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June 25, 2012
By Sarah Brown
How old is the information on your in-cockpit weather display? The picture you see may represent conditions up to 15 or 20 minutes older than the age indication in the cockpit, the NTSB warned pilots recently, and failing to account for the delay could have deadly consequences.
On Dec. 19, 2011, a Piper Cherokee Six broke up in flight near Bryan, Texas, while diverting to avoid weather. According to the NTSB, next-generation weather radar (Nexrad) mosaic images available to the pilot should have shown that he remained clear of precipitation. But the aircraft flew into a section of a developing rain shower, breaking up and leaving a trail of wreckage over an area about half a mile long by 200 feet wide. The instrument rated private pilot and four passengers were killed.
The last three Nexrad updates the pilot received should have displayed an age of one minute, the NTSB said. But the conditions represented on the displays were six, seven, and almost eight minutes old, respectively. The NTSB cited the accident in a recent safety alert, emphasizing that “Even small time differences between the age indicator and actual conditions can be important for safety of flight, especially when considering fast-moving weather hazards, quickly developing weather scenarios, and/or fast-moving aircraft.”
Nexrad mosaic images are available in the cockpit using flight information service-broadcast (FIS-B) and private satellite weather service providers. The age indicator on a Nexrad display shows the age of the mosaic image created by the service provider, not the age of the weather conditions, the NTSB explained. The discrepancy arises because of latencies in the processes used to detect and deliver data from radar ground sites and the time intervals used for the mosaic-creation process, the board added. Weather conditions depicted on your screen will always be older than indicated on the display, it said.
The alert serves as a reminder that, while useful, in-cockpit weather radar should not be a pilot’s only tool for weather avoidance, and should be used with an understanding of its limitations. “Datalink can be invaluable, but it doesn’t make you invincible,” warns the Air Safety Institute’s IFR Insights: Cockpit weather online course. The Air Safety Institute recommends avoiding thunderstorms by at least 20 nautical miles. Storms may move quickly, and holes between developing storms can close.
Discrepancies of 15 or 20 minutes are atypical, the NTSB said, but pilots should consider the potential delay when using in-cockpit Nexrad capabilities. “Understand that the common perception of a ‘5-minute latency’ with radar data is not always correct,” the alert advises. Pilots also should get a preflight weather briefing, use all appropriate sources of weather information to make in-flight decisions, and let other pilots know about the limitations of in-cockpit Nexrad, it says.
IFR Insights: Cockpit weather online course
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