Turbulence — Page 113, Lesson 100

I want to walk you through the topic of turbulence — specifically what happens at high altitude, around cumulonimbus clouds, near upper-level troughs and ridges, and how we report it. Let's start with high-altitude turbulence. If you encounter turbulence at high altitude, the first action is to reduce speed to the rough air penetration speed — that's the speed designed to give you the best structural margin in turbulent air. But there's a second critical step: you should also descend. Here's why. At high altitude, the margins between high-speed buffet and low-speed buffet can be quite small. High-speed buffet occurs when airflow separates over the wing due to excessive speed; low-speed buffet is the classic stall buffet. Turbulence can push the aircraft into either regime — a high-speed stall or a low-speed stall — because the gust loads change the angle of attack suddenly. By descending, you increase those buffet margins, which enhances the aircraft's safety. So the rule is: slow down to rough air speed and get lower. Now let's move to cumulonimbus clouds — we abbreviate them as CB. Turbulence inside a CB is driven by strong vertical currents. Those vertical currents do two things. First, they draw air in from around the cloud, and that inflow creates turbulence around the cloud itself. Second, the energy of the air rising inside the cloud is transmitted upward, above the cloud top, creating turbulence there as well. Below the cloud base, the inflow and outflow of air produces severe turbulence at low altitude. And there's a particularly dangerous phenomenon: microbursts, also called downbursts — a concentrated column of descending air that hits the ground and fans out. The excerpt calls them a potentially fatal hazard. We'll discuss microbursts in more detail from the AIC — the Aeronautical Information Circular — at the end of this chapter. Next, turbulence around upper-level troughs and ridges. Upper-level winds are stronger than surface winds. At an upper-level trough — that's a southward dip in the upper wind pattern — the wind direction changes sharply. Those sharp changes produce considerable horizontal wind shear, and that wind shear creates disturbance that you may experience as Clear Air Turbulence, or CAT. At an upper-level ridge — a northward bulge in the pattern — the curvature is gentler than at a trough. So the direction changes are less abrupt, and the resulting turbulence is less severe. Now let's talk about turbulence reporting criteria. Turbulence is an important operational factor at all levels, but particularly above Flight Level 150 — that's 15,000 feet in the standard pressure setting. The best source of turbulence information comes from pilots' Special Aircraft Observations. If you encounter turbulence, you are requested to report: the time, the location, the level (altitude or flight level), the intensity, and the aircraft type. You send that report to the Air Traffic Services unit you're in radio contact with. For high-level turbulence — normally above FL150 and not associated with cumuliform cloud, including thunderstorms — you report it using the code word TURB. You precede TURB with the appropriate intensity, or you can precede it with Light Chop or Moderate Chop. Note that EASA — the European Aviation Safety Agency — still refers to clear air turbulence as CAT, even though the reporting code is TURB. The excerpt includes a turbulence criteria table. Let me walk through the incidence categories first. Occasional means less than one-third of the time. Intermittent means one-third to two-thirds of the time. Continuous means more than two-thirds of the time. Now intensity. For Light turbulence, the aircraft reaction for a transport-size aircraft is: turbulence that momentarily causes slight, erratic changes in altitude and/or attitude — attitude meaning pitch, roll, and yaw. The indicated airspeed fluctuates between 5 and 15 knots, and the load factor at the aircraft's centre of gravity is less than 0.5 g. You report that as 'Light Turbulence'. Alternatively, if the turbulence causes slight, rapid, and somewhat rhythmic bumpiness without appreciable changes in altitude or attitude, and with no IAS fluctuations, you report that as 'Light Chop'. Inside the aircraft, occupants may feel a slight strain against seat belts or shoulder straps. Unsecured objects may be displaced slightly. Food service can still be conducted, and little or no difficulty is encountered in walking. That figure shows the turbulence surrounding cumulonimbus clouds — the vertical currents, the inflow and outflow, and the severe turbulence below the cloud. And there's also a figure for turbulence produced at upper troughs and ridges, which illustrates the sharper direction change at the trough versus the gentler curve at the ridge.