The weather gap

The pilot’s IFR flight plan from Columbia to Tuscaloosa, Alabama, specified an estimated time en route of two hours and 46 minutes at a cruise altitude of 6,000 feet msl, a cruise speed of 204 knots, six hours of fuel, and three souls on board. The flight path would be interrupted halfway by Georgia’s busy Atlanta airspace that required rerouting to the north or south of a 40-mile diameter circle centered on the Hartsfield/Jackson Atlanta International Airport (ATL).

A center weather advisory (CWA) had been valid for the area until 3:10 p.m. The advisory warned of an area of rain showers and thunderstorms with heavy to extreme precipitation and maximum tops to FL310. The CWA forecast covered an area from the Carolinas to Georgia, that included Charlotte, Columbia, and Atlanta. This weather was predicted to move from 260 degrees at 45 knots.

At 3:31 p.m., as N43368 climbed to 4,000 feet msl, Columbia Departure confirmed radar contact with the aircraft and subsequently cleared the flight direct Tuscaloosa on a 272-degree heading with instructions to climb and maintain 6,000 feet msl. At 3:40 p.m., N43368 was handed off to Augusta Approach. Nine minutes later, the pilot requested a 10-degree deviation to the left of course to avoid buildups. He then turned back on course four minutes later.The witness heard the engine noise first, then saw the airplane spinning toward the ground in a nose-low attitude until it disappeared.

Shortly after N43368 checked in with Atlanta Approach at 4:13 p.m., the controller issued a hazardous weather information broadcast valid until 10 p.m. local time for Georgia, Alabama, Tennessee, and the Carolinas. The hazards included IFR and mountain obscuration, turbulence from the surface to 16,000 feet, and moderate icing with freezing levels from 9,000 feet msl through 13,000 feet msl. The controller advised that further information was available on the flight service frequency.

At this point, the approach controller told the pilot that he needed to reroute N43368 around Atlanta airspace with the option to go north or south. It was now 4:15 p.m. The pilot said he’d prefer to go north and could fly at a higher altitude if that would help. Three minutes later, the controller issued an amended clearance that would route the flight via two intersections on the north side of Atlanta’s airspace, then direct Tuscaloosa.

At about 4:20 p.m., N43368 again needed to deviate 10 degrees left of course to avoid some buildups. After the controller approved the deviation, he mentioned that the flight would be past the line of weather in 15 to 20 miles.

Eight minutes later, the controller advised, “November Three-Six-Eight, in about eight miles there’s going to be a gap in this line. It’s just some light precipitation. We’re going to turn you north to get through it and then on the north side we’ll go back direct LOGEN.” The pilot agreed and at 4:30 p.m. the controller issued instructions to fly a heading of 300 degrees.

But a few seconds later, the pilot expressed concern, “Approach, Malibu Three-Six-Eight. That’s pointing us straight into a buildup, Three-Six-Eight.”

The controller explained that he would keep the flight south of the heavy precipitation and then turn it north through the line that had three miles of light precipitation. However, the pilot was worried that the area seemed to be closing in fast. “OK, if you don’t want to go that way Three-Six-Eight, we’re going to have to turn you due southbound and go well south around Atlanta, your choice.”

It was 4:31 p.m. The pilot of N43368 decided to continue with the northern route and the controller issued a 300-degree heading to keep the flight out of moderate precipitation. Seconds later the pilot responded, “I thought I was going to shoot this gap here, I got a gap I can go straight through.”

“All right, that’s fine, November Three-Six-Eight, if that looks good to you. I show you’re going to enter moderate precipitation in about one mile and that will extend for about four miles northbound.”

“Roger, Three-Six-Eight.”

“And November Three-Six-Eight, what are your flight conditions there?”

“Rain, Three-Six-Eight.”

“And Three-Six-Eight, I’m showing you in a right turn eastbound now, verify three-sixty heading. Malibu Four-Three-Three-Six-Eight, Atlanta. November Four-Three-Three-Six-Eight, Atlanta Approach.”

But there was no response from N43368. The airplane had impacted a wooded area behind a residential property and was destroyed by fire. All three on board were killed. The time was 4:34 p.m.

According to a witness, who was a pilot, visibility was about 20 miles, there were scattered rain showers in the area, and the cloud base was about 2,500 to 3,000 feet above the ground. There was no lightning or thunder. The witness heard the engine noise first, then saw the airplane spinning toward the ground in a nose-low attitude until it disappeared. Another witness heard the airplane, looked up, and noticed the airplane spinning to the ground. Two more witnesses saw debris falling from the airplane—likely as a result of an in-flight breakup.

Analysis

Satellite imagery at 4:30 p.m. and 4:40 p.m. indicated an extensive layer of cloud cover above the accident site that was cumuliform in nature. Approximate cloud tops at 4:40 p.m. were 22,000 feet msl. Weather radar indicated heavy to very heavy precipitation, with a growing band of rain showers moving southeastward.

A sigmet issued at 3:55 p.m., valid through 5:55 p.m., warned of thunderstorms moving from 250 degrees at 50 knots with tops to FL280. A previous sigmet issued at 2:55 p.m. warned of thunderstorms moving from 250 degrees at 50 knots with tops to FL330.

Before the accident, a special METAR at 4:06 p.m. for Athens/Ben Epps Airport (AHN)—which was closest to the accident site—showed winds from 300 degrees at 8 knots, 10 miles visibility or greater, a broken ceiling at 2,500 feet agl, broken clouds at 3,200 feet agl, overcast skies at 6,000 feet agl, and a temperature of 18 degrees Celsius with a dew point of 15 degrees Celsius. However, a review of the airport’s raw surface observations prior to the accident indicated IFR conditions because of low visibility with heavy rain, even though VFR conditions were reported at the time of the accident.

The pilot requested weather information via ForeFlight, which included warnings for thunderstorm and heavy rain shower activity in the vicinity of the accident site. Subsequently, the pilot filed an IFR flight plan via ForeFlight but he did not view any in-app weather imagery, although he could have accessed text information.

The aircraft was equipped with a Garmin GMX 200 that could display XM weather radar images. If the pilot had a subscription to SiriusXM weather data it is likely he would have used the information. He also had access to ADS-B Flight Information Services-Broadcast (FIS-B) weather information, including weather radar. But latencies inherent in the process to detect, deliver, and create a weather mosaic mean Nexrad data can age significantly by the time the mosaic image is created. The datalink weather radar display time stamp shows the age of the mosaic creation, not the age of when the weather was detected—a critical difference.

Although not a typical scenario, in extreme latency and mosaic creation situations, the actual age of Nexrad data displayed can be as old as up to 15 minutes for satellite weather and 20 minutes for FIS-B.

According to the final NTSB report, FIS-B data displayed on the regional Nexrad weather screen would have been updated once every five minutes based on composite radar data. The report also compared XM weather information with Doppler weather radar. If the pilot had viewed XM weather imagery before turning north at 4:30 p.m. it would have had a time stamp of 4:24 p.m. But there was a difference of between six to 12 minutes from when the weather radar scan was initiated.

Datalink weather radar is wonderful to aid in strategic weather planning, but the caveat applies that pilots should be cautious and not use it for tactical weather avoidance. Weather is dynamic and can change quickly, meaning those snapshots of conditions may have changed—for better or worse.

Three minutes before the accident, the controller advised the pilot that the weather the pilot referenced was the weather he had advised him of previously and that a 300-degree heading would keep him south of the heavy precipitation. But a minute later, the pilot saw a gap on his cockpit display he thought he could go straight through.

Unfortunately, the pilot elected to navigate the hazardous weather along his flight route based on the datalink radar data display instead of the routing suggested by the controller. That meant he saw where the weather had been, not where it actually was. This caused the flight to enter dangerous weather, including rain shower updrafts and severe turbulence that ultimately resulted in an in-flight breakup after the pilot lost control of the aircraft.

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