The National Transportation Safety Board (NTSB) is cautioning pilots to be aware that in-cockpit Next Generation Radar (Nexrad) information can be up to 15-20 min. older than indicated on display and that relying on such information for real-time reports can cause safety problems when aircraft are transiting fast-moving weather systems.

The safety board issued a safety alert that the actual age of Nexrad data “can differ significantly” from the age indicated on the cockpit displays.

A mosaic image of Nexrad information is displayed via flight information service-broadcast (FIS-B) and private satellite providers. The age indicator does not show the age of when Nexrad detected the actual weather conditions, but instead shows when the mosaic image was created by the service provider.

“Weather conditions depicted on the mosaic image will always be older than the age indicated on the display,” the NTSB safety alert warns. “Although such situations are not believed to be typical, in extreme latency and mosaic-creation scenarios, the actual age of the oldest Nexrad data in the mosaic can exceed the age indication in the cockpit by 15 to 20 minutes.”

Small time differences can factor into flight safety, especially when weather conditions change rapidly or with fast-moving aircraft, the agency adds.

The alert stems from investigations of two fatal accidents in which the Nexrad mosaic imager was available to pilots operating in fast-moving convective weather.

The first involved a Eurocopter AS350 B3, N855HW, which crashed new Brownsville, Tenn., on March 25, 2010. The image indicated the weather information was about 1 min. old and the severe weather was about 7 mi. away from the home base, where the pilot was attempting to land. But the weather information was 5 min. old, and the severe weather was crossing over the home base at the same time the display showed the weather information.

The second accident involved a Piper PA-32-260, N3590T, that broke up in flight near Bryan, Texas. The pilot had diverted to avoid weather, and NTSB believes the pilot had likely received a number of Nexrad updates leading up to the accident. The data would have indicated that the pilot was flying clear, but along the edge of precipitation the pilot flew into a section of a developing shower. The data would have been up to 7 min. older than indicated on the display.

NTSB notes that pilots need to consider the potential delay and that the common perception of a 5-min. latency is not always correct.