Turbulence — Page 113, Lesson 102

I want to walk you through the classification of turbulence intensity, starting with Moderate turbulence. Moderate turbulence is essentially a step up from light turbulence — it's similar in character but with greater intensity. There are two distinct forms to recognise. The first is Moderate Turbulence proper, where you get changes in both altitude and/or attitude — the aircraft's pitch, roll, or yaw — but crucially, you remain in positive control at all times. That means the pilot can still maintain commanded flight path without the aircraft departing from controlled flight. The indicated airspeed fluctuates between 15 and 25 knots, and the acceleration measured at the aircraft's centre of gravity is between 0.5 and 1.0 g. When you encounter this, you report it as 'Moderate Turbulence'. The second form is Moderate Chop. This is similar to Light Chop but more intense. It causes rapid bumps or jolts, but without appreciable changes in altitude or attitude — so the aircraft's path stays relatively steady, but the ride is rough. Indicated airspeed may fluctuate slightly, but not as dramatically as in the turbulence form. You report this as 'Moderate Chop'. Now, what does Moderate turbulence feel like in the cabin? Occupants feel definite strains against seat belts or shoulder straps. Unsecured objects get dislodged — things might fall off tables or out of overhead bins. Food service and walking become difficult. Let's move up to Severe turbulence. This is a significant step change. Severe turbulence causes large, abrupt changes in altitude and/or attitude. The aircraft may be momentarily out of control — that's a critical point: for brief periods, the pilot may not be able to maintain the desired flight path. Indicated airspeed fluctuates more than 25 knots, and the acceleration at the centre of gravity exceeds 1.0 g. You report this as 'Severe Turbulence'. In the cabin, occupants are forced violently against seat belts or shoulder straps. Unsecured objects are tossed about — not just dislodged, but thrown around. Food service and walking are impossible. Now, there are two important notes for your reporting procedures. First, when you encounter moderate or severe turbulence, you should report: the location and time in UTC, the incidence — that's how often it occurs, like intermittent or occasional — the intensity, whether you're in or near clouds, the altitude or flight level, and the type of aircraft. All locations must be readily identifiable. You make these reports when moderate or severe turbulence is encountered, or when requested. The excerpt gives two examples: "Over Pole hill 1230 intermittent Severe Turbulence in cloud, FL 310, B747" and "From 50 miles north of Glasgow to 30 miles west of Heathrow 1210, occasional moderate Chop TURB, FL 330, MD80". Second, and this is specific to UK operations: the UK does not use the term 'Extreme' in relation to turbulence. So you will never report 'Extreme Turbulence' in UK airspace — the top category is Severe. Let me show you a diagram that illustrates some of the conditions that produce these turbulence intensities.