In another fatal accident, this one in March 2010, the pilot of a Eurocopter AS350 was using his onboard satellite weather receiver to divert around a powerful storm cell near Brownsville, Tennessee. As he attempted to land, the Nexrad display would have indicated he was clear of precipitation based on a one-minute-old radar image. In fact, the image was at least five minutes older than that, and, unluckily, a severe storm was just passing over his home base as he arrived.
In these accidents, the pilots apparently failed to adhere to the 20-mile rule for operating near thunderstorms. Even if they had accurate data in the cockpit, they were still too close for comfort — and appeared to be using their datalink weather information for close-in tactical maneuvering. With the wrong information, they unwittingly set themselves up for disaster.
The fact is, the downlinked Nexrad weather data you see on the display in your cockpit is always older than what the time stamp indicates. The reasons make perfect sense: First, the radar station has to sweep past the storm that will eventually show up on your cockpit display. Then that data is transmitted to National Weather Service computers, which prepare the data for dissemination. Next, the data is transmitted to your service provider, such as the FAA in the case of FIS-B transmissions or to XM Satellite Weather or WSI InFlight in the cases of paid satellite services. Finally, the data is coded, rebroadcast and received in your airplane. All of this takes time — and maybe a lot more time than you would have guessed.
Makers of in-cockpit weather-data receivers have been warning about this phenomenon for years. Most pilots probably already understand the data they see on their display has some inherent latency, even if they don’t realize just how old those blobs of green, yellow and red can actually be. The bottom line is, pilots need to understand that weather avoidance is still their responsibility. Controllers can do only so much to help out since their primary responsibility is to ensure safe separation of IFR traffic. Just because you have that XM Nexrad display or the latest digital weather radar doesn’t mean you should take risks you wouldn’t in absence of such technology.
It’s important to understand that our weather equipment should be used for strategic planning and not tactical maneuvering. Once we accept this limitation of in-cockpit weather data, we can focus on the benefits — and there are a great many, obviously.