As the investigation proceeded, it was apparent that the pilot had attempted to pick his way around dangerous convective weather in night IMC and lost control of the aircraft. The airplane was equipped with a Garmin GPSMAP 696 and the pilot had a subscription to XM Sirius satellite weather Next Generation Radar (NEXRAD) data. The XM data was a weather radar service provided by XM Sirius and displayed on the Garmin 696. The Garmin GPSMAP 695/696 Owner's Manual specifically states that NEXRAD weather data should be used for “long-range planning purposes only” and not to “penetrate hazardous weather” as the “NEXRAD data is not real-time.”
Investigators were able to use radar records for the area and accident time to determine the latency for the XM display — the temporal difference between the original weather returns and the XM-display product. The pilot had likely received several NEXRAD updates in the minutes leading up to the accident. The Safety Board said:
According to the NEXRAD data the pilot likely should have received, he was flying clear of the precipitation along the edge of the rain. Near the end of the flight, the pilot flew into a section of the developing rain shower. His display would have shown that he still remained clear of the precipitation. The last three NEXRAD updates that the pilot received should have each said that they were 1 min. old at the time they were received; however, the actual weather conditions at the time the images were received in the cockpit were about 6, 7, and almost 8 min. old, respectively.
Stated plainly, the pilot may have been using 8-min. old data to guide his flight through a rapidly changing convective system.
Weather radar mosaic imagery from NEXRAD is available to pilots in the cockpit via Flight Information Service-Broadcast (FIS-B) and private satellite weather vendors. A mosaic presents radar data from multiple radar ground sites on a single image. Data from individual ground sites may not be updated with each new mosaic image. The age-indicator displayed to the pilot in the cockpit is not the age of the actual weather conditions as detected by the NEXRAD system. Instead, the age indicator refers to the age of the mosaic that is created by the service provider. The actual age of the oldest weather conditions is always older than the age indication on the display.
Due to latencies inherent in processes used to detect and deliver the NEXRAD data from the ground site, as well as the frequency of the mosaic-creation process used by the service provider, the NEXRAD data can age significantly by the time the mosaic image is created. “Although not believed to be typical,” says the Safety Board, “in extreme latency and mosaic-creation scenarios allowed by the service provider, the actual age of the oldest NEXRAD data on the display can exceed the age in the cockpit by up to 15 min. for satellite weather and 20 min. for FIS-B.”
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.