Definition

Wind shear is defined as the change in wind speed and/or direction from one flight level to the next. The shear rate is the rate of change of wind velocity per 1000 ft. If the wind direction changes with height, the absolute difference in velocities is used. For example, if the wind direction reverses in 1000 feet but stays the same speed, the shear rate is double the velocity.

Discussion

The Shear Rate (SR), or Shear Value, displayed on a flight plan alerts the crew to the potential existence of clear air turbulence (CAT) along the route of the flight. The number is always positive regardless of whether the wind speed increases or falls and the higher the number, the greater the degree of turbulence that can be expected. Pilots should always check other sources of information e.g. SIGMET charts and PIREPS, or consult an aviation meteorologist to determine whether there is a cause for concern.

There are different types of turbulence and the impact of this turbulence depends on the aircraft type. Max takeoff weight, wingspan, and wing loading all impact aircraft performance in relation to wind shear and susceptibility to turbulence.

Wind shear scales

Each operator will typically have their own set of guidance on how to interpret shear values. For example:

• 01 - 04 = Light Turbulence
• 05 - 09 = Moderate Turbulence
• 10 and above = Severe Turbulence

But it must be emphasized that shear rate does not necessarily correlate to flight turbulence and depends so much on aircraft type.