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, warns the NTSB. “The general issue of latency with in-cockpit NEXRAD is discussed in pilots' guides, in industry literature and on service providers' websites. However, the NTSB has not found that such guidance contains details about the potential time difference between the age indicator and actual conditions.”

The NTSB has issued a Safety Alert, “In-Cockpit NEXRAD Mosaic Imagery,” discussing the latency issue and asking pilots to spread the word among their colleagues. The Safety Board says it knows of two accidents in which display latency may have been a factor — the Texas crash discussed here and the March 25, 2010, loss of a Eurocopter AS350 B3 that crashed near Brownsville, Tenn. All three people on board were killed. Roughly halfway through that flight, the pilot's cockpit display received one NEXRAD image that indicated it was about 1 min. old; however, the weather conditions were actually about 5 min. old. The image indicated that the severe weather was about 7 mi. away from the home base where the pilot was attempting to land, but the severe weather was actually just crossing over the home base at about the time the display received the NEXRAD image.

The NTSB urges pilots to remember that the in-cockpit NEXRAD display depicts where the weather was, not where it is. The age indicator does not show the age of the actual weather conditions but rather the age of the mosaic image. The actual weather conditions could be up to 15 to 20 min. older than the age indicated on the display. Pilots should consider this potential delay, says the NTSB, when using in-cockpit NEXRAD capabilities, as the movement and/or intensification of weather could adversely affect safety of flight.

Understand that the common perception of a “5-min. latency” with radar data is not always correct.

The Safety Board also reminds us: “Having in-cockpit weather capabilities does not circumvent the need for a complete weather briefing before takeoff. Use all appropriate sources of weather information to make inflight decisions. Let your fellow pilots know about the limitations of in-cockpit NEXRAD.”