Déjà vu

ERA12FA376

By David Jack Kenny

No one knows how many pilots have flown light airplanes through thunderstorms and lived to tell the tale. It’s a pretty good bet that most of those who have weren’t tempted to try it again, though a few conspicuously failed to learn from the experience. What we do know is that storms powerful enough to bend airplanes usually break them. Recent years have only averaged about half a dozen thunderstorm accidents apiece, but three-quarters were fatal. In 99 percent of those, no one on board survived.

Pilots being pilots, we don’t want to ground ourselves, nor do we want to go hundreds of miles out of our way to get around the weather. Instead, we’ve looked to technology to help us get where we’re going. In-cockpit lightning detectors were once the state-of-the-art of weather avoidance in piston singles, and even before the advent of the Stormscope and Strikefinder, pilots used their automatic direction finders to point out the general direction of electrical discharges. Airborne radar became an option for larger aircraft, though the small dishes of most GA installations (and most pilots’ lack of experience in interpreting the returns) provide something less than transport-category performance. Now satellites can transmit weather data to anything from panel-mounted multifunction displays to the pilot’s telephone. Downloaded radar images won’t show you where extreme turbulence is now, but at least they’ll indicate where precipitation was as recently as 20 minutes ago.

Two accidents in recent years illustrate that improved technology can also increase the temptation to cut things a little too close. In October 2009, a King Air broke up in flight over south Texas as its relatively inexperienced pilot tried to use his on-board radar to find a gap through a squall line. He may have expected more help than ATC had time to give; that sector was simultaneously working 23 other flights, mostly jets seeking weather deviations. And in central Texas a little more than two years later, a Piper Cherokee Six whose pilot was relying on Nexrad downloads had a wing ripped off by a cell that hadn’t yet appeared on his display. Investigators concluded that ambiguity in the time stamp on the radar image, coupled with the respective speeds of the airplane and the storm, meant that the actual weather might have been as much as 15 miles closer than the downloaded image would have led the pilot to believe.

History repeats itself, and not just in Texas. On May 31, 2012, an A36 Bonanza broke up in a thunderstorm over Macon, Miss., after a sequence that combined elements of the two previous accidents.

The pilot took off from St. Petersburg-Clearwater International in Florida at 2:15 p.m. on an IFR flight plan for the Oklahoma University airport in Norman, a distance of some 875 nautical miles. A little more than three hours into the flight, he was handed off to Memphis Center. At that time the airplane was level at 20,000 feet, which must be close to the service ceiling of a normally aspirated Bonanza. The NTSB report doesn’t mention a supplemental oxygen system, but neither does it mention hypoxia, so oxygen was presumably in use.

The airplane was also headed directly for an extensive squall line, and the controller advised the pilot of “extreme precipitation … about 85 miles, extending north and south.” The pilot acknowledged, saying “Roger, I'm looking at that trying to see if there is any way I can get through it and I'm beginning to think there is.” When asked if he had on-board radar, the pilot replied, “Ah, I have Nexrad composite.” The airplane was also equipped with a WX-500 Stormscope.

The controller advised of “a break in the extreme portion of the precip, there’s still some moderate to heavy in that area.” At that point the apparent break was 115 miles from the pilot’s position on about a 330 heading, and the pilot requested and received clearance to deviate in that direction. Concerned about possible icing, he also requested several descents over the next 20 minutes, eventually leveling off at 12,000 feet msl. Twenty-eight minutes after the initial contact, the pilot advised that “I turned a little more to the right. I’m going due north now because that … cell’s filled in. There’s still a gap but I’m having to go about 10 miles to the north before I get to that gap.” The controller acknowledged and offered a further descent, which the pilot declined. Radar contact was lost less than four minutes later. The wreckage was eventually found scattered along a debris path a mile and a quarter long.

The NTSB reviewed the Nexrad images broadcast during this period. One transmitted less than four minutes before the last radio contact “depicted the airplane clear of precipitation.” The associated time stamp would have suggested it was six to seven minutes old, but accounted only for the time spent assembling the mosaic; the raw radar data could have been as much as eight minutes older than that. The next composite also showed the aircraft free of precipitation; it was probably broadcast at about the time the airplane began breaking up. By contrast, real-time ground-based radar showed that by that time the Bonanza had entered an area of very heavy (Level 5) precipitation.

This catastrophic error reinforces two lessons, one already familiar and one becoming more so. Trying to push a light airplane through a squall line is never a good idea. Even a relatively inexperienced instrument pilot (his logbooks showed 342 hours of total flight time that included 32 in actual IMC) should know this. And while datalink is great for showing you what to stay a long way away from, counting on it to show a way between violent, rapidly moving storms is asking for a whole world of trouble.