The Beechcraft Duke was in visual conditions at Flight Level 270 (FL270). The pilot reported he observed a 10-mile wide opening between a large thunderstorm cell and a smaller cumulonimbus (CB) build-up that was developing just to the southwest of the large cell. As the airplane approached the gap, the pilot noticed that the CB was developing faster than he had anticipated. The airplane’s onboard weather radar was depicting only light precipitation, while the large thunderstorm cell northwest of the airplane was depicting extreme precipitation. As the airplane entered the northeast side of the CB, it encountered light to moderate turbulence followed by an extreme downdraft. The pilot initially lost control of the airplane, then recovered at an altitude of about 14,500 feet, and the airplane sustained substantial damage. A reconstruction of NEXRAD weather data displayed to the ATC controller revealed that the airplane had entered an area of extreme (VIP 6) echoes.
We all should know the extreme hazard of flying through thunderstorms. In recent years, we’ve learned about the hazard of misinterpreting weather data uplinks, especially NEXRAD radar images that can be as much as 20 minutes old by the time they are visible in the cockpit – when 20 minutes is a significant percentage of an air mass thunderstorm cell’s entire lifespan. The National Transportation Safety Board even issued Safety Alert #017 reminding pilots that cockpit NEXRAD weather uplinks are not accurate enough to plot a course between or in the vicinity of thunderstorms… that a display of the “latency period” is the time since the NEXRAD image was transmitted, not the time that the individual radar images were recorded, which will always be older.
There are times, however, when airplanes crash and it at least appears from the investigative process that the pilot was maneuvering around thunderstorms but did not actually penetrate a radar return that correlates to a storm cloud. What might cause that? There are actually several possibilities.
Hail. One explanation might be the hail threat. Hailstones have been reportedly encountered as far as 20 miles downwind of major cumulonimbus clouds. Generally, storms of this magnitude have well-developed anvil tops; one of the “classic weather text” warnings about thunderstorms is to avoid “severe” thunderstorm clouds by at least 20 miles, and to avoid flying beneath anvil clouds at any distance from the cloud boundary. “Regard as severe any thunderstorm,” the FAA tells us in Advisory Circular 00-06A, Aviation Weather, “with tops 35,000 feet or higher whether the top is visually sighted or determined by radar.” In my experience, that’s the vast majority of all thunderstorm cells. From a hazard-to-flight standpoint, we should avoid almost all storm clouds by at least 20 miles.
New cell development. Individual storm cells can develop quite rapidly, in three stages:
• The Developing or Updraft stage, marked by vertical updrafts of up to 3,000 feet per minute, creating intense wind shear turbulence for the airplane that encounters this rising air.
• The Mature stage, with updrafts and downdrafts measuring in the thousands of feet per minute. By meteorological definition, the Mature stage begins when precipitation begins to fall from the cloud.
• The Dissipating or Downdraft stage, when most air movement in the cell is downward…at thousands of feet per minute.
Critical to a pilot’s interpretation of NEXRAD or airborne radar is that, by definition, dangerous Developing stage turbulence occurs before rain begins and therefore before the cell may appear on radar.
Clear Air Turbulence. Clear Air Turbulence, or CAT, is a generic term for any significant turbulence that occurs outside the boundaries of a cloud. CAT is most frequently associated with high-altitude flight in the vicinity of the jet stream. But, it can also describe the rolling and swirling flow of air along a front in an area of cumulonimbus cloud development. Clouds can be thought of as forming as whitecaps on waves of air. That doesn’t mean that the waves between the whitecaps can’t be rough as well.
The guidance to avoid storm cells by at least 20 miles means that storm clouds must be at least 40 miles from one another, with no overhanging anvil, for you to split the difference and fly between them. That’s a big gap in a line of storms – much larger than the 10-mile-wide gap the Duke pilot was trying to use. The classic texts actually advise us to avoid trying to fly in areas with more than 6/10th coverage of thunderstorm cells…even with active airborne weather radar.
Turbulence in the gap
What about crashes that follow an attempted deviation around storms? How might we explain an airplane that seems to be flying along just fine, then fails to respond to Air Traffic Control transmissions and suddenly plunges downward into the ground? The late centenarian pilot Captain Johnny Miller wrote prolifically about his storied career as an aviator “from Jennys to jets,” as he put it. John once wrote about an experience he had while crewing an Eastern Air Lines DC-8 in an area of strong thunderstorms. He noted that the airplane unexpectedly hit severe turbulence so intense that, even strapped in with two shoulder harnesses, he was flung upward and sideways, and he hit his head on the upper sidewall hard enough that he was momentarily dazed. John speculated that in a light airplane it would be possible for a pilot to hit the cabin sidewall or headliner with enough force to knock him/her unconscious and be unable to control the airplane…or perhaps even break the pilot’s neck. This, John suggests, might explain airplanes that lose control and crash for no apparent reason, when they have strayed too close to a thunderstorm but may not have actually penetrated the storm cloud. A pilot knocked unconscious by turbulence would explain many cases when an airplane went down when flying in the vicinity of thunderstorms.
Just as we intellectually understand we should avoid flying within 20 miles of a thunderstorm, we also can repeat – often simply by rote – that we should slow the airplane to its Turbulent Air Penetration Speed (TAPS) to avoid overstressing the airframe in turbulence. The TAPS is usually considered to be the same as the Design Maneuvering Speed, or VA. Maneuvering Speed is an indicated air speed at or below which the wing will attain critical angle of attack and stall before it exceeds the airplane’s load limit. Momentarily stalling “unloads” the wing and prevents catastrophic damage in turbulence or aggressive maneuvering.
VA, however, is defined solely at the airplane’s maximum gross weight. In reality, what we might call the “VA Effect” happens at a lower-than-maximum-weight speed when the airplane is at lower weights. Some manufacturers publish a VB speed, or range of speeds, for use at less-than-maximum weights. Most manufacturers that publish VB say that the speed decreases about 1% – 2% for every 100 pounds of weight below the airplane’s maximum gross weight. If your airplane does not have a published VB this at least gives you some guidance.
Two things are not often taught about TAPS, however: First, VB is not your target airspeed when flying in turbulence. It is the maximum speed you should fly, a sort of unofficial new VNE or “barber-pole” speed. Your target airspeed in turbulence must be a good bit lower, so those 10- or 20-knot increases in gusts result in a speed that is still below VB. Second, you must slow to TAPS before you enter turbulence. You don’t get “one free bump” before you slow down, because the airplane will exceed its structural limit in that first bump if you’re not already below VB. You need to anticipate when you might encounter heavy turbulence and slow the airplane down before the first bump.
Running the gap
Running the gap between storm cells is a calculated risk. First, the gap should be at least 40 miles wide, with you splitting the difference. Second, although airborne radar is better than NEXRAD uplinks, and NEXRAD is better than nothing at all, if you’re cutting it close (i.e., 20 miles from cells) it’s best to avoid clouds visually, in case a new, updraft-stage cell is in your path. And third, it’s a good idea to slow down before you fly between strong thunderstorm cells, because you don’t get a free bump before you decelerate to turbulent air penetration speed.