use each tool?

Nexrad image
The radar picture at the FBO showed a “popcorn” day in the Southeast.

A recent flight from Hilton Head, South Carolina, to Cincinnati, Ohio, offered me the opportunity both to compare the options and to dodge some healthy thunderstorms. The results were surprising.

Preflight Briefing

It was a hot summer afternoon in the Southeast, one of those days when the radar is clear at 11am and lit up like a Christmas tree by 3pm. There was a large cold front moving into Cincinnati from the northwest, but otherwise the developing thunderstorms were not organized into lines. In fact, most of the the route of flight was dominated by a large area of high pressure.

This big picture understanding is critical when evaluating any trip, but especially if there are storms along the route. Some pilots are slaves to the radar picture, canceling a trip almost anytime there’s red on the screen. If you follow this rule, get ready to cancel most of your trips–it’s simply too conservative.

This day was a classic example. While the radar was busy, there was plenty of room between the storms and plenty of good options en route if a diversion was needed. With this plan in mind (yes, I do believe in “taking a look”), we launched.

The Pilatus PC-12 features an onboard Bendix/King color radar and an Avidyne EX500 multi-function display with XM satellite weather. I also carried an iPad with a Stratus portable ADS-B weather receiver. We would use all three systems over the course of the next two hours.

Onboard Radar

For decades, the only option for in-flight weather (besides calling Flight Watch) was onboard weather radar. While this has worked quite well for airliners and business jets, the cost and weight of these systems prevented them from being installed on most piston airplanes.

But even if you’re lucky enough to fly with onboard radar, using it effectively takes serious training and practice. This is not a “set it and forget it” tool. Modern radars like the one in the Pilatus have simplified some things, but they still require constant attention to make sure the tilt is set properly. In addition, the effective range of small radars is not much more than 40 miles, making it hard to plan very far in advance.

For all these reasons, I only use the onboard radar about once a year for real storm avoidance. I have it turned on much more often than that, it’s mostly valuable as a close-up tool–for picking your way through a line of storms or skirting the edge of a cell in IMC. Fortunately, those days are pretty rare and even then it’s not magic. For everyday flying, there are simply better options.

On this flight, I didn’t even turn on the radar for the first hour of the flight–avoiding the big storms was much easier done visually and with datalink radar. But the last 40 miles presented a broken line of weather that we would have to pick through, so I turned on the radar and brought it up on the pilot’s EFIS display (keeping the Avidyne MFD free to show XM weather). This should have been a perfect case for the real-time, high resolution picture that only an onboard radar can deliver.

It did work, painting a number of cells of varying intensity in clear outlines, especially as we got closer. Plus, the high resolution nature of this image was quite helpful for determining the gradient of each storm–that is, how fast it goes from green to red, with a steep gradient generally indicating a more severe storm. As the picture below shows, there was an area of lighter returns in between two well-defined storms.


But in the end, the onboard radar was simply a supporting tool. It didn’t tell us anything we couldn’t learn elsewhere, and what it did tell us was incomplete. In the picture above, the radar suggests a gap may be there, but I would never fly through this area without verifying the gap with another source. Among other questions, what’s behind this line?

For the expense and amount of work required, onboard radar wasn’t a great value on this trip.

XM Datalink Weather

When XM Satellite Weather was first introduced, most pilots realized it was a big deal. But with the launch of the Garmin 396 portable GPS in 2005, the concept of in-flight radar for light piston airplanes really went mainstream. For the first time, even renters could have a nearly real-time radar picture in their cockpit, complete with their airplane’s position on a moving map. It was truly a breakthrough.

Besides being affordable and portable, pilots fell in love with XM weather because it is easy to use (no tilt knob to mess with), it can show the entire United States (no range limitations), and it displays high resolution radar (no blocks or shadows). Because it is satellite-based, there are also no limitations on coverage areas or altitude, so literally every pilot in the US can use it effectively.

Of course there’s no free lunch, and XM weather does require a monthly subscription (anywhere from $35 to $100 per month). This seemed like a small price to pay at first, although a certain amount of “subscription fatigue” seems to have set in with some pilots. It’s a fair comment–that $2000 XM receiver/GPS does cost another $600/year to keep active. That’s still a great deal overall, but with $6/gallon avgas and monthly database updates, every dollar counts.

The only other criticism is not an XM problem: not all implementations are the same. For example, some panel-mount systems don’t display all the available XM weather products and others, like the Avidyne in the Pilatus, don’t allow you to pan the map. Also, there isn’t a good option for displaying XM weather on an iPad (it can be done, but it’s messy), which is a serious limitation for many pilots who do everything on their tablet.

On this trip, the XM performed flawlessly. Before takeoff, the MFD was already showing a current radar picture, and once in cruise we got regular 5-minute updates. It’s not “real-time” like an onboard radar, but it’s pretty close, and we spent a lot less time with our heads down tuning the radar display.

Most importantly, because of the range we were able to make weather avoidance plans hundreds of miles in advance. This is my favorite feature of XM weather–you don’t get backed into a corner, because you have the whole picture. On some trips I’ve taken detours of 100 miles or more to avoid nasty weather, and I’ve never regretted it. Without XM, I never would have the confidence to take these types of deviations.

It’s also easier on ATC. Instead of asking for 10 degrees left, then 20 right, then 30 left, we can simply change our flight plan route to avoid the weather. While you should never sacrifice safety to please ATC, in busy airspace like the Northeast or the Washington, DC area, it’s good aviation manners to consider their needs.

The picture below, which shows the same storm system as the onboard radar picture above, is a good example of the power of XM radar. It clearly shows a gap in the rain, but it also shows what’s happening behind these cells.


You may notice one other difference between the XM picture and the onboard radar picture: there isn’t much consistency in the intensity levels. What’s yellow on XM may be green on the onboard radar or red on another display. This is an important detail to understand, especially as you transition into new avionics. On some datalink systems, I’ve flown through yellow and even some red with hardly a bump; on others I avoid any return stronger than green. Learn the levels of reflectivity for your system (measured in dBZ).

ADS-B Weather

The newest option for datalink weather is Flight Information Services-Broadcast (FIS-B), although it’s more commonly called ADS-B weather because it’s delivered over the FAA’s Automatic Dependent Surveillance-Broadcast (ADS-B) network. There are two major differences between ADS-B and the well-known XM system: it’s ground-based not satellite-based, and there are no monthly subscriptions (your tax dollars paid for it).

Because the weather data is transmitted from a network of ground stations, ADS-B weather is not available on the ground in most locations and not at all in some western states where the network is not built yet (the system should be completed by early 2014). In practice, I usually start receiving ADS-B weather by 300 ft. AGL in most areas east of the Mississippi and throughout Texas and the West Coast.

ADS-B weather products are updated roughly every 5-10 minutes, just like XM, and most of the same weather products are available on both services, including radar, METARs, TAFs, TFRs, AIRMETs, SIGMETs, Pilot Reports and Winds Aloft. ADS-B adds NOTAMs but does not include lightning or satellite imagery.

One other detail: ADS-B radar isn’t the same as XM radar. In fact, ADS-B radar is made up of two different resolutions–a medium resolution regional image that is within a 150-250 mile radius of your airplane, and a lower resolution national radar image. So while you have radar everywhere, it’s not always perfect. The three screens below show XM, ADS-B regional and ADS-B national radar images. The national image on the far right is clearly blocky, but the other two are fairly similar.

xm v adsb

The screenshot below is another example, and shows how the difference in resolution plays out over the course of a flight. From 200 miles away, the national radar was so blocky that it was hard to make detailed decisions about what was happening, but at least we knew there was some serious weather to consider.

As we got closer, the national radar became the higher resolution regional radar and it was detailed enough to make more specific choices. We found that one red cell on the ADS-B national picture became a green cell with just a little red when it turned to regional. The national picture was showing the highest intensity in that block, so it was red. But that overstated the severity of the weather.
adsb both

The real magic isn’t the weather data but the way it’s displayed. While it’s not specifically an ADS-B feature, I’ve found the ability to display ADS-B weather on the iPad to be a critical advantage (all the major apps support ADS-B weather). It’s easy to zoom out, pan around and zoom in on specific weather systems. And it’s not just radar–the option to overlay other weather products, terrain or even traffic makes for a glass cockpit display in the palm of your hand.

Weather also lends itself quite well to the touchscreen interface of the iPad. Tools like the measuring ruler in ForeFlight really come in handy when dealing with weather. Take the screenshot below–how far apart are those two cells? Instead of guessing, we could measure and find out.

adsb measure

Is the lower resolution of ADS-B radar a practical limitation? Judge for yourself. The shot below is zoomed in fairly tight, so the pixelated radar image is exaggerated, but there’s no doubt the resolution is lower than XM. Having said that, the takeaway is the same–there is a line of convective weather with a small gap.


On this flight, ADS-B was the tool we used most often, probably because the iPad made it easier. I found myself planning on the iPad, then confirming my plan on the XM or onboard radar display. It’s certainly not perfect, but easy beats perfect most of the time.

Which Is Best?

If you could have any one of these options, which is the most accurate and most reliable? There’s probably a different answer depending on the type of airplane and the type of flying you do. But given the choice, I’d take datalink over onboard radar every time. That may elicit some eye-rolling from old-timers, but in my 10 years of flying with both options, I’ve used datalink much more frequently and successfully.

Datalink radar is just about dummy-proof, and it requires almost no interpretation. There is no attenuation and no radar shadows. Its unlimited range makes it easy to plan diversions far in advance.

What about the time delay that’s inherent in any datalink product? It’s certainly there (anywhere from 5 to 20 minutes, depending on whom you ask), but it’s not a major factor as long as you recognize the limitations and plan accordingly. Thunderstorms can build fast, but that’s why you give the big ones a wide berth. At the speeds most general aviation airplanes fly, 5 or 10 minutes should not make the difference between missing a storm and hitting one if you’re flying with the proper margins.

And then there’s this old saw: onboard radar is tactical, datalink is strategic. Maybe, but who cares? Datalink radar allows you to make deviations so far in advance that you rarely get into tactical deviations.

Finally, datalink weather is much more than just radar. Many times, the combination of radar and a pilot report is much more valuable than a radar picture by itself. Weather in context is powerful, and this is a feature that no onboard radar can offer.

So given my preference for datalink weather over onboard radar, is XM or ADS-B the better choice? The reality is they’re very similar. That doesn’t mean they are the same–XM has higher resolution radar and better coverage–but it does mean they produce the same outcome most of the time.

I typically get ADS-B radar shortly after takeoff and have good coverage everywhere but Wyoming, Utah and western Nebraska. Since I look at the radar picture on the internet before takeoff, that means I’m without weather from the time I walk out the door of the FBO until the time I’m out of 500 ft. That’s not long; if there’s a thunderstorm between those places, I shouldn’t be flying. Practically speaking, coverage isn’t an issue for my trips.

As for the difference in radar resolution, it’s another difference that doesn’t matter too much. XM radar is undeniably better, with high resolution radar, echo tops, storm tracks and plenty of other features that ADS-B simply doesn’t offer. For a truly awful weather day or for weather geeks, these products are fantastic. But most of us are interested in avoiding the red.

If money were no object, the XM radar picture is hard to beat and would get my vote. It’s a premium product. But ADS-B is 80% of the quality for 100% less subscription fees. For many GA pilots, that’s a pretty good deal.

The Eyes Have It

But as good as these tools are, none of these options was the ultimate winner on our flight to Cincinnati.

When it came time to pick through that line of weather, we followed our eyes and avoided all the clouds. Even in a high performance airplane on an IFR flight plan with a two-pilot crew, the smart choice was to stay visual. We did and we enjoyed a perfectly smooth ride as we descended through the weather, weaving back and forth around buildups. The onboard, XM and ADS-B radar pictures certainly helped to verify what we saw out the front window, but none of them were as detailed, real-time and valuable as the human eye. After all, the goal isn’t just to avoid the rain, but to avoid all the nasty bumps.

It certainly helps to know your air mass, which is why I’m a big proponent of Dick Collins’ advice to always learn the big picture during a weather briefing. If you’re flying through a messy occluded front with a lot of embedded thunderstorms, staying visual may not be an option. In this case, your best bet may be to stay on the ground, but if you absolutely have to fly that day, this is where an onboard radar can pay off. For most non-professional pilots, though, these days are few and far between.

My advice: if you can afford to fly cross countries, you can afford to fly with datalink weather. ADS-B is subscription-free and perfectly fine for most GA pilots. If you don’t mind the subscriptions, XM weather is a bit better. Regardless, consider these technologies to be backup for the Mark I Eyeball. It’s still your best weather avoidance tool.


  • I enjoyed this article as the author has acess to all these useful tools and also has the ability and time to describe the advantages and disadvantages of each.
    I was surprised, however, that no mention was made of tools such as Stormscope. I understand that the devices mentioned in the article also can display lightning. Were they not used in this way?

    • Believe it or not, the Pilatus has a Stormscope in it. But with all the other options, I almost never look at it. I like Stormscope, and I’ve flown a number of airplanes where that was the only option. It’s certainly better than nothing. To me, though, datalink weather is much more accurate and complete. For example, the range on most Stormscopes is usually not much more than a rough guess–it does fine on azimuth but not too well on range, and that’s an important detail.

      I wouldn’t remove a Stormscope if it was in the airplane, but I also wouldn’t pay money to install one right now.

      • Apropos of your comments about the interpretation of colors varying as it does on the different displays, I have found my Stormscope useful in discriminating dangerous red from non-dangerous red. Would you agree with that?

        • That’s a fair point. But my issue with “safe red” vs. “dangerous red” is more between different displays/receivers, not on the same one. On the same display/receiver, once I’ve learned the colors, they’re a pretty reliable indicator.

          It does all go back to knowing your air mass–if you’re in stratus yellow may be just fine; building cu and it probably isn’t.

      • Unless flying outside the U.S. NEXRAD images end about 100 nm beyond our national borders. Flying along the Caribbean chain or in Central America, which have frequent afternoon thunderstorms, means either a Stormscope or praying while penetrating.

        That said, the article is a good one!

  • Nice, informative review John! I’ve not yet flown with either onboard weather radar or ADS-B, but do have a lot of hours of cross-country with XM Wx as my cockpit companion, which has proved itself extremely helpful, even if not cheap. Perhaps with the competition from the FAA’s ADS-B systems, maybe the sat subscription rates on XM will come down a bit some day?

    It’s starting to get a bit tiresome reading the repetitive warnings against failure to recognize the built-in time delays on XM Wx data … as if the product is really not all that useful, which could not be futher from the truth. There seems to be an assumption amongst too many aviation experts that most pilots are complete imbeciles.

    In my experience flying in relatively close (but not too close!) proximity to the cells depicted on my portable XM Wx display, the data as depicted have been reasonably representative of what I see by looking out of the cockpit.

    No, I wouldn’t depend on the pixels on a screen to navigate between two red cells only a few miles apart … I’ll use my eyes, thank you, and stay the heck away from same … while I observe the 20-mile rule on clearance around monster cells with steep gradients. Using NexRAD to “pick one’s way between cells” is certainly courting danger.

    As you say, the biggest advantage of XM is the big picture – what am I dealing with, how far does it extend, and therefore can I get around this cell or this system without getting trapped by a massive converging system? Or should I just head for my alternate destination, or the nearest airport, or simply make a 180? There’s tremendous utility as well as peace of mind in being able to see the big picture. It’s also very nice to get updated METARs en route.

  • Great article and with the addition of the Stormscope info, a really excellent review of the weather avoidance options available. Similar to the PC-12, we have onboard weather radar in our Aerostar. Older and only monochrome, but it works. We’ve also had XM pretty much since it came out. And we’ve got access to ADS-B weather on our iPad. Lastly, a number of friends have sferics devices, so we’ve also been able to try various combinations and have come to the identical conclusions:
    – Anything is better than nothing.
    – XM and ADS-B weather gives the best overall weather avoidance because the big picture is complete and always available. I give XM the edge because the resolution is the same regardless of how far away you happen to be flying from the current location.
    – Stormscopes are an acquired taste and take some training.

    A technique that I like to use is when there is potential weather at or near our destination, I’ll leave our 2nd navigator radio (we have a GNS 530 and 430) on the destination zoomed in on the expanded weather page typically using around a 50mile range. Although it may be a couple of hours away, every few minutes NexRad updates and it’s easy to tell whether storms in the area are building, static, or dissipating, as well as how fast are they are moving. Helps a lot with planning and whether a diversion is going to be necessary.

    We also work hard to stay visual as much as possible. Being pressurized helps a lot. I’ve found that flying in the mid to upper teens works quite well although higher is usually better. I’m not thinking of penetrating a storm at those altitudes, but being above the haze layer gives a great view of the storms and what they are doing. For non-pressurized aircraft, particularly turbo-charged plane, getting on oxygen and getting high is a good idea.

    I try not to think about the “old days” (pre XM) flying in airplanes with no onboard radar or anything else. We did inadvertently fly through some storms in those days and feel fortunate to be here to talk about it. I really don’t want to do that again and with these tools used properly there’s no reason to repeat the experience.

  • Now that WingX Pro 7.0 is offering weather on my Ipad 2, I’m thinking I can do a pretty good job of navigating through weather using that in combination with my Stormscope. I’ve had the Stormscope for quite a while and have learned how to evaluate the display (changing the range, and clearing the display frequently to assess activity.

  • This is one of the best Airfacts articles I’ve read; thank you.

    I fly a 4-seat single in Florida. It is hard to fly in Florida during the summer unless you are willing and able to deal with thunderstorms. I rely on my eyes, the data link weather, and the trusty storm scope. Although instrument rated, I often fly VFR. I just traded in my XM for an I-pad and Stratus receiver. I feel the “usability” of the I-pad, coupled with the storm scope, more than make up for the lower resolution.

    Also, it seems to me the ADS-B updates the METARs and other information quicker than XM. Can anybody confirm this?

    • Ken I agree. The specs say ADS-B and XM have similar update times, but in my experience the METAR and radar updates almost always show up on ADS-B first.

  • John, I also use the two source approach for weather decisions.

    The basics are eyeballs and Flight Watch. As an Skylane owner for 14 years I started with a Stormscope and then added a Garmin GDL 49 (low orbit satellites). I was not very reliable and later Garmin replaced this unit, but it was a start.

    My wife summed up weather depiction very well once when the original Garmin weather link failed in moderate weather. At this point in IMC conditions she looked at me and in a non negotiable tone said “CHARLES, NexRad is a NO GO item!”

    I find the XM weather and Stormscope to be more complimentary that your experience. With the XM big big picture I have made a number of 200 mile deviations on long trips that did not materially add to the total distance.

    With a large fractional operation I used one other source, company meteorologists. At FL410 you could see a building line up to 200 miles in front or your route with no information on whether to divert left or right. Get of the Flightfone and call home to get an outstanding brief on which direction to turn while still a long way from the line was a big help.

    Always have to sources to make your weather decisions.

    Fly Safe

    • You’re so right about those big deviations. They always seem so costly when you’re flying, but when you get home and do the math you realize that the huge detour cost you about 5% of your total flight time. It’s a good reminder to take the sure bet, even if it’s longer.

  • I appreciate the article. As an older but no longer bolder pilot (nearing 70 and nearing 41 years of flying), I’m still a newbie when it comes to in-the-cockpit weather info.

    Years ago I regularly flew a Mooney 231 which had a King color radar with an inky dinky antenna mounted in the wing, and its limitations were significant: very short range, frequent fiddling with tilt required, etc. Not useless, but not very useful. I wouldn’t pay for onboard radar today (didn’t have to then–a friend owned the airplane).

    Since then, for roughly the last 25 years or so, I’ve flown with the old tried and true weather sources, Flightwatch, eyeballs, and ATC (which is a whole lot better now than it was when I was earlier in my flying “career”).

    But just a couple of months ago, I acquired an iPad Mini, Foreflight Pro, and Stratus II. I mounted the Mini on the yoke of my 50-year old C-P172D, the Stratus II on the lower left windshield, ran power to both so that I never worry about batteries, and suddenly I’m in love.

    This combination is incredibly easy to use. My first real use of it was my annual OSH trek, from northern Colorado across Nebraska to southwestern Minnesota to OSH, returning through Iowa and mid-Nebraska. Although I flew the entire route VFR, it helped me determine my exact routing well in advance of any problems.

    This past weekend I flew to the annual Colorado Pilots fly-in to Marble, CO, a tiny grass strip in a canyon in the middle of the Rockies. I had weather from TPA at Greeley all the way to Eagle, until I descended toward the canyon at Carbondale. On the way home, I used the weather from Eagle all the way home. By watching the movement of the cells with each update, I was able to make those strategic decisions, well in advance of any critical timing.

    With ADS-B soon to be available throughout the country and the cost of the hardware being pretty reasonable, the increased safety of having onboard weather is within the reach of just about every pilot. Amazing progress.


  • Good review – thanks. Can anyone comment about the utility of the Foreflight radar display? Although I have neither XM nor ADS, I use Forefight routinely both for planning and inflight. If seems to be a product that is excellently integrated with many other weather layers and combined with the ease of route planning and ability to immediately review emergency alternative destinations on a single screen ( minimising hand- eye distraction from aviating ) seems to be a near- ideal all round compromise to gaining a big picture perspective of the ogres lurking out there..

    • ForeFlight (and most of the apps these days, for that matter) has great radar and weather graphics. It’s unbeatable for big picture planning–the rubber band tool is great for planning deviations. Just note that you would need a Stratus or XM receiver to get that radar image in the air (no cell connection most of the time).

  • I too have used Stormscopes, XM and ADS-B. Stormscope only scared the wife and provided little information other than there was something “out there”. XM was great when it worked but slow to update & the cost was annoying as I didn’t use it regularaily, tho I’ve flown 150+ hrs the last six years and all across the country. With a NavWorx ADS-B, tablet and/or IPad & WingX weather & traffic, I’m good to go anywhere. Had to negotiate a front from DFW area to OSH last month and knew what I was in for as I left DFW. ADS-B Wx and the old eyeballs work great in combo to get around the bad stuff – IFR or VFR.

  • A most interesting & comprehensive article. Nice coverage of on board radar, XM &
    No mention of WSI, which I’ve flown with for 8 years, with never a “down” moment
    & brief update intervals. How does that compare with XM ?

  • Great article! After recent cross-country flights in IMC (heavy rain but no cells), with both XM and ADS-B, I formed a similar opinion. The big picture is important to me and I do not try to thread the devil’s needle. In that regard, I found the ADS-B display with the Garmin Pilot (on a Nexus 7) more informative, because it showed the motion of the weather over the previous 30 minutes vs. the stationary XM display (on a Windows8 tablet running AnyWhereMap). These were my first flights with both XM and ADS-B and I do not know whether other display software provide the same feature. As I was descending out of the clouds to land, Mark I eyeballs could also correlate the movement of the clouds with what I saw on the ADS-B display, which certainly increased my confidence – both in the ADS-B display and my safety!

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