April 1, 2008
By Ian J. Twombly
The pilot training curriculum devotes little time to learning how to effectively use air traffic control as a weather resource. There’s a good reason for that. Sure, there are hourly weather reports, center weather advisories, and in some cases, weather radar feeds, but for the most part, pilots are on their own to judge what weather looks good and what doesn’t after leaving the ground. For all of the emphasis placed on obtaining a preflight weather briefing and making that critical go/no-go decision, once the decision has been made to go, airborne weather resources are particularly sparse for most pilots. Besides, even if air traffic controllers had every weather product available to a flight service specialist, their primary job is clearly defined as providing aircraft separation. Beyond that, almost every task a controller takes on is based upon that controller’s free time, including issuing weather advisories, rerouting for weather, and obtaining weather reports en route (an exception is when the safety of the flight is an imminent issue).
If the U.S. air traffic control system is thought of as a hierarchy, then air route traffic control centers are at the top. Even so, they have been historically behind when it comes to accurate weather data. But that situation is changing through the use of new technologies.
Centers use a type of long-range radar with limitations that prevent them from accurately detecting weather. Because their radar is designed to show airplanes at long distances, it does an inherently poor job of picking up weather returns. In recent history, most center controllers only had the ability to detect precipitation in two intensities—heavy and light. In fact, the old displays used to show a slash in the direction of the antenna for light precip and an H for heavy precip. It wasn’t uncommon for pilots looking to receive precipitation information to hear, “no weather information available,” or “I’m painting it as heavy precip.” Either way, the information wasn’t terribly useful to pilots who could most likely tell as much by looking out the windshield.
In the past couple of years, centers have gained much better weather technology. The national Nexrad weather radar system that’s commonly seen online, on most local news broadcasts, and in certain cockpit weather displays, is now uploaded to a central computer system that distributes the returns to each individual controller. It’s called Weather and Radar Processor (WARP), and because controllers work a very specific section of the airspace, WARP logically only shows weather radar for that sector.
Precipitation appears on the scope in three levels—moderate, heavy, or extreme. Light precipitation isn’t shown. Previously, a controller would call out the weather in terms of a number, say level three. Now controllers call out moderate, heavy, or extreme. Each term equates to the rate of precipitation per hour in inches: moderate is 0.175 to 0.5, heavy is 0.5 to 2.0, and extreme is 2.0 to more than 16.
Tracons, or Terminal Radar Approach Controls, work in much the same fashion, but with vastly different equipment. And despite centers being at the top of the ATC hierarchy, tracons have among the best weather radar capability of all ATC facilities with their Airport Surveillance Radar (ASR), although the capabilities vary widely from facility to facility. Through the years, ASR has become more advanced, and the newest version, ASR-11, has far greater capability than the oldest ones still in service. Most of the bigger and newer tracons use ASR-9. One such example is the Potomac Consolidated Tracon that serves the greater Baltimore, Maryland/Washington, D.C., metro area. Potomac Approach was commissioned in 2002, and the controllers working the scopes have some of the best weather tools available.
ASR-9 shows weather on the controller’s scope much the same way Nexrad radar shows up on a map. Pilots would immediately recognize the different intensities shown on the screen, although it’s only shown in six levels, compared to Nexrad’s 16. Tracons describe weather radar returns in the same manner as the centers, even though ASR will show light precip. But because ASR has a shorter range, usually around 60 miles, it does a great job of showing weather returns directly. What does that mean to the pilot? Timely information. Whereas with WARP, a computer system has to compile data from the National Weather Service (provider of Nexrad), and then disseminate it to each controller, ASR takes the weather return directly and displays it on the controller’s screen. In real terms, that means center weather returns are up to 11 minutes old, and tracon scopes show weather that’s only seconds old. While that may not be an important distinction when you’re talking light or moderate weather, it’s a major consideration when a massive thunderstorm is building at your 12 o’clock position. But both WARP and ASR have their limitations.
First, it’s important to remember how radar operates. Because a radar return is simply the indication that the outgoing signal bounced off an object and back to the radar antenna, and not what type of object it bounced off, there’s no way to determine what type of precipitation might be occurring. This is compounded by the fact that rain gives the same return as sleet or snow. So while the controller’s scope may show moderate precipitation, that could mean everything from virga to light sleet or very heavy snow. There’s simply no way to tell without seeing it firsthand.
Also, older ASR systems in use at the smaller tracons have less capability. Instead of six different levels of precip, the controller’s scope will only show that precipitation is occurring, with no delineation of level. That means a pilot is likely to hear the term “intensity unknown” in weather reports from these facilities. Even the newer ASR-9s have quirks. The radar is programmed in such a way that different antennas will show the same weather in different places. A controller may have three separate sources of weather on his scope—two ASR antennas and one Nexrad compilation, in the case of Potomac Approach. Switching between sources can result in slightly different weather pictures. And in the case of a center controller, he or she has the ability to turn WARP on or off, leaving the older-style light or heavy display. But even with limitations, radar controllers are much better equipped to handle weather requests than their tower counterparts.
As a flight comes out of the en route environment and down through the terminal environment, one could argue the need for accurate weather information becomes even greater. Many accidents happen during the landing phase of flight and weather is often a factor. Despite this, most air traffic control towers have little to no weather capability. Depending on the tower’s certification level, the weather tools can be as advanced as the Integrated Terminal Weather System (ITWS, more on that later) or as basic as a certified weather observer’s pair of eyes.
Major hub airports generally have the best weather detection capability. ITWS, a sophisticated weather reporting and forecasting tool, is in place at many hubs around the country, and its ability to forecast and detect low-level wind shear is a good resource. But smaller towers serving general aviation pilots lack many of the tools that other ATC facilities have.
In some cases, towers have radar feeds from local tracons showing traffic, but their weather information is practically nil. Almost all towers, the smaller VFR ones included, also have an automated weather reporting system that controllers use to generate the ATIS, and pilots can often tune in directly. But at some, weather capabilities literally come down to a ground reference at a fixed distance (to determine visibility) and a certified weather observer estimating cloud bases.
Aside from basic weather radar, center and tracon controllers do have other resources. They range from the simple, such as passing on a pilot report, to the advanced, such as ITWS. Relaying pireps, as controllers often do when a pilot asks for weather information, is a useful, basic way to determine weather ahead. Because a pilot is in the best position to observe weather, pireps are a good way to determine the direction of a deviation or the level of ice or turbulence at a particular altitude. As an example, if a pilot were to ask a center controller about a particular cell looming at 12 o’clock, that controller could look at his radar and combine that scope picture with a pirep. The controller could then say a preceding aircraft turned left 20 degrees and reported a smooth ride, or whatever the case may be. Maybe the preceding pilot reported a smooth ride, and maybe they made a mistake. Either way, the information can prove invaluable to the pilot.
Centers also have one key resource that tracons and towers don’t—a Center Weather Service Unit staffed by a National Weather Service specialist. Every center in the country has a specialist, and that person has advanced weather tools he or she can indirectly pass on to the controllers. NWS specialists create both center weather advisories and meteorological impact statements (MIS).
Both products are of a broad nature and advise the controllers of more substantial weather in the area. Center weather advisories are a warning of thunderstorms, severe icing or turbulence, and low IFR ceilings and visibility. The MIS is a two- to 12-hour report of weather expected to affect each individual center. Although center weather advisories are specific enough to be of general use to the pilot, the MIS is more of a traffic-planning tool for the center.
Only in rare cases can the center weather service unit help a pilot directly. If a pilot gets in trouble, a controller can call on the specialist for weather information pertaining to cloud tops, and possible holes so a pilot can remain VFR, among other things.
ITWS is also available to some radar facilities. The system monitors everything from thunderstorms to low-level wind shear. Usually only set up for the traffic management officer, it gives a broad view of the area weather for traffic planning purposes. Cell movement, tornadoes, and other severe weather can be shown on the screen. Though usually not directly given to pilots, the information from ITWS is a way to institute large-scale reroutes to keep aircraft out of the bad stuff far in advance.
With both centers and tracons having weather radar capability, one would think that ATC has accurate, timely, and useful weather resources for pilots. And to a certain extent, that’s true. But the most important thing to remember when using ATC as a weather resource is that giving a pilot weather information is a favor of sorts. In most cases it falls under the “optional” category of controller duties, and the information is given strictly on a time-available basis. That’s not to say a controller won’t give good deviation advice for cell buildups. ATC can be a great resource for such a request. But in general, flight watch is a better resource of both broad and detailed weather information, and most controllers are happy to let a pilot change frequency to chat with them.
Training is also a key issue. Because controllers are busy separating traffic, little to no time is spent on training them for weather situations. Even traffic management controllers who spend all day digesting weather data have very little formal weather training. With that in mind, pilots shouldn’t rely on controllers for detailed weather briefings, forecasts, or other information generally reserved for flight service professionals.
Finally, ATC is a system that allows a large amount of flexibility for pilots, provided they ask for it. Although controllers will sometimes offer deviation information unprompted, it takes a slow sector to do so. But, even if a controller is busy, he or she will usually be happy to accommodate those deviation requests, or any other type of request for that matter. The pilot is in the driver’s seat, and it’s up to him to take control of the situation.
E-mail the author at firstname.lastname@example.org.
Flight Training Editor Ian J. Twombly joined AOPA in 2003 and is an instrument flight instructor.
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