NTSB Warns GA Pilots of NEXRAD Delays
June 21, 2012 - The NTSB has issued a safety alert to remind pilots that NEXRAD mosaic images delivered via satellite to airplane flight decks can be many minutes old by the time pilots see the image. We have written about this topic many times, but apparently the NTSB believes at least some pilots don't understand the "latency" issue in the delivery of radar mosaics.
Virtually all satellite weather receivers display a time stamp with a radar mosaic. In the best case, that time stamp - at least a minute or two old by the time the mosaic is received - represents the time at which the mosaic was created. However, the indicated time may represent when the file was sent, not when the mosaic was made.
The radar mosaics image sent to the flight deck is made up of the radar returns from many radar ground stations. In most parts of the U.S. the NEXRAD sites overlap so more than one radar "sweeps" the same piece of terrain. The radar mosaic provider uses computer software algorithms to pick and choose among the multiple radars to make a single radar image that is transmitted.
The ground radars are not synchronized, so they don't scan in unison. And a single NEXRAD scan can take five minutes or more, depending on the mode the radar operator has selected. So when a mosaic is made some areas of radar coverage could come from a scan already five minutes old. Then there is processing time to assemble the mosaic, and at least a brief period to transmit the image to the satellite receiver.
Add up all of the normal delays and a "new" mosaic image could show radar returns 10 or more minutes old when the image is received in the flight deck. Add in other common delays and the "new" image could be showing radar returns 15, 20, or more minutes old.
During a rapidly building and moving convective weather situation, a few minutes can make a huge difference in the intensity of a storm and its location over the ground. The NTSB joins others in cautioning pilots to use satellite-delivered NEXRAD weather radar mosaic images only for large scale strategic weather avoidance, not for up-close, tactical maneuvering around storms.
The NTSB cites a helicopter crash near Brownsville, Tennessee, and the in-flight breakup of a Piper Cherokee Six near Bryan, Texas, as accidents where the pilots may have been viewing satellite NEXRAD images without understanding the age of the image. The NTSB acknowledges that pilot's guides and aviation publications such as EAA Sport Aviation have discussed the "latency" issue, but the Board does not believe there has been enough detail in those discussions.
So, no matter what the time stamp shows on your just-received NEXRAD image, you should know that some or all of the radar return information is at least five minutes old, and often several minutes more than that out-of-date. And always give thunderstorms or other heavy precipitation a wide berth.