In early November, my husband and I had traveled to Pensacola, Fla., to celebrate our daughter’s completion of a phase of her pilot training at the Naval Air Station and catch the Blue Angels final performance of 2018. After a memorable few days, we turned our attention to the flight home to Kansas City.
As often typical this time of year in the northern hemisphere, the struggle between seasons is underway as the polar jet moves south and brings the season’s first significant cold fronts. Thus, this was the case on the day of our intended flight. After much analysis, we determined that the front was slow moving with widely scattered precipitation anticipated. The front was forecasted to run out of energy mid-afternoon and become stationary. Closer to the low-pressure center located in Iowa, convection was forecasted down through eastern Missouri to Memphis. Sure enough that morning, cells were starting to manifest north of our route on the radar. We decided to launch midday as the front ran out of gas.
At about 140 nm from the front, the ride was smooth with light rain and pockets of moderate rain. From a strategic point that far out, Nexrad confirmed what we were expecting to see: large areas of green and smaller yellow returns…nothing that looked terribly concerning (read: convective). By the time we reached the front in VMC between layers, our Memphis Center frequency was inundated with aircraft – from FL450 down to 6,000 feet – reporting moderate turbulence. A Citation 40 nm north of our route climbing through 11,000 reported wind shear and severe turbulence. I pulled back the power in anticipation, and within minutes it hit: some of the worst wind shear we’ve experienced in three decades of flying.
In addition to the effects of turbulence, we observed tremendous variations in indicated wind direction – shifting from 10 kts tailwind to 48 kts headwind. Groundspeed varied wildly, and the autopilot got a workout to maintain heading and altitude. All the time, I kept my hand on the yoke and eyes on airspeed, ready to hand-fly if necessary. Meanwhile, our ForeFlight map became heavily populated with PIREPs all around us, reporting moderate turbulence. We added ours to the collection. A good friend who happened to be passing overhead at FL450 in a CJ3 told us later that the turbulence “rocked their world” as they passed over the front.
We have all been taught that whether or not wind shear conditions are expected, the pilot must be able to recognize it when it occurs. What are the indications you’re in a wind shear event?
- 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 microbursts. At higher altitudes, clear air turbulence is often associated 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 associated 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.