If you are a pilot who has a GPS that receives NEXRAD weather radar data, the NTSB wants you to know that the time-stamp on the image is the time that the mosaic image was created. It is not the time when the weather was painted by radar.
The time difference between where the weather was when the radar painted it and the time stamp of the image you see can be as much as twenty minutes. So while you may think that you are in the clear and a safe distance from that line of thunderstorms, those storms might we right where you're going to be.
If you fly and rely on NEXRAD, click on the above link.
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4 comments:
When I quit flying the pipeline in 2007 (flunked the physical but I was ready anyway)I did not have anything as fancy as that. I knew about it and could see the weather radar at home but in the aeroplane I had no such thing. I had that new-fangled GPS, a cell phone(barely)and a radio. New Orleans had a working radar and that nice lady over there steered me more than once 25 miles out of my way to get around some nasty weather. She would describe to me the colors on her radar screen and let me decide.
Being a pipeline patrol I really liked being independent of government control so I really lusted after one of those phones with a screen. At the wage level I worked that was not going to happen. Just as well.
There's a basic disconnect between today's connected world and the flying world's weather data. We've become conditioned to instant data...look online or at the local TV weather for a "current" radar picture and you start to "assume" the "radar display" in the cockpit is the same.
Problem 1: NEXRAD takes at least 6 minutes to scan weather because it takes multiple passes at different levels to create the picture. Then that has to be integrated, uploaded, downloaded, viewed and understood.
Problem 2: Pilots never seem to get it explained to them during training on equipment that the "weather radar" from NEXRAD is actually a picture of weather at a given time (a time that can be up to 20 minutes old, it turns out).
Problem 3: Current connectivity induces an unconscious "understanding" that the displayed data is real time, simply because everything else is "live". Ever notice how little the phrase "live" is used anymore on TV simply because it's understood.
Now, from the controller's perspective, we have similar issues. The older controllers remember using "primary radar" to show weather, and know that it was "accurate" in that it updated every 6-12 seconds. A couple of years ago, the FAA finally briefed everyone that NEXRAD lag could result in displayed weather lagging real-time weather by as much as 15 miles with rapidly moving cells. Those older controllers still punch up the primary display now and then to see how much NEXRAD is lagging, but the FAA is working to decommission primary radar.
It is also inherent that controllers picture the pilot as having a view on the world that lets them judge weather in real-time. A lot of the newer controllers have never flown in a cockpit, and don't realize that various weather phenomena can hide cells from a pilots view. They simply assume that a pilot can see what he needs to avoid, and that everyone has weather "radar"...not understanding NEXRAD weather is delayed, despite the briefings. Even older controllers fall victim to this "understanding" of the world unless the pilots mention they can't see the cells because of clouds or rain.
Noted! It's thunderstorm season after all. Always a good idea to check that "last update" time - I've seen XM wx just drop out, stop updating entirely - so the picture can be way older than 5 minutes. Even when it's working, it's a good idea to consider it a long-range tool ... a way to plan deviations, not a way to pick your way through CBs you can't see. Shudder.
Going to brave the 101 degree midwest tomorrow with nothing more than an airplane, radios, GPS and datalink nexrad. How did they do it in the old days!? Maybe it's all in what you're used to.
I tried to tell 'em.
Say Again? #71: Weather Radar
Don Brown
http://gettheflick.blogspot.com/
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