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How heavy is the rain?

ATC rolls out new precipitation descriptors

When the new year dawned, air route traffic control center controllers began using new terms to describe precipitation for pilots: moderate, heavy, and extreme. Previously, the descriptors were moderate, heavy, and...heavy.

A rash of accidents brought about this change in terminology. According to the AOPA Air Safety Foundation's 2005 Joseph T. Nall Report, nearly 25 percent of fatal weather-related accidents in 2004 were caused by encounters with thunderstorms. Were the pilots stupid, or just careless and reckless? No. All the pilots involved in these accidents were in contact with ATC, but they still flew into severe weather conditions. In many of these accidents, confusion over terminology played a role.

The FAA is standardizing communication between pilots and controllers to help decrease these types of accidents. The changes took place at centers in January and are expected to carry over soon to all facilities that display digital Nexrad weather--approach controls and flight service stations.

So what's the implication for flight instructors? We owe our students a working knowledge of weather radar and ATC weather radar systems.

In my experience, students fail to understand the basics of weather radar, much less how ATC can use it to help them stay out of trouble. It's fine to say that radar shows precipitation, but that may not be enough; we may have to say what radar is not. It's not a detector of instrument meteorological conditions or turbulence, and it has limitations. Students can memorize the fact that radar works via line of sight, but do they understand the implications, such as limited coverage at lower altitudes and in mountainous terrain?

The weather radar display used at centers is called Weather and Radar Processor (WARP). Like all weather radars, WARP has some limitations that we need to share with our students.

WARP compiles precipitation information from multiple Nexrad sites and overlays it on a controller's scope. This takes time, so the weather information displayed can be six to 11 minutes old. In a benign weather situation, this delay may not mean much, but can you imagine the delay's potential for mayhem in an area of rapidly building cumulonimbus activity? Also, WARP does not show light precipitation or display the tops of precipitation. (The weather display at approach facilities does show light precipitation.) For a center controller to find out about light precipitation or tops, he must extrapolate from what he does see. For instance, are pilots deviating around an area where no precipitation is being displayed? Are they asking for altitude changes because of weather or turbulence, or relaying pireps that include light precipitation?

For those of us who remember those not-so-long-ago times when ATC had little or no weather information, WARP--even with its limitations--is a great improvement. Once capabilities and limitations of weather radar equipment are understood, the next step is to remind students of their responsibilities when using ATC to work around weather. Using the new terminology, controllers, on a workload-permitting basis, will provide a description of the weather that includes intensity, location, and size. For example, ATC may say "Moderate weather areas between six o'clock and 10 o'clock, three zero miles. Weather area is five zero miles in diameter." The new terminology does not include the precipitation's direction of movement, which could be helpful when requesting a deviation around weather (or deciding to land).

Pilots who want a weather deviation must request it. For example, "Request heading two four zero for 10 miles for weather"--or even a more general "request deviation west of course for weather." A new pilot is frequently in awe of the commanding voice on the radio, and he may not understand that it's his responsibility to ask for a deviation. By extension, our responsibility as flight instructors is to ensure that students understand not only what help ATC can and cannot provide, but also the proper roles of ATC and pilots in command.

To help pilots understand how miscommunications can cause an accident, ASF has developed a free online mini-course, Thunderstorms: A Case Study. This five-minute course, which follows and explains a recent thunderstorm tragedy with actual pilot/controller audio, graphically demonstrates the importance of understanding exactly what radar services are being provided.

ASF will be expanding its online offerings to include a full-length online course that provides in-depth detail on the radar systems that ATC uses, communication procedures for pilots when working with ATC, and rules of thumb to help pilots with in-flight decision making. Check back often in the ASF Online Safety Center.

Leisha Bell is manager of safety education programs for the AOPA Air Safety Foundation. A former pilot for US Airways Express, she is a CFII and MEI with more than 2,000 hours.

By Leisha Bell

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