From Max Trescott

NTSB Alert—In-Cockpit NEXRAD Weather Age Misleading and Killing Pilots

Posted: 20 Jun 2012 10:22 PM PDT

Today the NTSB issued an alert warning “pilots using in-cockpit FIS-B and Satellite Weather display systems that the NEXRAD "age indicator" can be misleading. The actual NEXRAD data can be as much as 20 minutes older than the age indication on the display in the cockpit. If misinterpreted, this difference in time can present potentially serious safety hazards to aircraft operating in the vicinity of fast-moving and quickly developing weather systems.” To put it more bluntly, pilots are dying because they’re unaware that NEXRAD data is much older than the 1 or 2 minute “age” they see on the screen.

This is old news, but somehow pilots seem to believe anything they read on a GPS or computer screen. I discussed the issue in detail in my Max Trescott’s Garmin G1000 Glass Cockpit Handbook, now in its 4th edition. Chapter 8 is devoted entirely to Onboard Data Link Weather, such as the services provided by SiriusXM, previously known as XM Weather, that pilot view on their portable GPSs and glass cockit displays. 

NEXRAD Radar data, one of the most used yet most misunderstood in-cockpit weather services, is described in detail.  My section on the age of NEXRAD radar data says in part: “In the best case, some of the data you view in a NEXRAD image is at least eight minutes old. In precipitation mode, it takes five minutes to complete a scan of the atmosphere at the radar site. The data is sent to a central NWS computer where it’s processed for a couple of minutes and then sent to SiriusXM®, which distributes the data your G1000 or Perspective system receives. Updates are broadcast to your system every five minutes.

“While eight minutes may not seem like a long time, consider that cumulus clouds can grow at up to 3,000 feet per minute. Thus, in eight minutes, cloud heights could have increased by 24,000 feet and evolved into a serious thunderstorm sending hail and turbulence a long distance from the clouds.” 

I vividly recall giving a presentation on the Garmin G1000 at a local flight school and explaining that the age of NEXRAD data is significantly longer than the age displayed on the screen. An otherwise bright young flight instructor countered “yes, but the screen says the data is only one or two minutes old.” I repeated my explanation of why the data is older than what’s displayed on the screen and he repeated, “yes, but the screen says….” 

I fear we live in an age where many pilots—even some technically literate ones—believe everything they read on a computer screen. Sadly, failing to understand the limitations of data on a GPS or moving map can kill pilots. Just two weeks ago, I made a note to write a blog story on this topic when I ran across the following two accidents. In both cases, pilots were killed trying to pick their way through a line of thunderstorms, most likely with NEXRAD radar data they didn’t know was at least 8 minutes old.

Bruce Landsberg of the AOPA Air Safety Institute described one of the accidents. The accident aircraft, a 1992 Turbo Bonanza, was equipped “with dual Garmin 430W units and a satellite weather service subscription. It’s reasonable to assume that the pilot probably was viewing datalinked Nexrad radar, although that is not confirmed.”

Landsberg continues “The controller provided a pilot report from a Cirrus that had passed through the area about 20 minutes earlier. The Cirrus pilot reported light turbulence and heavy rain for about a minute. It had flown through a gap in the line “that was yellow to green on our onboard radar, versus red on either side of it. It was fairly good.” 

Even the Cirrus pilot apparently didn’t understand his equipment, as I’ve never seen a Cirrus with onboard radar. Undoubted, he too was using data link weather. Sadly, the Bonanza broke up in flight and the 790-hour pilot was killed.

The second accident, which occurred just last month on May 31, 2012, also involved a Beech Bonanza. According to the NTSB’s preliminary report, “center controller advised the pilot of extreme precipitation at the airplane's 12 o'clock position… The pilot acknowledged the information and added that he was looking at it, and evaluating if there was any way to get through it…. At 1633, the controller asked the pilot if he had weather radar onboard, and the pilot replied that he had ‘Nexrad Composite’." Aircraft wreckage was spread over 1.25 miles, possibly as the result of an in-flight breakup.

In my G1000 book, I talk about the differences between NEXRAD and airborne radar. “You should use NEXRAD radar to develop strategies for avoiding wide areas of weather, not for determining where to penetrate a storm. It’s highly complementary to airborne radar… In contrast, airborne radar data is real-time, so it can be used tactically to help determine where to penetrate an area of storms, though it does have limitations.”

To sum it up, NEXRAD data tells you were the storms WERE, not where they ARE. Pilots need to STOP USING NEXRAD TO PICK A PATH THROUGH THUNDERSTORMS—under penalty of death.