Page 7 - Dec18T

Let’s review:
• Indicated airspeed variations in excess of 15 kts;
• Groundspeed variations (decreasing headwind or
increasing tailwind, or a shift from head wind to
tail wind);
• Vertical-speed excursions of 500 fpm or more;
• Pitch attitude excursions of five degrees or more;
• Glideslope deviation of one dot or more;
• Heading variations of 10 degrees or more.
Wind shear may be vertical or horizontal, or a mixture of
both types and is usually associated with fronts, jet streams, thunderstorms or convective clouds, mountain waves or micro- bursts. At higher altitudes, clear air turbulence is often associ- ated with wind shear and can be violent. In the worst cases, it can lead to loss of control.
As Advisory Circular No 00-30C notes, wind-shift areas associ- ated with pressure troughs and ridges are frequently turbulent. This accurately describes what we flew through on that day.
While prediction tools, forecasting models and onboard turbulence detection are getting better and better, wind shear and severe turbulence continue to bend aircraft and cause onboard injuries. Beyond the severe jolts, which can cause structural damage, airspeed fluctuations and G-loading can lead to a high-altitude upset.
If you aren’t already, get familiar with the NWS’ Graphical Turbulence Guidance Model; it provides an analysis and forecast for clear air turbulence as well as mountain waves, and it includes turbulence information beginning at 2,000 MSL all the way to FL450. Also, turbulence guidance is now available as an ADS-B product, so that you have access when connected to an ADS-B input source. (On ForeFlight’s map, there is a slider that allows you to view the layer at the altitude of your choosing.) It also uses a more nuanced color gradient scale based on the eddy dissipation rate, or EDR, which is a universal measure of the rate at which energy dissipates in the atmosphere. These are automated forecasts without human input like you’ll find with AIRMET Tango and SIGMETs for turbulence. ForeFlight has some excellent blog posts on their website that fully explains the NWS GTG products and how they are created. It’s worth a read or review.
In addition, we have access to real-time conditions through graphical PIREPs – an incredibly useful tool for in-flight decision- making. Finally, queries to ATC can help draw out more details on what others have reported. Together, all of these tools give us the ability to better flight plan and make in-cockpit decisions to minimize our exposure to dangerous turbulence. However, the most important tool is the one between your ears: that’s where you can interject experience, knowledge plus a measure of conservatism and humility to your decision.
Today, I have the benefit of hindsight to analyze whether I should have launched. But perhaps more importantly, I now have some valuable experienced-based insight that I can apply to future go or no-go decisions.
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December 2018
TWIN & TURBINE • 5
Dianne White can be contacted at editor@diannewhite.com