Windshear and Jetstream

Windshear

Windshear is a rapid variation in wind velocity and/or direction along the flight path of the aeroplane. Pilots must remain alerted to the possibility of wind shear, and be prepared to make relatively harsh control inputs and power changes to offset its effects. Immediately after take-off and on the final approach the pilot’s course of action is critical, since they will normally be at the recommended speed for the configuration, and close to the ground.

When encountering windshear conditions, pilots shall report such conditions to ATC as soon as practicable via PIREP, stating the loss or gain of speed and the altitude at which it was encountered, along with the type of aircraft.

Conditions that indicate the possibility of windshear and their associated probability are represented on the following table.

These guidelines apply to operations in the airport vicinity i.e. within 3 miles of the point of take-off or landing, along with the intended flight path and below 1.000 ft AGL. The clues should be considered cumulative. If more than one is observed the probability weighting should be increased. The hazard increases with proximity to convective weather. Weather assessment should be made continuously.

Currently, no quantitative means exists for determining the presence or intensity of microburst windshear. Pilots are urged to exercise caution in determining a course of action. The observation weighting is categorised according to the following scale:

• Low Probability: Consideration should be given to this observation, but a decision to avoid is not generally indicated.

• Medium Probability: Consideration should be given to avoiding. Precautions, as specified in the OM Part B, are appropriate.

• High Probability: Critical attention need be given to this observation. A decision to avoid (e.g. divert or delay) is appropriate.

Although encountering weather conditions described in the foregoing table at an altitude above 1.000 ft AGL may be less critical in terms of the flight path, such encounters may present other significant weather related risks. Pilots are therefore urged to exercise caution when determining a course of action. Use of the table above should not replace sound judgement in making avoidance decisions. Windshear clues should be considered cumulative. The probability for each single observation is given. However, if more than one windshear clue is observed, the total probability rating may be increased to reflect the total set of observations.

Windshear on Take-off

If there are reports of severe windshear, take-off should be delayed. If after take-off the presence of shear is indicated by rapidly fluctuating airspeed and/or rate of climb/descent, or on receipt of a windshear warning apply maximum thrust and aim to achieve maximum lift and maximum distance from the ground without changing aeroplane configuration until safety is assured. Advise ATC as soon as possible.

Windshear on Approach

If there are reports of severe windshear, consider holding or diverting. If conditions are such that windshear might be encountered the Commander should brief that a go-around may be required. If windshear is experienced on final approach a go-around shall be carried out if the approach becomes unstable. For detailed operating procedures refer to OM Part B.

Jetstream

Near the tropopause, there can be narrow bands of the wind with extremely high speeds (up to 300 kts) called Jetstream. The extent in length is up to several thousand miles, the width can be several miles. The main direction is south-west to the north. In mid-latitudes, there is a common area for clear air turbulence (CAT) around the jet stream, above the jet core and to the polar side.

Taking a cross section of a jet stream looking downwind, the turbulence region would be to the left of the core in the Northern Hemisphere and to the right in the Southern Hemisphere. To avoid or leave the area of CAT the following procedures should be applied:

• Reduce speed, to reduce acceleration due to wind shears;
• When flying parallel with the jet stream changing altitude up to 1.000 ft;
• When flying perpendicular to the jet stream, changing altitude by 1.000 ft from the warm side to the cold side downwards, from the cold side to the warm side upwards;
• If the temperature is changing in the CAT area the flight should be continued on course, the CAT area is likely to be crossed in a short time;
• If the temperature remains constant the course should be altered in order to leave the CAT area.