WX Watch: Dodging with datalink

One such memorable flight was from Wichita to Leesburg, Virginia, in a factory-fresh Cessna 152. I was ferrying the airplane to a flying club, and there would be two en-route fuel stops—one at the Rolla National Airport in Missouri, and the other at the Cincinnati Municipal/Lunken Field. New airplanes back then came with one nav/com and, well, nothing much else except a six-pack of standard-issue avionics. This was in June 1978, when brand-new lightplanes weren’t even delivered with transponders. For radar identification I was told to make some three-sixties after leaving Wichita airspace so ATC could tag me with a “skin paint,” which is slang for when the radar detects an airplane by seeing a return from its aluminum—not a transponder.

After a morning departure, the IFR trip to Rolla went well. The preflight briefing mentioned turbulence and a chance of thunderstorms later in the day, but after all, the day’s legs would be in a huge warm sector between a strengthening cold front running northeast-southwest from Wisconsin to Texas, and a newly formed warm front running from Lake Michigan to West Virginia. At 7 a.m., temperatures and dew points in this sector were in the high 60s. It would be a sticky late-spring day, but Rolla had light winds and a high cirrus layer.

On the next leg, all hell broke loose. It started out well, but a layer of broken, low cumulus clouds began to grow to my 7,500-foot cruising altitude by the time I reached the Evansville, Indiana, area. ATC said the layer would continue to Lunken. I pressed on.

In a few minutes the cloud tops rose even more, and the gaps between them closed. I climbed to get on top. But alas, the 152 wasn’t up to the task. The clouds quickly towered above me, and if I was to remain visual, I’d have to top them. Ultimately, I climbed to 12,000 feet, but even that wasn’t enough. The clouds were heading for the flight levels, and my only hope of avoiding buildups was to get above them. I knew that wasn’t going to happen, so the only option was to somehow thread my way through the cloud canyons during a descent to Lunken. So I started down.

Sometimes my bank angles reached 45 degrees or maybe more, as the towering, darkening cumulus clouds went by. It was turbulent, with downdrafts and updrafts. Then came a broadcast over my VOR’s voice channel. It confirmed what I already suspected—that the whole Ohio Valley was under a convective sigmet featuring signs of tornadoes. Somehow, I completed the descent and prepared for an approach into Lunken. That’s when I heard the next news. Tornadoes had formed west of Cincinnati.

The landing was uneventful, but there was no time to lose. It was very dark west of the field, the wind was picking up, and I needed this brand-new airplane in a hangar before hailstones could total it. Some helpful soul came running out to direct me to one. That night I slept on a bench in Lunken’s art-deco terminal building while thunderstorms passed by in waves.

Five minutes is a long time when the weather goes convective. Spend some time watching aggressively boiling, building cumulus cloud tops and you’ll understand how they can grow at rates of 5,000 fpm.That was then. Preflight and inflight weather avoidance services existed, but these were primitive by today’s standards. Sure, there was flight service, but on 122.20, 122.60, and 122.65 MHz. There was no dedicated weather advisory, like Flight Watch. You could call flight service on 122.1, then listen over your VHF nav radio’s audio channel (along with confusing crosstalk from its Morse code identifier). There was no national network of Doppler weather radars, and no Center Weather Service Units based at ATC facilities. ATC could give advice, but none based on high-quality, high-definition, color-contoured radar returns.

These days, thunderstorm avoidance is much better informed. True, the FAA did away with Flight Watch in 2015, but 122.20 MHz and other FSS frequencies have taken over that role. In 1988, the nation’s network of 160 Doppler weather radars—and 45 terminal Doppler weather sites—was initiated, and Doppler displays and meteorologists were assigned to air route traffic control centers to advise controllers and pilots of fast-breaking convective activity. We now have aviation weather apps for our smartphones and iPads, and television’s AccuWeather and the Weather Channel are there to forecast and analyze any time of day.

But the real revolution came with datalinked weather products. There are a lot of them, but most pilots lock on to cockpit displays showing Doppler weather radar precipitation depictions, datalinked from that network of ground-based radars. These give us near-real time situational awareness of any storms or precipitation. It’s almost like having airborne weather radar, right there on your panel. Note the emphasis on “near” and “almost.”

There is always a delay between when these datalinked images are received by the Doppler radars and when they appear in your cockpit. That’s because it takes time for the radar’s antennas to finish rotating through several elevation angles, time for the datalink providers to collect and process the returns into mosaics (so there are no gaps in coverage), and time to transmit the finished product to your cockpit. The time stamp on your display may say that the datalinked images are five minutes old, but this is the broadcast interval. The actual update interval—the time to prepare the imagery for broadcast—may be 15 minutes or more. This means you’ll be seeing precipitation returns from the past.

In June 2012, the NTSB issued Safety Alert SA-017 (amended in December 2015), which asserted that cockpit imagery could be 15 to 20 minutes older than the posted time. The safety alert included two accident summaries illustrating the dangers of image latency. In one accident, a helicopter crew saw an image posted as being one minute old. The Nexrad returns on the cockpit screen showed that severe weather was seven miles from the destination airport. In truth, at the moment it was posted on the display the image was five minutes old, and the weather was in fact crossing over the destination airport.

There are two providers of datalinked Nexrad imagery: SiriusXM, and the government’s Flight Information Service-Broadcast (FIS-B) services. FIS-B’s Nexrad presentations are compiled only after the radar antennas complete a full scan of all their elevation angles; this can take up to 10 minutes. SiriusXM Aviation weather runs a check of radar sweeps every five minutes, makes changes to accommodate the new imagery, and puts out “rolling” updates every five minutes. This lets SiriusXM say that its imagery is never more than five minutes old.

Five minutes is a long time when the weather goes convective. Spend some time watching aggressively boiling, building cumulus cloud tops and you’ll understand how they can grow at rates of 5,000 fpm. Thunderstorm internal dynamics can also change precipitation rates, cloud coverage patterns, updraft and downdraft strengths, and cause icing conditions within cloud masses. Not to mention lightning. A storm cell, or line of precipitation echoes, moving at 20 knots can cover five nautical miles in 15 minutes. If you’re flying at 120 knots you’ll cover that distance in 2.5 minutes.

And yet, datalink Nexrad can tempt us with thoughts of finding a “soft spot” (a passage having low, light-green precipitation rates) where we can make our way safely between precipitation echoes. It’s a bad bet. Sure, datalinked Nexrad is a great tool for safely avoiding storm cells. But it doesn’t give you invincibility, or let you fly at will around what you think are buildups.

The standard cautions apply today, as they did in pre-datalink times. Collect all the weather information you can during your preflight weather briefing. Keep abreast of late-breaking convective sigmets while en route. Keep your distance from cumulus buildups. Don’t weave around active storm cells; that’s a recipe for inadvertent flight into instrument conditions. Being on an IFR flight plan can help. ATC’s weather radars update with every sweep of the antenna, controllers know your location and track, and can issue vectors to avoid storm cells.

Even so—VFR or IFR—your ability to circumnavigate an active front, line of storms, or large cluster of cells depends on your speed, the growth of the cells, your fuel state, and the distance you need to fly to put buildups behind you. Finally, be prepared to accept that without enough speed or fuel for a massive reroute, your only options may be to reverse course or land as soon as safely possible. Last but not least: Do you really need to get there right now?

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