Turbulence

Question 1

How does mechanical turbulence occur?

Answer 1

• When air flows near the surface, it will be disturbed by irregular features like hills and buildings.
• An irregular pattern of eddies will be superimposed on the streamlined airflow, causing gusts and lulls.

Question 2

What does the amount of mechanical turbulence depend upon? (2)
How high do light winds and strong winds produce turbulence respectively?

Answer 2

• Wind speed and roughness of the terrain.
• Light winds may produce a layer of turbulence only a few hundred feet thick, but stronger winds will produce increased turbulence up to higher levels.

Question 3

What is the largest cause of mechanical turbulence?

Answer 3

Mountains.

Question 4

How does thermal turbulence work? (3)

Answer 4

• Surface air is heated by the Earth and rises as convective currents.
• This warmer air rises and cools as a function of height, until it reaches a level of ai with the same temperature, where it will spread horizontally.
• Descending air over those areas that are relatively cold, will replace the ‘convected’ air from the ground.

Question 5

How do thunderstorms cause wind shear?

Answer 5

• Through updraughts and downdraughts inherent in the thunderstorm.
• The updraughts and downdraughts will cause the aircraft to experience gusts, which are localised irregular air flows within the draught.

Question 6

When are updraughts the strongest in a thunderstorm?

Answer 6

When the TS is still developing.

Question 7

What are the dangers of a downdraught during a thunderstorm?

Answer 7

• Downdraughts are deflected horizontally at the ground under the cloud and spread ahead of the cloud.
• The deflected down draught will appear as a cold gusty wind.

Question 8

How do thunderstorms cause windshear?

Answer 8

Due to its divergent flow near the ground, and its powerful downdraughts, windshear can occur.

Question 9

When is windshear mainly experienced?

Answer 9

Most critical and mainly experienced during take-off and landing.

Question 10

What is positive windshear?

What is negative windshear?

Answer 10

• Positive windshear is an increase in IAS.

- Negative windshear is a decrease in IAS.

Question 11

What happens if there is a reduction in headwind speed during landing? (2)

Answer 11

• Aircraft will increase its rate of descent.

- Aircraft will fall below intended glide path because the sudden drop in headwind results in loss of lift.

Question 12

What happens if there is a reduction in headwind speed during take-off?

Answer 12

Windshear will reduce the amount of lift available to the aircraft, sinking the aircraft below the intended climb out path.

Question 13

Where is low-level windshear usually located? (2)

Answer 13

• Junction of warm and cold air.

- Sea breeze fronts.

Question 14

What is Clear Air Turbulence?

Answer 14

Turbulence produced by windshear at higher levels due to the formation of clouds.

Question 15

How does the leeward side of mountains cause windshear?

Answer 15

• Due to strong winds from leeward slopes, this can cause windshear.
• Stagnant air is also found over the plane in the lee of a mountain range, causing windshear.

Question 16

How does ‘stagnant air’ affect an aircraft on approach? (2)

Answer 16

• Severe windshear will be encountered when aircraft penetrate stagnant air from above.
• Loss in altitude can occur as well as IAS decreasing.

Question 17

How is wake turbulence formed and what does it trigger?

Answer 17

• Caused by pressure differential between upper and lower wing surfaces of aircraft.
• Triggers a roll up of the airflow, aft of the wing, resulting in swirling air masses trailing downstream of the wing tips.

Question 18

What is the strength of wake turbulence determined by? (2)

Answer 18

• Weight, speed and pitch of the aircraft’s wing.

- Weight of the aircraft.

Question 19

What are the dangers associated with wake turbulence?

Answer 19

• Vortex from a large aircraft sinks at a given rate and levels off below the flight path of the generating aircraft.
• Wake can impose rolling moments (roll the aircraft like a boat, imagine it) exceeding the roll control capability of aircraft, which may cause damage to equipment.

Question 20

How do you avoid wake turbulence? (4)

Answer 20

• Change altitude.
• Move laterally i.e. to the side away from the preceding aircraft’s position.
• Obey ATC instructions.
• Following the correct separation limits.