Turbulence

Turbulence can be defined as small-scale, short-term, random and frequent changes to the velocity of air. In other words, when there are rapid changes to either the air’s speed or its direction of movement or both, conditions are said to be turbulent. (Wagtendonk, 2003) When an aircraft flys through this disturbed air, it will experience turbulence.

## Types of turbulence

There are four fundamental causes of turbulence:

Thermal:
When the surface is sufficiently warm, vertical currents of air form. As an aircraft flys through these, the occupants will experience turbulent conditions.

Mechanical:
This is caused by interference of surface features on the horizontal flow of air. This could include mountains, Tall buildings, Trees, etc.
The amount of turbulence depends on speed of wind, the size of the obstruction, the shape of obstruction and atmospheric conditions.
An aircraft will experience turbulence as it flys over and behind this object.

Shear:
When the direction or speed of wind changes dramatically within a short horizontal or vertical distance, an aircraft will experience turbulence when it flys through this boundary.

Aerodynamic:
Turbulence caused by an aircraft as it flys through the air. Also known as wake turbulence. If another aircraft were to fly behind this, it would experience turbulence.

 Turbulence from aircraft swirling clouds below. Picture embedded from NY Times on Aug 03, 2009 Picture showing area affected by aerodynamic turbulence created by an aircraft. Picture embedded from Pilot Friend on Aug 03, 2009
 Picture showing the different types of turbulence that can affect an aircraft. In the first segment the aircraft is experiencing Thermal turbulence. When the aircraft flys over the mountain it is then experiencing Mechanical turbulence. As it flys through the thunderstorm cloud it experiences Shear turbulence as it passes through the different flows of air within the thunderstorm (image embedded from Aviation Explorer on 3 August 2009

## Intensities of Turbulence

Turbulence is separated into four levels of intensity. Each different level of intensity can be described from both ‘reaction of the aircraft’, as well as the ‘reaction inside the aircraft’. These four levels are described below.

 LEVEL Aircraft Reaction What Occupants feel Level 1, Light Turbulence that momentarily causes slight, erratic changes in altitude and/ or attitude (pitch, roll, yaw) Occupants may feel a slight strain against seat belts or shoulder straps. Unsecured objects may be displaced slightly. Food service may be conducted and little or no difficulty is encountered in walking. Level 2, Moderate This will cause changes in accelerometer readings of around 0.5 – 1.0g at the aircraft’s centre of gravity. Changes in altitude and/ or attitude occur but the aircraft remains in positive control at all times. It usually causes variations in indicated airspeed Occupants feel definite strains against seat belts or shoulder straps. Unsecured objects are dislodged. Food service and walking are difficult. Level 3, Severe This will cause changes in the accelerometer reading of greater than 1g at aircraft’s centre of gravity. Large, abrupt changes in altitude and/ or attitude. It usually causes large variations in indicated airspeed. Aircraft may be momentarily out of control Occupants are forced violently against seat belts or shoulder straps. Unsecured objects are tossed about. Food services and walking are impossible. Level 4, Extreme Turbulence in which the aircraft is violently tossed about and is practically impossible to control.

These four descriptors are used to distinguish between the four levels of intensity of atmospheric turbulence. It is crucial for every pilot to fully understand these turbulence descriptors, as they appear often in weather forecasts and reports. In terms of avoidance, the pilot needs to know both their personal and the aircraft's turbulence limit, and never fly into a turbulent situation beyond personal and aircraft capability. The pilot should also consider passenger comfort before flying into an area of known turbulence.

# Preventing Passenger Injury

In-flight turbulence is the leading cause of injuries to passengers and crew. Occupants injured during turbulence are usually not wearing seatbelts, ignoring recommendations to keep seatbelts fastened even when the signs are not illuminated. It is recognised that passengers need to move around the cabin to use restroom facilities or to exercise on long flights. However you should keep your seatbelt fastened at all times when seated.

From 1981 through 1997 there were 342 reports of turbulence affecting major air carriers. Three passengers died, two of these fatalities were not wearing their seat belt while the sign was on. 80 suffered serious injuries, 73 of these passengers were also not wearing their seat belts.

# Turbulence related incidents

The following are recent jet airliner mishaps from around the world. In each event, at least one passenger/flight attendant was injured during an unexpected turbulence encounter. (Infomation taken from CASA website)

• During a flight from Singapore to Sydney with 236 passengers and 16 crew, the airplane encountered turbulence over central Australia. The plane hit an "air pocket" which caused it to drop 300 feet. Nine passengers including one pregnant woman and three crew members suffered various neck, back and hip injuries, with one of the passengers requiring surgery. Those who were injured were not wearing seat belts.
• During a flight from Japan to Brisbane 16 passengers were injured when a large aircraft encountered turbulence. Passengers had been advised to keep their seatbelts fastened while seated. The pilot in command reported that flight conditions were smooth prior to encountering the turbulence. The weather radar did not indicate adverse weather, so the crew did not turn on the seatbelt signs. A number of the passengers who were not wearing their seatbelts were injured when they were thrown from their seats.
• A jet hit air turbulence shortly before it landed at a Hong Kong airport, injuring 47 people, seven of them seriously. "It happened very suddenly and everything was very chaotic," one of the 160 passengers aboard the flight said. "The plane just dropped and I saw things flying all over."
 Video explaining the causes of turbulence, aimed at those who have a fear of flying, or who get nervous when they experience turbulence while flying (embedded from YouTube on 03 August 2009)

# Bibliography

Wagtendonk, W. (2003). Meteorology for Professional Pilots. Bay of Plenty, New Zealand: Aviation Theory Centre (NZ) Ltd.

# Want to know more?

'Atmospheric Turbulence' Britannica Online
More explanation on the term 'Atmospheric Turbulence'.
Wikipedia, Turbulence
Wikipedia, CAT
Information on Clear Air Turbulence, Definitions and when an aircraft can experience this phenomenon
CASA
Australian Civil Aviation Safety Authority, Article on Turbulence
Pilot Friend
In depth information relating to wake turbulence, how it is created, and how pilots can avoid flying into it.
Aviation Knowledge - Wake Turbulence