Turbulence

Question 1

Define wind shear.

Answer 1

A sudden change in wind speed and/or direction over a short distance, either horizontally or vertically.

Question 2

What motions do wind shears induce in the atmosphere and what does this generate?

Answer 2

Tumbling motions. These are experienced as turbulence.

Question 3

What atmospheric conditions are wind shears associated with (other than turbulence)? (2)

Answer 3

Strong temp gradients or density gradients.

Question 4

List two problems which may be encountered on take-off when low-level wind shear is present.

Answer 4

A moderate to strong cross-wind component e.g. terrain channeling at Wellington airport.
A major wind-shift during the take-off roll or shortly after.

Question 5

What wind forecast is available in NZ to help pilots detect the possibility of low-level wind shear?

Answer 5

The forecast 2000 ft wind added to all domestic TAFs. The greater the speed deferential and/or the greater the difference in the angle of the wind, the greater the chances of encountering low-level wind shear.

Question 6

What are the major hazards associated with low-level wind shear on the approach path?

Answer 6

The shear may produce considerable, even severe turbulence. Conversely, flight conditions may be relatively smooth, but an aircraft may suffer a marked change in airspeed as it penetrates the shear. This is turn may induce a change in vertical speed that may cause difficulties, especially if operating close to the ground.

Question 7

At what altitude can wind shear’s occur?

Answer 7

Any. The higher occurrences are classed under the term clear air turbulence and are often associated with jetstreams.

Question 8

List the four circumstances which commonly may cause low-level wind shear.

Answer 8

Surface friction.
Thunderstorms.
Temp inversions.
Frontal activity.

Question 9

Describe how surface friction can lead to wind shear.

Answer 9

Large obstructions (buildings, mountains etc) can create disruptions to wind flow leading to induced turbulence within the friction layer. This causes localised wind shears.

Question 10

Define first gust.

Answer 10

They are the rapid shift and increase of wind experienced just before a thunderstorm hits. Caused by strong downdraughts from mature thunderstorms striking the ground and spreading out horizontally.

Question 11

Define downburst.

Answer 11

An extremely intense downdraught from a thunderstorm. Usually much closer to the thunderstorm than a first gust.

Question 12

Define microburst.

Answer 12

Similar occurrence to a downburst but a much smaller (less than 4 km) and shorter lived event.

Question 13

Can down bursts and micro bursts occur in phenomenon other than thunderstorms?

Answer 13

Yes. They can occur in relatively high based CB cloud (6000 - 10,000 ft). Rain evaporates before reaching the ground. A downdraught is generated and maintained or enhanced as it descends, by the evaporating rain cooling the air in the downdraught.

Question 14

Describe how temperature inversions can lead to wind shear.

Answer 14

Overnight cooling can create temp inversions a few hundred feet above the ground. If this is coupled with strong upper winds, significant wind shear can be produced close to the ground.

Question 15

Describe how frontal activity can lead to wind shear.

Answer 15

Only occur where the temp difference across the front is 5 ºC or more or if the front is moving at 30 kt or more.

Question 16

Define turbulence.

Answer 16

An irregular motion of air characterised by gusts or sudden surges of fast moving air.

Question 17

Define moderate turbulence (ICAO classification).

Answer 17

Moderate changes in attitude or altitude but a/c remains in positive control. Small variations in air speed and changes in accelerometer readings of 05. - 1 G. Occupants feel strained again seat belts and loose objects move about.

Question 18

Define severe turbulence (ICAO classification).

Answer 18

Abrupt changes in a/c attitude or altitude; a/c out of control for short periods. Large variations in air speed with changes of 1 G + at the C of G being shown. Occupants forced violently against seat belts. Loose objects tossed about.

Question 19

List four types of turbulence.

Answer 19

Convective.
Mechanical.
Wake turbulence.
Clear air turbulence.

Question 20

Describe convective turbulence.

Answer 20

Caused by raising and descending air (convective currents), which is the result of the unequal heating of the earth’s surface. For every rising column of air there is a compensating down-current of comparatively cooler air which develops into a circular motion. An aircraft flying through this area will experience upward and downward currents.

Question 21

Describe mechanical turbulence.

Answer 21

Occurs when air near the earth’s surface flows over an obstruction which transforms it into a vastly complicated snarl of eddies. The severity depends on wind speed and the obstruction. There is little effect on the upward side.

Question 22

What is convective cloud over a mountain or along a ridge a good indicator of?

Answer 22

That turbulence is present on the windward side and over the crest (turbulence is likely to be worse on the leeward side).

Question 23

Is it likely that a METAR report of OVC018TCU is accurate?

Answer 23

No. TCu is a convective cloud and convection implies both upwards and downwards motion. The downwards motion would cause some cloud to evaporate, which would suggest that perhaps BKN would be about as much could be coded for TCu.

Question 24

Given that there is a strong wind blowing at 2000 ft AGL, explain the turbulence and wind shear implications of landing at an airfield on a cool clear morning.

Answer 24

On a cool clear morning, a radiation inversion will have formed off the surface. This inversion will have decoupled the stronger winds aloft from the surface winds which will have become light. With little wind at the surface, and strong winds at 2,000 ft, the wind shear will have been concentrated into a narrow band towards the top of the inversion, thus concentrating and increasing turbulence.

Question 25

On final approach, you notice a Cb cell immediately ahead of you at the far end of the airfield. From which direction do you anticipate the first gust coming from?

Answer 25

The first gust will come directly down the runway towards you. If encountered on short finals, it will cause the a/c to balloon on flaring.

Question 26

An Ohakea METAR reports a surface wind of 32024G40KT. Which category of turbulence is the likely source of this gustiness? From the gust strength, estimate the wind at about 2000 ft.

Answer 26

This is mechanical turbulence generated in the wind flow over the buildings, trees and sloping ground to the NW of the tower. The 2000 ft wind will be approximately 29040KT.

Question 27

Describe the the most significant visual indicator of wind shear.

Answer 27

Cb cloud. Thunderstorms and high level Cb.