Scoping out storms: Is it worth the risk?
A Missouri businessman, 54, and his dog, who accompanied him everywhere, died when his Piper Cherokee Six broke up in flight over Cuba, Missouri, in 2015.
The 1,200-hour pilot had filed an instrument flight plan from Branson, Missouri, where he had a vacation home, northeastward to St. Louis. He was cruising at 5,000 feet. When he first checked in with Kansas City Center, the controller warned him of “moderate to extreme precipitation from your 10 o’clock to 2 o’clock position, beginning in 50 miles, about 30 miles in diameter.” The pilot acknowledged, and added that he had “a scope on board.”
He could have flown around the weather; a 20 nm dogleg would have added only a few minutes to the 180 nm trip. Instead, he chose to continue on course.
Thirty minutes later, the pilot was in the middle of the area of precipitation. He told the controller that the ride was smooth. His Stormscope was showing some returns to his right, and he said he was seeing “a little bit on Nexrad.” Less than a minute later, the Cherokee began a right descending turn; then its radar return disappeared.
Witnesses on the ground reported hearing the sounds of an airplane seemingly in distress, and then seeing the plane come out of the clouds in pieces. Debris was scattered over more than half a mile, beginning, curiously enough, with pieces of the windscreen.
A Cape Air Cessna 402 happened to be at 7,000 feet, in the same vicinity, at the same time. The pilot later told a National Transportation Safety Board investigator that his trip had been routine, in solid IMC, with light to moderate turbulence, some moderate to heavy rain and some small returns on his weather radar — nothing that gave him cause for concern.
Radar imagery from St. Louis, northeast of the accident site, showed returns “consistent with convective activity” along the Cherokee’s flight path, but no lightning was recorded by local lightning-detection networks. A three-dimensional reconstruction of weather at the time of the accident, based on radar data, showed the Cherokee in a clear area, having just turned to the right, possibly to circumnavigate an area of “moderate-high reflectivity” that had moved into its path from the northwest. That area would have been visible to the pilot as a yellowand-red patch on his Nexrad display.
(The pilot told his controller that he had Nexrad, but accident investigators did not find “any evidence of Nexrad capabilities” in the wreckage. Since debris from the disintegrating airplane was scattered over a large wooded area, however, it seems more probable that the pilot
Witnesses on the ground reported hearing the sounds of an airplane seemingly in distress, and then seeing the plane come out of the clouds in pieces.
had a handheld display that was never found than that he gratuitously reported a nonexistent Nexrad capability to center. The fact that he flew precisely through a narrow low-reflectivity corridor between two more active areas suggests guidance from ground-based weather radar.)
The NTSB’s probable cause was “the pilot’s continued flight into thunderstorm activity.” To be sure, all that could be known from radar returns was that there was heavy rain, which was “consistent with” thunderstorms, but also consistent with just plain heavy rain. The pilot’s Stormscope showed no lightning ahead, and the ride was smooth. At the time of the breakup, the Cherokee was in an area of zero radar reflectivity. The only evidence that he was in danger was the breakup itself. But the NTSB often falls into the logical fallacy of using the accident as evidence that the pilot must have done something reckless.
The probable cause adds that a contributing factor was “the pilot’s reliance on onboard weather equipment to navigate through severe weather.” Actually, that is what FAA-approved onboard weather equipment is for. Presumably, the pilot’s aim was to avoid severe weather by using his onboard equipment, not to go “through” it. The Cape Air 402 pilot was apparently guilty of exactly the same behavior, but was not chided for it.
In the synopsis of the accident, the report’s author declares that the pilot “was attempting to use onboard weather radar to maneuver through an area of storms that he should not have been flying through in the first place.”
Never mind that the pilot was not using onboard radar at all. What actually happened here was that a controller called a pilot’s attention to an area of possibly heavy weather ahead of him; the pilot acknowledged the warning and said he would use his lightning detector and Nexrad to guide him. Not only was he within his rights to do this, but it was what many pilots would have done, considering that weather is as you find it, not as a controller tells you it may be. There is no reason to suppose that the pilot, who presumably had flown this route many times, would not have turned back or deviated if he had seen clear evidence of hazardous weather ahead. He didn’t, and so he continued.
On the other hand, at the time of the first warning from the controller 30 minutes earlier, he would have seen on his Nexrad display that large clear areas surrounded the patch of heavy rain. This was not a case of feeling one’s way between storms in a squall line too long to circumnavigate. The inconvenience of bypassing the weather would have been minimal. Why did he choose to aim straight at the center of the bad weather? Was he so in love with his onboard equipment that he could not pass up a chance to use it?
Evidently, the pilot unluckily stumbled into an area of severe local turbulence, lost control of the airplane and overstressed it while trying to recover. The turbulence could not have been identified or forecast; it just happened to be there. The Cape Air flight, cruising nearby, did not encounter it.
At the time of the accident, the Cherokee was making a groundspeed of 163 knots. The winds at his altitude were southwest at up to 30 knots, so it appears that the pilot had taken the precaution of slowing down to a few knots above his maneuvering speed, which would have been around 114 kias. Maneuvering speed, however, is no guarantee of safety.
Manufacturers of transport-category aircraft are required to define a turbulence-penetration speed, Vb, whereas general aviation airplanes like the Cherokee Six are provided only with a maneuvering speed, Va. While the two speeds are arrived at differently, the underlying principle is the same: There is a speed at which wings or control surfaces will stall before reaching their limit load. In other words, if you are flying slowly enough, no maneuver or gust will be able to tear your wings off.
It is cold comfort, however, that the only thing that stands in the way of losing a wing is a stall, which is normally something you would try to avoid. Unfortunately, turbulence or gusts powerful enough to break an airplane are also powerful enough to upset it. An upset in IMC is likely to bring about a rapid increase in speed in a downward direction, and that is almost sure to be followed, in turn, by a desperate and terrified pilot overcontrolling the airplane.
This accident can be seen as a case of a pilot taking a reasonable risk and the chances happening to turn out very badly for him. He gambled and lost. On the other hand, you can consider the gamble itself — however small the risk — to have been completely unnecessary. Take your pick.
The probable cause adds that a contributing factor was “the pilot’s reliance on onboard weather equipment to navigate through severe weather.”