Windshear and Microburst

Low altitude windshear is a sudden change of wind velocity along the final approach path or along the runway and along the take-off and initial climb-out path. Vertical windshear is the change of wind vector with height. Horizontal windshear is the change of wind vector with horizontal distance. The most potent examples of windshear are associated with thunderstorms (cumulonimbus clouds), but windshear can also be experienced in association with other meteorological features such as:

•The passage of a front.

•A marked temperature inversion.

•A low level wind maximum, or

•A turbulent boundary layer.

Topography or buildings can exacerbate the situation, particularly when there is a strong wind.

The effect of windshear is an abrupt displacement from the flight path and the need for substantial control action to counter it. A windshear encounter is a highly dynamic event which can be extremely uncomfortable which can strike suddenly with devastating effects. An encounter may cause alarm, a damaged landing gear, or a total catastrophe. The most vital defence is avoidance.

Before a pilot can apply the recovery techniques he will have been taught in training, he must be able to recognize that the aircraft is encountering windshear. There is an unavoidable time lag between the pilot first seeing the signs, recognizing them, applying the appropriate recovery techniques and the aircraft responding accordingly. Reducing the time lag to a minimum means early recognition of the windshear condition by the pilot and the unhesitating application of the recommended windshear recovery techniques. Recognition and reaction times are largely a function of training, by giving the pilot the knowledge to quickly spot the first sign of windshear and the confidence to apply recovery techniques without hesitation.

Indications that an aircraft is encountering windshear may be derived from the flight deck instruments, from special on-board windshear warning equipment, from windshear warnings or other pilots’ windshear reports or from external MET clues. Moreover, whether and how quickly the pilot recognizes the instrument and MET signs for windshear depends on factors such as whether the pilot has been forewarned to expect windshear and is therefore alert to the possibility, and the extent to which windshear has figured in the pilot’s training.

Where an aeroplane encounters an abrupt decrease in headwind component or an increase in tailwind component, the indicated airspeed will decrease commensurate with the loss of headwind component with no decrease in ground speed. This will result in loss of lift and an increased sink rate during any approach phase. In this energy loss situation, the only remedy is to apply engine power to compensate for the energy loss and accelerate the aeroplane back to the approach reference speed.

An increase in headwind component or a decrease in tailwind component is an energy gaining situation. IAS will increase with no increase in ground speed resulting in greater lift. The gain in energy will be temporary because the gain will be compensated in additional lift. Because the energy gain is temporary, and the flight phase is such that a gain in height is required, no control action would be required as the desired climb rate v IAS will quickly be re-established.

Windshear and Microburst 13

Microburst and Windshear 13

Recognition of external MET clues to the possible presence of low-level windshear near an airport permits the pilot to make an early decision to avoid an encounter by going around or by delaying the approach or take-off until conditions improve. External clues that may be directly visible to the pilot include the following:

(a)Strong, gusty surface winds, especially where the aerodrome is located near hills or where there are comparatively large buildings near the runway,

b)Lenticular cloud (smooth lens-shaped altocumulus) indicating the presence of standing waves, usually downwind from a mountain,

c)Virga, i.e. precipitation falling from the base of a cloud but evaporating before reaching the ground (especially under convective cloud) because downdraughts may still exist and reach the ground even though the precipitation itself has evaporated,

d)Roll cloud girding the base of a thunderstorm and advancing ahead of the rain belt, indicating the presence of a gust front,

e)Areas of dust raised by wind, particularly when in the form of a ring below convective clouds, indicating the presence of a downburst,

f)Wind socks responding to different winds;

g)Smoke plumes sheared, with upper and lower sections moving in different directions; and

h)Thunderstorms, which should always be assumed to have the capability of producing hazardous windshear.

Entering a downdraught from a horizontal airflow, the aeroplane’s momentum will at first keep it on its original path relative to the new direction of flow. In addition to a loss of airspeed, the change in relative airflow will also affect the angle of attack of the wing. The resulting decrease of angle of attack will cause a loss of lift which is not desirable near the ground. The risk of a downdraught will be more likely than an updraught below 1000 ft. The vital actions to counter the loss of airspeed (and lift) caused by windshear near the ground are:

•Briskly increase engine power. (Full power.)

•Raise the nose to check descent. (EU-OPs recommends 15°, unless otherwise stated.)

•Co-ordinate power and pitch.

Be prepared to carry out a missed approach rather than risk landing from a de-stabilized approach.

In a microburst situation, the combination of increasing headwind followed by a downdraught, followed by increasing tailwind will result in a temporary energy gain followed by increasing energy loss. The effect during any approach profile will be to cause the aeroplane to sink below the glide path although the first indication is the ‘ballooning’ of the energy gain. Any action to counter the energy gain will be potentially disastrous as this will compound what will happen shortly afterwards. A successful escape will depend upon an adequate reserve of engine power, height and speed. If the flight path is through the periphery of the microburst, lateral displacement will add to the problem if attempting to line up with the runway. The best defence is to expect the unexpected and know the signs of potential microburst/windshear activity.