From time to time, the National Transportation Safety Board issues safety alerts in response to hazards its members feel merit more attention. It’s fair to say that the aviation community has been more receptive to some of their recommendations than others. A June 19 alert warning of unreported latencies in Nexrad images deserves the attention of every pilot who’d like to use this technology to help avoid violent weather.
This alert was motivated in part by the in-flight break-up of a Piper Cherokee Six (PA32-260) near Bryan, Texas, some six months earlier. In the late afternoon of Dec. 19, 2011, the Cherokee made a fuel stop at the Jackson-Evers International Airport in Jackson, Miss., en route from Hampton, Ga., to the TSTC Waco Airport outside Waco, Texas. It took off on the final leg about 10 minutes before 6 p.m. Four passengers were on board along with the 400-hour instrument-rated private pilot.
The first two and a half hours were uneventful. However, weather was starting to build in east and central Texas, the result of an outflow boundary extending from the Houston area into the northeast corner of the state. The air throughout the region was warm and moist, with temperature/dew point spreads of three degrees or less, and the instability triggered by the outflow created “a moist conditionally unstable environment, which would have been supportive of cloud formation, rain showers, and thunderstorms.” Two convective sigmets were in effect.
At 9:19 p.m., the pilot checked in with Houston Tracon, reporting that he was level at 8,000 feet. The controller advised that he could hold his present heading for about another 40 miles, after which he should expect to turn right. The pilot acknowledged and added, “I was looking at my Nexrad, is up ahead … taking about a two-five-zero heading for a little while, that be all right?” The controller replied, “that two-fifty will put you in some moderate to heavy precip” and suggested that about 20 miles north of College Station, “you can start bending it to the right, there is some pretty good gaps in the weather once you get around that area.” Thirteen minutes later the controller advised the pilot that radar showed the Cherokee skirting an area of light to moderate precipitation. The pilot replied that he saw the same thing and was maneuvering south to find a hole through the showers.
Just one minute later, the controller warned of a “heavy to extreme cell at your one to two o’clock and about eight miles.” The pilot acknowledged, adding, “If it’s all right with you we will hold this heading right here until we get south of that.” The controller approved the request; four minutes later, he asked the pilot if he was “getting any lightning” off his right side. The pilot was not, but altered course another 15 degrees to the left, saying that his display showed “a pretty good storm” off to his right. Once clear of that, he’d make his right turn toward Waco.
It was about five more minutes before the controller contacted the pilot again, this time to warn of “pretty heavy weather” moving to the northeast. He then said “It looks like you just made a left three-sixty on me; what’s going on? … I show you headed right into heavy weather, now uh I would suggest you turn back right to about a two-two-zero heading.” The pilot’s final transmission was, “Okay, yeah, we’re turning right. We’re in some bad weather here. I’m going to try to get out of it.” Radar contact was lost two minutes later; the left wing was found 50 yards from the rest of the wreckage. The main spar had been broken upward at the root. Metallurgic analysis attributed the failure to overload, not fatigue.
Overlaying radar track data on the precipitation echoes showed that the Cherokee’s southwest course had converged with an area of “strong to intense” echoes moving northeastward. The last radar contact with the aircraft came in an area of “very strong to intense echoes.” However, the most recent Nexrad mosaic available to the pilot was based on images acquired more than eight minutes earlier. The investigators also noted that the display in the cockpit would not have made this obvious. The time stamp shown on the radar mosaic was the time at which the mosaic had been assembled; the individual site images from which it was built could, on occasion, be as much as 15 minutes older.
Eight minutes may not sound like much, but it’s plenty of time for a normally aspirated piston single to fly another 15 miles—enough to obliterate the recommended 20-mile buffer from thunderstorm cells that were themselves moving at 45 knots. The time lag is one reason in-cockpit radar downloads, helpful as they are in avoiding areas of bad weather altogether, aren’t up to the challenge of finding a way between cells in the dark.