Crossing the Line: Datalink Weather and Thunderstorms

by HPA · August 9, 2017

Crossing the Line: Datalink Weather and Thunderstorms

by Paul Volk

It’s summer, and one way to tell is that it’s very easy to find “how not to fly into thunderstorms” articles. These are important reminders to GA pilots of the hazards of convective weather. Flying into a thunderstorm is certainly one of the top two things you don’t want to do (flying into icing conditions without anti-ice being the other). If you operate a technically-advanced aircraft, especially one equipped with datalink NEXRAD imagery in the cockpit, you’ve probably read, with some discouragement, that you are NOT to rely on this data for tactical avoidance of convective weather. OK, Debbie Downer, how DO I use this expensive equipment in the summer?

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I hope that this article helps, at least a little, by sharing some anecdotal uses of datalink weather in thunderstorm season. I spent the first couple of decades of my flying career without cockpit weather. Even after I got my instrument ticket, I can recall more than a few trips delayed, not completed, or not even attempted because of weather. Since 2002, however, the capability to continuously monitor meteorological conditions on my route has increased my success rate dramatically. I think that this helps justify the time and money that I spend on flying.

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Figure 1. NEXRAD Imagery on an MX20

Obligatory Warning

A few years ago, I was part of a team that assessed the “State of the Cockpit” of datalink weather technology for the FAA. The FAA requested that we include typical operational scenarios in the report. Since I was the designated GA expert, I suggested including a scenario of a GA pilot faced with deciding whether to cross a line of convective weather. To me, this was the “elephant in the room” regarding datalink weather and couldn’t be ignored. Naturally, the FAA was somewhat horrified at the suggestion, because it was already well established that NEXRAD imagery, because of the inherent delays in delivering it to the cockpit, is NOT a tactical tool for weather avoidance. (See my article, “10 Thing Pilots Should Know About Datalink Weather” on this website.) Ultimately, the scenario was included, with an acknowledgment that NEXRAD in the cockpit was responsible for a “paradigm-shifting advance in aircraft utility,” but “could be misused without adequate training…”

At the time, I had already completed several trips that started with convective weather in front of me, and ended with it behind me, safe and sound at my destination. While I saw plenty of thunderstorms during those trips, I used the NEXRAD imagery to manage my flight around the weather I saw, not to escape from it.

Everything you’ve learned about flying around thunderstorms still applies with datalink weather: keep your distance, stay visual around towering cumulus, don’t fly into a “sucker” hole, etc. The major difference is that you know where the storms are (or were, not too long ago), and often where they are headed relative to your current flight plan. This allows you to adapt to moving, changing weather and formulate an avoidance plan. Your avoidance plan needs an “out,” — a sure-fire alternative to avoid the weather, such as landing or turning around. If you’re in contact with ATC, then you have a collaborative partner to help you with that plan.

Scattered Storms

A very common scenario for convective weather is scattered thunderstorm activity. This can look like a large minefield of poorly-organized storms on long-range settings, but as you zoom in, very navigable gaps that don’t violate the 20-mile rule of storm clearance can become apparent. Your NEXRAD image can help you make the best deviation choices quite easily. I don’t consider this “crossing a line,” and I have often gotten from point A to point B through weather like this without getting wet.

It’s hard to overemphasize how important it is to use your eyes while flying in this type of weather. Convective storms strengthen and weaken, sometimes quite rapidly. Since the NEXRAD image you are looking at is delayed, a small area of precipitation on the screen may have already blossomed into something more formidable in your windshield. The converse can also be true. This makes your eyes the tactical tool, while the NEXRAD gives a strategic overview to plan with. If you are IFR, ask for deviations around buildups and stay visual.

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Figure 2. Flyable gaps? Not sure, but I might take a look, with an “out” plan

Frontal Lines

An organized line kicked up by a front can be another matter entirely. These tend to be more tightly-packed areas of intense storms, and of course, can’t be taken lightly. Although I have seen navigable gaps in lines like these, you can’t put your faith in the NEXRAD image because the gaps can fill in quickly. The better strategy is not to penetrate, but to use your speed advantage over the weather to circumnavigate it. Successfully employing this strategy requires both the fuel to spare, and the willingness to burn it.

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On an IFR trip from Virginia to Austin, TX my planned route was going to take me into a line of developing storms extending from Mississippi into Texas. Shortly after departure from Meridian, Mississippi (where I like to stop for fuel, free chili dogs and soft-serve ice cream), I coordinated a deviation with ATC, which, unexpectedly took me 75-100nm north of my intended route to avoid the line. The extra miles required me to stop in College Station to refuel, but I considered that a good trade-off. As I approached Austin, I could see that the weather was filling in behind me. I had not crossed the line, but I HAD outrun it.

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Figure 3. Using NEXRAD to plan a diversion around convective weather

On another trip, I was attending a college football game in Charlottesville, VA that was punctuated by a strong thunderstorm in the fourth quarter. I knew I’d probably meet that storm again flying home. Sure enough, soon after launching, it became apparent on NEXRAD that I would have to make a 100-mile diversion to get around a solid line between me and home, which was less than 100 miles away. A quick conversation with ATC confirmed that there were no other viable options, so I used my “out,” landed and waited the 3 hours for the line to get past my destination. Not ideal, but it was the right decision.

Behind the Storm

By the way, I can confirm that the flying weather BEHIND a line of thunderstorms is very nice; much preferable to the downwind side. Far less stressful, flying behind the line also avoids the chance of running into hail spewing downwind out of the anvil head. If you must fly the downwind side, look for indications of the speed of the storm’s movement on your system. You should factor these into your diversion plan; simple math (speed x time = distance) will tell you how much clearance to give that advancing storm.

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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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).


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.

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