Chapter

15

Windshear

Introduction (Ref: AIC 84/2008)					489
Microburst					489
Windshear Encounter during Approach					491
Effects of Windshear . . . . . . . . . .	. . . . . . . . . . . . .	. .	.	.	492
“Typical” Recovery from Windshear . . . .	. . . . . . . . . . . . . .	. .	.	.	494
Windshear Reporting					495
Visual Clues . . . . . . . . . . . . .	. . . . . . . . . . . . .	. .	. .		. 495
Conclusions . . . . . . . . . . . . .	. . . . . . . . . . . . .	. .	. .		. 495
Questions . . . . . . . . . . . . . .	. . . . . . . . . . . . .	. .	.	.	496
Answers . . . . . . . . . . . . . .	. . . . . . . . . . . . . .	. .	.	.	500

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Windshear 15

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Windshear 15

Introduction (Ref: AIC 84/2008)

Windshear is a sudden drastic shift in wind speed and/or direction that occurs over a short distance at any altitude in a vertical and/or horizontal plane. It can subject an aircraft to sudden updraughts, downdraughts or extreme horizontal wind components, causing sudden loss of lift or violent changes in vertical speeds or altitudes. Windshear will cause abrupt displacement from the flight path and require substantial control action to counteract it.

A windshear encounter is a very dynamic event which can strike suddenly and with devastating effect which has been beyond the recovery powers of experienced pilots flying modern and powerful aircraft. An encounter may cause alarm, a damaged undercarriage or a total catastrophe. The first and most vital defence is avoidance.

The most powerful 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, or a marked low-level temperature inversion. The meteorological features of windshear will be dealt with fully elsewhere.

Microburst

Microbursts are associated with thunderstorms and are one of the most dangerous sources of windshear. Microbursts are small-scale intense downdraughts which, on reaching the surface, spread outward in all directions from the downdraught centre. This causes the presence of both vertical and horizontal windshear that can be extremely hazardous to all types and sizes of aircraft, especially when within 1000 feet of the ground.

A microburst downdraught is typically less than 1 mile in diameter as it descends from the cloud base to about 1000 to 3000 feet above the ground. In the transition zone near the ground, the downdraught changes to a horizontal outflow that can extend to approximately 2.5 miles (4 km) in diameter.

•Downdraughts can be as strong as 6000 feet per minute.

•Horizontal winds near the surface can be as strong as 45 knots resulting in a 90 knot shear as the wind changes to or from a headwind across the microburst.

•These strong horizontal winds occur within a few hundred feet of the ground. An individual microburst seldom lasts longer than 15 minutes from the time it strikes the ground until dissipation.

These are maximum values but they do indicate how it is possible for large and powerful aircraft to become uncontrollable when they meet such examples of the microburst.

Windshear 15

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A microburst intensifies for about 5 minutes after it first strikes the ground, with the maximum intensity winds lasting approximately 2 to 4 minutes. Sometimes microbursts are concentrated into a line structure and, under these conditions, activity may continue for as long as an hour. Once microburst activity starts, multiple microbursts in the same general area are not uncommon and should be expected.

			STRONG DOW NDRAUGHT
		Increasing Tailwind		Increasing Headwind
		Outflow		Outflow
	15			2
			3	1
	Windshear	4
			Figure 15.1 A microburst encounter during take-off

During take-off into a microburst, shown in Figure 15.1, an aircraft first experiences a headwind which increases performance without a change in pitch and power (1).

This is followed by a decreasing headwind and performance, and a strong downdraft (2).

Performance continues to deteriorate as the wind shears to a tailwind in the downdraft (3).

The most severe downdraft will be encountered between positions 2 and 3, which may result in an uncontrollable descent and impact with the ground (4).

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