Windshear 15

Windshear Encounter during Approach

The power setting and vertical velocity required to maintain the glide slope should be closely monitored. If any windshear is encountered, it may be difficult to stay on the glide path at normal power and descent rates. If there is ever any doubt that you can regain a reasonable rate of descent, and land without abnormal manoeuvres, you should apply full power and go-around or make a missed approach.

Windshear can vary enormously in its impact and effect. Clearly some shears will be more severe and consequently more dangerous than others.

When countering the effects of windshear, it is best to assume ‘worse case’. It is impossible to predict at the first stages of a windshear encounter how severe it will be, and it is good advice to suggest that recovery action should anticipate the worst.

Figure 15.2 Indications & recovery actions for windshear encounter during approach

Referring to Figure 15.2, this table gives guidance should you encounter windshear during a stabilized landing approach. Approaches should never be attempted into known windshear conditions.

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15 Windshear

Effects of Windshear

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Windshear 15

'ENERGY LOSS' - Effect of downdraught.

10 kt

Figure 15.5 “Energy loss” due to loss of headwind

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15 Windshear

“Typical” Recovery from Windshear

The combination of increasing headwind, followed by downdraught, followed by increasing tailwind should be considered, as this is the sequence which might be encountered in a microburst on the approach, or following take-off.

• The presence of thunderstorms should be known and obvious, so the increase in speed caused by the rising headwind should be seen as the forerunner of a down-burst or microburst; any hope of a stabilized approach should be abandoned and a missed approach carried out

aircraft beginning to escape from the effects of the microburst.

When there is an indefinite risk of shear, it may be possible to use a longer runway, or one that points away from an area of potential threat. It may also be an option to rotate at a slightly higher speed, provided this does not cause undue tyre stress or any handling problems. The high power setting and high pitch angle after rotate already put the aircraft into a good configuration should a microburst then be encountered. The aircraft is, however, very low, there is little safety margin and the ride can be rough. If there is still extra power available, it should be used without hesitation. Ignore noise abatement procedures and maintain the high pitch angle, watching out for stick shaker indications as a signal to decrease backpressure on the pitch control.

In both approach and take-off cases, vital actions are:

•Use the maximum power available as soon as possible.

•Adopt a pitch angle of around 15° and try and hold that attitude. Do not chase airspeed.

•Be guided by stick shaker indications when holding or increasing pitch attitude, easing the back-pressure as required to attain and hold a slightly lower attitude.

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