Insider Series: How Do Air Traffic Controllers Help Planes Avoid Bad Weather?

Sep 9, 2015

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TPG Contributor “Vic Vector” is an air traffic controller at a major ATC facility in the United States. In this installment of our Insider Series, he guides us through what it takes to keep planes — and their passengers — safe in all kinds of weather.

On August 7, 2015, Delta flight 1889, an Airbus A320 flying from Boston (BOS) to Salt Lake City (SLC) encountered severe turbulence and hail that caused substantial damage to the aircraft’s windshields and nose cone. However, the pilots were able to divert to Denver (DEN) and make a safe landing, and there were no injuries. The incident made news on many major outlets, including CNN.

No worries — that’ll buff right out. Image courtesy of Beau Sorensen.

While the above image of the aftermath is certainly striking, I think it’s important to acknowledge the professionalism and skill of both the flight crew and the controllers who oversaw their safe emergency landing. Pilots and controllers regularly train on the execution of emergency procedures, and the outcome of this situation is certainly a credit to that preparation.

Still, you might wonder — how does something like this event happen?

First, it’s helpful to understand the basics of weather radar used by both controllers and pilots. Weather radar transmits a directional pulse of microwave energy, waits for that signal to bounce off something and return to the receiver, then analyzes the return signal and interprets it into a picture of sorts, detailing the size and shape of whatever the signal bounced off of; in the case of Delta flight 1889, the “whatever” was hail. Weather radar excels in detecting large, wet types of precipitation such as raindrops or sleet, but is less successful at seeing things like snow or mist. It can’t see clouds or turbulence at all — only precipitation — and requires line of sight.

Image courtesy of Shutterstock.

Pilots and controllers use different types of weather radar. Each has its own respective strengths and weaknesses, though used in conjunction with each other, they can offer an effective and comprehensive picture of dynamic weather systems.

Pilots utilize weather radar located in the nose cone of their aircraft. They can point it up or down to see things above or below their altitude, though it has a rather limited lateral scope, usually looking less than 15 degrees on either side of the aircraft. It can range out to upwards of 150 nautical miles, though the most accurate pictures are displayed closer in, within the range of roughly 80 nautical miles.

The weather radar display is the second screen from the left. Image courtesy of Shutterstock.

While a pilot’s weather radar paints a fairly narrow picture, the fact that it’s in real time means that it displays the most up-to-date information, making it useful for quick, tactical moves. However, since it’s derived from a single source, it’s subject to a phenomenon known as radar attenuation; this is when a large cell of precipitation is so powerful and reflective that it bounces back 100% of the radar signal, effectively masking all of the area behind in what’s called a “radar shadow.”

23 3 Attenuation
The area behind this large thunderstorm cell might be clear, or there might be even more severe weather — unfortunately, weather attenuation doesn’t allow us to see what’s behind the first cell. Image courtesy of Langley Flying School.

Air traffic controllers use weather radar that’s based on the NEXRAD system operated by the National Weather Service. Unlike the single-source radar used by airliners, this system uses multiple reporting sites to construct a mosaic image, preventing attenuation and allowing for a more comprehensive, large-scale picture of the weather that better enables ATCs to make strategic decisions.

enroute weather radar
An en-route radar display with weather. Image courtesy of MIT.

However, it takes about 10 minutes to compile the information necessary for this multiple-source snapshot, which means that the picture this NEXRAD-type radar paints is actually a short look into the past. And when you’re dealing with a rapidly developing or quickly moving weather system, 10 minutes can make an enormous difference.

Essentially, pilots’ radar allows them to see a real-time image of what’s directly in front of them, while mine shows me a slightly delayed but larger-scale picture of the entire weather system; these two types of radar enable us to keep most planes safe in bad weather. For instance, each thunderstorm season, an aircraft will invariably request permission to deviate around an approaching storm because on its radar, one side or another appears to be the fastest route around that particular cell; however, sitting at my radar scope, I can see that if they deviate, they’ll get stuck behind an even worse line of storms that extends for hundreds of miles. Working together, we can utilize the best of both systems.

delta weather GIF
Image courtesy of Ken Hoke.

So, back to the question of how Delta flight 1889 was pummeled by hail.

See that hole in the weather in the center of the radar image above? I can only speculate, but I’m assuming that on both the pilots’ and controller’s weather display, that gap looked large enough to safely traverse. It’s likely that many aircraft ahead of Delta flight 1889 had made it through that hole and reported to the controller that it was just fine. That is, until the hole closed up, an occurrence that isn’t uncommon — but which in this case appears to have happened exceptionally quickly.

The delay in the controller’s weather display means he or she likely wouldn’t have seen the hole close until after the aircraft flew through it. While the pilots would have had more real-time display, they were traveling in excess of seven miles a minute — so it’s possible that by the time they realized how quickly the gap was diminishing, they were simply too close to turn around.

shutterstock_77635738 (1)
Image courtesy of Shutterstock.

Though the safe outcome of this incident was ultimately favorable, the incident itself was not. However, the fact that it became a newsworthy story is a credit to the rarity with which these mishaps occur. When it comes to the unpredictable nature of weather, nothing is perfect — but for the most part, the two different weather radar systems allow pilots and ATCs to work together and avoid these types of situations. I’m confident that we in the aviation industry can learn important lessons from this event in order to decrease the likelihood of it occurring in the future — come hail or high water.

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