Safety Pilot:

In-cockpit weather

July 1, 2011

One of the biggest safety advancements for light general aviation aircraft has been the availability of weather information datalinked into our cockpits via satellite. No matter whether it’s a portable or panel-mounted device, the ability to get TAFs, METARS, and—most important—Nexrad precipitation mosaics has been a huge step forward. It allows us to take trips that otherwise would have been impossible or much more uncertain.

Step back with me a few decades to the bad old days before Nexrad was in the cockpit. I was flying a Cessna T210 back to Wichita from the East Coast. An active front extended almost the length of the Mississippi Valley. We’d refueled in Mount Vernon, Illinois; filed IFR; and were looking at an increasingly angry western sky. ATC wasn’t able to provide too much help so it was off frequency to Flight Watch for a strategic view.

Flight Watch can give you a good picture but there are two downsides: the information is perishable, as you’ll see in a moment, and when there’s real weather to deal with the party line on 122.0 MHz can get mighty busy. We’ve all faced the weather version of Armageddon 50 miles ahead and needing information right now while some jet type at Flight Level 350 is checking out a sun-filled forecast for West Palm two hours downstream. ATC usually gives us three minutes and then you’d better be back on frequency.

The situation was “active” but a southern deviation of about 70 miles was recommended. I returned to Center frequency but, out of morbid curiosity, decided to monitor Flight Watch to see who else was going to get bad news. About 10 minutes later, the Flight Watch advisor called me in the blind to report that the southern hole had closed up solid and a northerly route over St. Louis was much the better deal. Glad I’d been listening, so we reversed course and penetrated the front with only a slight bouncing and light rain. It worked but wasn’t particularly elegant by today’s standards.

Today, with Nexrad in the cockpit, the workload is lower and a picture is worth at least 1,000 words. Flight Watch is essential if you’re not equipped for an in-cockpit view but is clearly from the last century. After the privilege of flying datalinked-equipped aircraft since the beginning it would be hard for me to give it up. There is a cost to hardware, with a subscription required in some cases, so if you’re not doing a lot of cross-country flying there’s little reason to invest. However, if the aircraft is used regularly for transportation the rewards are significant. Like all tools, datalink has to be used intelligently and despite all the marketing promises, there is interpretation, limitation, and nuance.

In the early 1990s, an FAA associate administrator was dead set against the idea of providing Nexrad pictures because pilots would use the information tactically to maneuver around storms. I mentioned to the administrator that airborne weather radar had occasionally been misinterpreted but we certainly wouldn’t give that up. Irrelevant! Catastrophic carnage was predicted, but fortunately it hasn’t worked out that way. A few datalink-equipped aircraft have had weather mishaps but thousands more trips were completed in greater safety and confidence.

Why not use datalink close in to convective cells? Start with latency. That’s a technical term that simply means the time delay from when the radar scans the storm and the picture appears in your cockpit. It can be a little as a minute or two to or as long as 10 to 12 minutes, although this is unusual. In slow-moving or fairly static weather this is not an issue but in powerful systems, where there can be explosive growth and cells may be moving at 40 to 50 knots, what you see on a 10-minute- old display is not at all what you get. A strong cell moving at 40 knots will have moved about four miles in six minutes—not that far. But if it’s faster or the delay is longer. Watch out!

Two key points: Maintain at least 20 miles from any strong cells, preferably upwind. That means the hole between them must be at least 40 miles wide and, secondly, understand the general nature of the weather system in which you’re flying. A warm front will have different characteristics than a strong cold front, different from a tropical system, or scattered air mass storms. They all have their personalities and while they’re all bad, some are much nastier than others.

I’ve been reminded on several occasions that despite having all of the gear, it doesn’t make unflyable weather flyable, and sometimes there’s temptation to cut corners. If you need a little more motivation, remember thunderstorms release energy levels that exceed nuclear weapons. The tornadic spring of 2011 is a reminder of the power.

I could go on for pages on this topic but strongly recommend you take the Air Safety Institute’s new online course available later this summer.

AOPA Foundation President Bruce Landsberg is an active pilot with more than 6,000 hours.