The NTSB warns NEXRAD users that displayed information is not real-time data.
A 33-year-old, instrument-rated private pilot and his four passengers were killed on Dec. 19, 2011, when their Piper PA-32-260 broke up in flight near Bryan, Texas, while attempting to divert around an area of thunderstorms. The cross-county IFR flight had originated at Clayton County Airport in Hampton, Ga., at 1345 that day. The pilot stopped for fuel at Jackson-Evers International Airport in Jackson, Miss., then departed at 1750 bound for Waco (Texas) Airport, the ultimate destination.
Weather briefings for the area covered by the flight's final leg included two SIGMETs valid for the area west of the accident site at the time of the accident. Convective SIGMET 5C and Convective SIGMET 7C advised of a line of storms moving from 200 deg. at 45 kt. with embedded severe thunderstorms. Thunderstorm tops were forecast above FL 450 with wind gusts to 50 kt. possible.
ATC recordings revealed that the pilot was diverting around an area of thunderstorms at 2133 CST when he reported that he was in “bad” weather and was going to try to get out of it. Radar track data showed the airplane had been traveling on a heading of about 230 deg. at an altitude of 8,000 ft. (msl). At 2137, the airplane turned south to a heading of 193 deg. Two minutes later, the airplane turned back to a southwesterly heading. Then at 2142, the airplane made a right turn toward the northeast and began descending at a rate of about 600 fpm. The descent continued until radar contact was lost at 2144. At that time, the airplane was at 6,800 ft., descending at a rate of 840 fpm, and on a heading of 315 deg. The airplane's position was 18 nm northeast of College Station, Texas.
About 2150, an area resident in her home heard a sound resembling an explosion. The witness reported that at the time she heard the noise, the rain was falling as a light drizzle. However, by the time she and her husband got outside to see the source of the explosion, the rain started pouring down. The witness's husband located the main airplane wreckage approximately 450 ft. southwest of their house.
The airplane wreckage was spread over an area approximately one-half mile long by 200 ft. wide. The main wreckage was near a clump of trees, and the left wing was located about 200 ft. north-northeast of it. The wing spar was broken upward at the root. Several bends, tears and paint transfer marks were noted along the outboard leading edge. An examination of the fractured spar surface showed that the wing broke upward and twisted aft in positive overload. The left wing aileron cables found with the main wreckage showed unraveling and fracturing consistent with overload failure where the wing separated from the fuselage.
An examination of engine, propeller and fuel system revealed no abnormalities that would have prevented normal operation or production of rated horsepower. Investigators also determined that the cockpit gyro instruments had been functioning properly and that the aircraft weight and c.g. were within limits.
All on board died of multiple blunt force injuries. Toxicology tests of the pilot's tissues were positive for marijuana, but below the levels where impairment would have been a factor.
The pilot had accumulated 392 total flying hours, with 347 hr. as pilot-in-command, and 378 hr. in the PA-32. He had recorded 14 hr. of actual instrument time, and 40 hr. simulated instrument time. The pilot had flown 46 hr. in the previous 90 days, 15 hr. in the 30 days prior to the accident, and 2 hr. in the previous 24 hr.
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.”