Page 19 - Volume 19 Number 10

DecisionPart II: by Thomas P. TurnerFigure 2: Flying in an area that looks like this? Avoid all the radar returns...even the green.extremely lucky to have survived. The airplane was totaled.Radar and the life cycle of a thunderstormOne of the challenges of making an informed, confident “go” decision when thunderstorms are developing is precisely how to plan your route to avoid the threat. It’s easy to look at the greens and yellows and reds on a radar screen or uplink display and think: “As long as I stay out of the yellows and reds I’m okay.” After all, green often indicates very light precipitation. Certainly a piston twin or light turbine can handle that.The pilot who lost control and miraculously recovered asked me how he encountered such significantOCTOBER 2015turbulence while flying outside the yellow and red areas of his iPad’s NEXRAD map, and those Center told him were painted on their screen. I told him this is because radar doesn’t indicate the earliest, but still dangerous, phase of a thunderstorm’s development: the Updraft Stage. I took the experienced pilot back to his knowledge test preparation and asked him to name the three stages of a thunderstorm’s development. He recalled them as the Cumulus orUpdraft stage; the Mature phase; and the Downdraft or Dissipating stage. I then asked him–and this is a key point–what defines the beginning of the mature stage. The answer: it’s when precipitation begins to fall from the cloud.Then I queried: What’s the greatest hazard of flying into a thunderstorm? He correctly said “turbulence.” I have recently been asked “How much rain can an airplane fly through?” The answer is “it depends”–some engines may have air filters or inlets that don’t stand up to rain. Some canard surfaces are known to have adverse aerodynamic reactions to rainflow, and high-efficiency laminar airfoils may lose some of their lift-generation capability in the rain. But, for most of us, the beating of rain against the airplane makes noise but does not alone create a hazard. What is dangerous is the wind shear effect of a boiling, fluid atmosphere.The critical concept: Radar doesn’t detect turbulence, it displays the precipitation that sometimes (but not always) coincides with areas of strong turbulence. It’s turbulence, not precipitation, that is the hazard of flying near a thunderstorm. And, by definition, the Updraft stage of thunderstorm development is invisible to radar, despite the potentially damaging wind shear it contains–as soon as the storm appears on radar it is no longer in the Updraft stage.Further, Center air traffic control radars are optimized for aircraft detection, not weather returns. Often, they cannot detect areas of light precipitation that may be the first indication of a mature- stage thunderstorm. The ability to paint weather on ATC scopes varies widely from one ATC facility to another. And, the rate at which individual thunderstorms can build is so great that what appears to be a clear path can close in before an airplane can traverse the space between the cells. That may include your escape path, too.TWIN & TURBINE • 17