TURBULENCE

Question 1

Define windshear

Answer 1

A sudden change in wind direction over a short distance, either horizontally or vertically

Question 2

What are severe windshears created as a result of

Answer 2

Strong temp or density gradients

Question 3

How does windshear affect us in the takeoff

Answer 3

Moderate to strong XWC
Sudden TWC results in increase in takeoff ground roll required.
Sink shortly after getting airborne

Question 4

How can we detect low level windshear.
What should cause alarm bells?

Answer 4

2000ft wind on TAFs/ATIS are there for comparison to surface winds.
Any difference in wind angle and/or speed, anything greater than 35kts difference should cause alarm bells.

Question 5

The lower the Shear zone (or change between HWC and calm winds) on approach, the higher the?

Answer 5

Chance of experiencing unrecoverable sink prior to touchdown.

Question 6

What is low level shear usually caused by?

Answer 6

One of the below four;
1. Surface friction
2. Thunderstorms
3. Temp inversions
4. Frontal activity

Question 7

Examples of how surface friction causes windshear?

Answer 7

Wind flow over large buildings near runways
Strong wind flow down river valleys

Essentially, bigger the object (right up to mountain ranges), the greater the induced turb within the friction layer = larger difference between strong, laminar wind flow above, and turbulent air below

Question 8

What causes a first gust from a TS?

Answer 8

Caused by strong downdrafts from mature TS striking the ground & spreading out horizontally.
Wind direction can change by 180deg & speeds up to 100kts as far as 16km ahead of a TS.

Question 9

What is a downburst

Answer 9

Extremely intense downdraught from a TS. Can exceed >720fpm

Question 10

Define/describe a microburst

Answer 10

A smaller version of a down burst (less than 4km across). These can originate in the downdraughts in active TS, but can also occur in high-based CBs.
Incredibly dangerous in the takeoff/landing environment.

Question 11

How do temperature inversions cause windshear

Answer 11

Essentially the inversion separates strong upper winds and calmer winds closer to the surface, particularly at night.

Question 12

RoT wrt fronts & when to expect windshear

Answer 12

Temp difference across the front >5deg or if front is moving faster than 30kts.

Question 13

ICAO classification of turbulence: Moderate

Answer 13

Small variations in airspeed and changes in G of 0.5-1G. moderate changes in aircraft attitude/altitude but a/c remains in positive control.

Question 14

ICAO classification of turbulence: Severe

Answer 14

Large changes in airspeed & >1G . Abrupt changes in a/c attitude/altitude, some loss of control for short periods

Question 15

Turbulence causal factors?

Answer 15

1. Convective
2. Mechanical
3. Wake turb
4. CAT

Question 16

Describe convective turbulence - what is it caused by?

Answer 16

Is the result of unequal heating of the earths surface.
I.e. tar seal heats faster than a pond, therefore more lilting & expansion of air happens over the tar seal in comparison to the pond.
As air lifts, the rising air must be replaced, therefore for every parcel of rising air, a compensating down-current of cooler air exists.
A circular motion develops.
In large cases & when moisture is present, CBs will develop.

Question 17

Describe Mechanical turbulence

Answer 17

Air flow near the earth’s surface flows over/around objects on the ground (similar to water & rocks in a river).
Upwind of obstacle there is no effect, downstream however the stronger the airflow/larger the object, the more pronounced the turbulent flow.