There are three categories of turbulence: Light, Moderate and Severe. The best way to learn these questions is to look at key language for each level and think about the differences, rather than trying to remember the whole thing.
Light turbulence: Turbulence that momentarily causes slight, erratic changes in altitude and/or attitude (pitch, roll, yaw). IASIAS —Indicated Air Speed fluctuates 5-15kt (<0.5g at the aircraft CGCG —Centre of Gravity) Occupants may feel a slight strain against seat belts or shoulder straps. Unsecured objects may be displaced slightly. Food service may be conducted and little or no difficulty is encountered while walking.
Moderate turbulence: Turbulence that is similar to light turbulence but of greater intensity. Changes in altitude and/or attitude occur but the aircraft remains in positive control at all times. IASIAS —Indicated Air Speed fluctuates 15-25kt (0.5-1.0g at the aircraft CGCG —Centre of Gravity). Occupants feel definite strain against seat belts or shoulder straps. Unsecured objects are dislodged. Food service and walking are difficult.
Severe turbulence: Turbulence that causes large, abrupt changes in altitude and/or attitude. Aircraft may be momentarily out of control. IASIAS —Indicated Air Speed fluctuates more than 25kt (>1.0g at aircraft CGCG —Centre of Gravity). Occupants are forced violently against seat belts or shoulder straps. Unsecured objects are tossed about. Food service and walking are impossible.
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