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?
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.
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.
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, but 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, until recently there wasn’t a good option for displaying XM weather on an iPad (it can be done now), 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).
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 areas of the western US where the network is a little sparse. 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.
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.
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.
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’s 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.