Turbulence — Page 113, Lesson 99

I want to walk you through some important operational guidance and phenomena related to turbulence. Let's start with a set of practical avoidance rules, then move into rotor streaming, and finally jet streams. First, the book gives us several specific flight-level and altitude rules for avoiding severe turbulence near mountains and stable layers. The first point: avoid flight levels within 5,000 feet of a stable layer where severe turbulence is most likely. That means if there's a stable layer—a layer of air where temperature increases or remains constant with height, resisting vertical motion—you want to stay at least 5,000 feet away from it in terms of your assigned flight level, because that's where the worst turbulence concentrates. Next, allow a height clearance above the highest ground at least equal to the height of that ground above the local terrain. So if the highest ground in an area is, say, 2,000 feet above the surrounding terrain, you need to give yourself at least 2,000 feet of clearance above that ground. This is a rule of thumb to keep you out of the worst mechanical turbulence near the surface. Then, avoid low-altitude flight towards the mountain range from the lee side. The lee side is the downwind side—the side sheltered from the wind. If you fly low towards the mountains from that side, your aircraft's height variations will be out of phase with the waves and downdraughts will be hazardous. In other words, the mountain waves and downdrafts won't match your aircraft's natural response, and you could get caught in a dangerous descent. Also, avoid high-altitude flight on the lee side of the mountain range downwind. The buffet margin at high level may be small, meaning you have little room before the airflow separates from the wings and causes buffeting. And the speed of approaching standing waves will be high, with subsequently greater loads applied to the airframe. Standing waves are stationary wave patterns in the airflow downwind of mountains; if you fly into them at high speed, the forces on the structure increase significantly. Finally, be prepared for icing in cloud. Whenever you're flying in cloud near mountains or in turbulent conditions, ice can form on the airframe, so you need to have your anti-icing or de-icing systems ready. Now let's move to rotor streaming. This is a specific phenomenon that can occur when the winds approaching a mountain range are strong only at lower levels and fall off or reverse direction at higher levels. Under those conditions, rotor streaming may result. This comprises violent rotors moving downwind from the ridge. Unlike the stationary rotors described earlier in the chapter—which stay in place over the lee slopes—these rotors travel downwind after forming on the lee slopes. So they're not fixed; they drift away from the mountain. Figure 8.4 shows rotor streaming, and you can see that depicted on screen. Finally, let's talk about jet streams. Jet streams are narrow, fast-moving currents of air which occur just below the tropopause—the boundary between the troposphere and stratosphere. They'll be discussed in detail in the chapter on upper winds, but for turbulence purposes, here's what you need to know. Generally, the associated turbulence is found on the cold air side of the jet stream just below the core, where the greatest windshear occurs. Windshear is a rapid change in wind speed or direction over a short distance. There's also a secondary area above the core extending into the stratosphere, as the winds rapidly decrease in strength. The turbulence will be more severe with curved jets, developing and rapidly moving jets, and in mountainous areas, particularly when mountain waves are present. So if you're flying near a jet stream that's curving, intensifying, or moving quickly, or if you're over mountains with wave activity, expect stronger turbulence. Figure 8.5 shows a vertical cross-section through a jet stream, which you can see on screen.