The businessman-pilot took off at 3:15 in the afternoon from St. Petersburg, Florida, in his company’s A36 Bonanza, bound for Norman, Oklahoma. He filed IFR, with a cruising altitude of 10,000 feet and a speed of 185 knots. The 900 nm trip would have been at the very limit of the airplane’s range, but he was evidently counting on a tailwind.
He followed Florida’s western shore to the north-northwest until it, and he, bent westward. Skirting the southern edge of a line of convective weather in the Florida Panhandle and southern Alabama, he encountered in Mississippi a more formidable obstacle: a squall line that extended, practically unbroken, all the way from the Gulf of Mexico to Chicago.
He had left his initial cruising altitude of 10,000 feet, climbing first to 16,000 feet then to 20,000. He had been airborne for a little over two hours when a Memphis Center controller greeted him with a warning of extreme precipitation ahead.
“Roger,” the pilot replied. “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.”
“There’s a break in the extreme portion of the precip,” the controller said. “There’s still some moderate to heavy in that area though, at your, ah, about 325, 330 heading and about 115 miles.”
The Bonanza was equipped with a Stormscope and Nexrad weather, and so the pilot was seeing almost the same thing as the controller but not quite: Nexrad weather is always several minutes old and cannot provide a synchronous picture of rapidly changing conditions.
“Roger, I can see that, that’s the best spot I can see, so, ah, if you don’t mind I’ll change course and go towards that and see, if I get up there, if I can see through it. At this altitude, if I get any precip it’s going to be ice.”
After hearing an airliner’s pirep of moderate to severe mixed icing in Arkansas, the pilot requested a precautionary descent into warmer air. Half an hour after first contacting Memphis Center, he was level at 12,000 feet and flying north, converging gradually with the leading edge of the squall line. One gap he had hoped to thread had filled in; he reported that he was continuing a few miles farther north toward another. Center offered him a lower altitude, but he declined, saying, “It’s 40 degrees right now and no ice or precip — well, actually it’s a little wet precip, but it’s liquid, so I’m OK right now.”
That was his last transmission. Three minutes later, wings, ailerons, empennage and fuselage fragments tumbled out of the stormy sky and scattered over a mile of bosky terrain.
Early news reports said the airplane had been struck by lightning and had exploded, but the source of that interpretation is unclear, and it is physically unlikely. Lightning passes through metal airplanes, leaving superficial burns; it does not make them explode. At any rate, the final report of the National Transportation Safety Board cited no evidence of an explosion, merely reporting an inflight breakup and blaming it on “the pilot’s decision to continue flight into an area of known thunderstorms.”
“Contributing to the accident,” the probable cause went on, alluding to the 53-year-old pilot’s 342-hour total time and 32 hours of actual instrument experience, “was the pilot’s lack of experience in actual instrument meteorological conditions and his reliance on datalink weather radar imagery for tactical avoidance of convective weather.”
The NTSB superimposed the recorded flight track on stored weather radar images, which are of higher resolution than the pilot’s Nexrad display. While the cockpit display was showing the airplane on the edge of the band of weather but still clear of precipitation, it had, in fact, flown into a building Level 5 cell. The pilot appears to have been completely unprepared for the sudden violence he encountered. He said nothing to the controller, and the memory of the Bonanza’s JPI 700 engine monitor showed that he had made no effort to reduce power or slow down prior to the breakup.
It is commonly said that pilots, not storms, break airplanes. This is true when a panicked pilot is trying to recover from a spin or spiral dive entered in instrument conditions. It is true in ordinary frontal weather, and when VFR pilots stray into IMC and become disoriented. But thunderstorms are a different story. The sheer force of thunderstorm winds can break an airplane, even if the airplane is flying below its maneuvering speed.
Maneuvering speed is the speed at or below which an airplane’s flying surfaces will stall before they generate aerodynamic forces sufficient to break them. Most pilots of smaller airplanes do not make a clear distinction between maneuvering speed (VA), which depends on the airplane, and turbulence-penetration speed (VB), which depends on the atmosphere, and they use VA for both.
The A36 Bonanza is certificated as a Utility Category airplane. At its gross weight, it can withstand a positive load factor of 4.4 G without permanent deformation and 6.6 G without failure — 16 percent more than is required for Normal Category certification. The Bonanza’s maneuvering speed is 141 kias at 3,650 pounds, but with only one person aboard and having been airborne for nearly three hours, its weight would have been 3,000 pounds or so, and its maneuvering speed would have fallen to 128 kias. At the time of the breakup, its indicated airspeed was around 140 knots.
Strength requirements for certification are based on a 50 foot per second vertical gust. The effect of such a gust is both to increase the indicated airspeed slightly and, more important, to change the wing’s angle of attack. An airplane moving horizontally at a true 170 knots would experience an effective increase in angle of attack of about 10 degrees. This would be equivalent to a 4.7 G pull-up — enough to wrinkle the A36 but not to break it.
What kind of gust would be necessary to produce the ultimate load for which the A36 is designed? One at around 90 to 100 feet per second. Is such a gust plausible? Certainly. Vertical velocities in strong thunderstorms cannot be measured directly or predicted, but meteorologists have published estimates of 150 to 220 feet per second, or 90 to 130 knots, for upward drafts — upward ones are somewhat stronger than downward — at 25,000 feet. At 12,000 feet they are about a third slower, but in any case such gusts are several times more powerful than the ones on which structural designs are based. Normal Category airplanes, whose ultimate strength is 5.7 G rather than 6.6, are obviously less likely to survive thunderstorm winds than the A36 was.
Loss of control is probable, if not certain, in extreme turbulence. The airplane may experience unforeseeable attitudes and freakish combinations of forces. No mishandling by a panicked or inept pilot is needed, however, witness the 2006 breakup in a Georgia thunderstorm of the Cessna 210 of Scott Crossfield, one of the coolest and most capable test pilots of his generation.
Pilots cross squall lines between cells all the time, usually with weather radar that gives real-time information about what lies ahead. But it’s hard to do so safely when weather is active and gaps in the line are small. With its inevitable delays, datalink weather requires a more cautious approach. If the A36 pilot had circled for a few minutes to the east of the spot where he hoped to cross the line, he would have seen that its apparent tranquillity was temporary. Its leading edge was, in fact, boiling with new cells. He might have cursed his luck and kicked himself for not acting more swiftly. But then he would have landed, slept soundly, and continued the next day under clear skies.
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