Turbulence Flashcards
Define wind shear.
A sudden change in wind speed and/or direction over a short distance, either horizontally or vertically.
What motions do wind shears induce in the atmosphere and what does this generate?
Tumbling motions. These are experienced as turbulence.
What atmospheric conditions are wind shears associated with (other than turbulence)? (2)
Strong temp gradients or density gradients.
List two problems which may be encountered on take-off when low-level wind shear is present.
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.
What wind forecast is available in NZ to help pilots detect the possibility of low-level wind shear?
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.
What are the major hazards associated with low-level wind shear on the approach path?
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.
At what altitude can wind shear’s occur?
Any. The higher occurrences are classed under the term clear air turbulence and are often associated with jetstreams.
List the four circumstances which commonly may cause low-level wind shear.
Surface friction.
Thunderstorms.
Temp inversions.
Frontal activity.
Describe how surface friction can lead to wind shear.
Large obstructions (buildings, mountains etc) can create disruptions to wind flow leading to induced turbulence within the friction layer. This causes localised wind shears.
Define first gust.
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.
Define downburst.
An extremely intense downdraught from a thunderstorm. Usually much closer to the thunderstorm than a first gust.
Define microburst.
Similar occurrence to a downburst but a much smaller (less than 4 km) and shorter lived event.
Can down bursts and micro bursts occur in phenomenon other than thunderstorms?
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.
Describe how temperature inversions can lead to wind shear.
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.
Describe how frontal activity can lead to wind shear.
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.
Define turbulence.
An irregular motion of air characterised by gusts or sudden surges of fast moving air.
Define moderate turbulence (ICAO classification).
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.
Define severe turbulence (ICAO classification).
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.
List four types of turbulence.
Convective.
Mechanical.
Wake turbulence.
Clear air turbulence.
Describe convective turbulence.
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.
Describe mechanical turbulence.
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.
What is convective cloud over a mountain or along a ridge a good indicator of?
That turbulence is present on the windward side and over the crest (turbulence is likely to be worse on the leeward side).
Is it likely that a METAR report of OVC018TCU is accurate?
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.
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.
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.
