Flying with XM Weather-equipped AV8OR

 - January 5, 2009, 4:47 AM

Bendix/King’s new AV8OR handheld GPS receiver can display XM Weather’s Aviator LT and Aviator products, making it a handy cockpit tool. During a recent trip, I used the Aviator product provided by WxWorx of Huntsville, Ala. Although WxWorx also offers XM Weather’s Aviator Pro, the AV8OR can’t display the additional features available in Pro. The regular Aviator service ($49.99 per month) includes Nexrad, TFRs, metars, TAFs, winds aloft, echo tops, freezing level, surface analysis and more.

Although the weather was nearly perfect during the trip, I had an opportunity to learn more about how Nexrad works and some of its limitations. The conventional wisdom is that Nexrad in the cockpit should be used strategically and not for tactical in-flight picking-the- way-through-thunderstorm decisions. A building storm south of Flagstaff, Ariz., underscored the value of that advice.

On the leg from Gallup, N.M., to Sedona, the air traffic controller warned everyone on the frequency about a building storm between Sedona and Flagstaff. I checked the Nexrad display on the AV8OR, but in the hour between the controller’s warning and my arrival at Sedona, nothing showed. It wasn’t until after I landed that the storm finally lit up the AV8OR’s screen.

Nexrad does have limitations. One is that the radar beam is oriented five degrees above the horizon to avoid ground clutter. Thus, the farther from the antenna, the higher the area scanned by the beam. At 124 nm from the radar, the beam is already at 14,700 feet and higher. The radar that covers the Sedona/Flagstaff area is about 20 miles NNE of Payson, Ariz., so the coverage in that area should be pretty good.

But in general, Nexrad coverage in the mountainous western U.S. is not as good as in the flatter central and eastern part of the country, explained Bob Dreisewerd, senior vice president of research and development and forecast operations for WxWorx. “There are quite a few gaps and holes.” Mountains can also block the radar beam. “That’s pretty common in the western U.S.,” he said.

Dreisewerd looked at records of that particular storm, however, and said that he doesn’t think blocking is what caused the delay in displaying the storm. “There was a shower that did develop into a thunderstorm detected at that location,” he said. What could have happened is that the system I was using didn’t receive some updates, but also, Nexrad doesn’t display rain with a reflectivity below 10 dBZ, which is fairly light precipitation. “That’s pretty common,” he said. “It might not show up immediately because the reflectivity is not heavy enough. It was a shower for quite a while then developed into a storm.”

Another reason that Nexrad shouldn’t be used strategically is because there is a delay between the information being received and delivered to airborne units, usually from four to seven minutes. The latency wasn’t the problem in this case, Dreisewerd said.

The bottom line for airborne Nexrad users, he said, is that when flying in the western U.S., “be aware that radar coverage isn’t as good as in the East.”