What is mid-air turbulence?

• An Air India flight from Phuket to New Delhi faced mid-air turbulence and the aircraft suddenly lost 300 feet in altitude on August 4.

• At least 17 people onboard were injured.

• The narrow-body A320 neo aircraft, operating flight AI2379 from Phuket, landed safely at Delhi airport. 

• There were 137 passengers and eight crew members on board.

• The Directorate General of Civil Aviation (DGCA) is probing the incident.

• The aircraft has been moved to the hangar and the Flight Data Recorder and Cockpit Voice Recorder have been secured for detailed examination. 

What is turbulence?

• Turbulence is air movement that normally cannot be seen and often occurs unexpectedly. 

• It may be as insignificant as a few annoying bumps or severe enough to momentarily throw an airplane out of control or to cause structural damage. 

• It can be created by many different conditions, including atmospheric pressure, jet streams, air around mountains, cold or warm weather fronts or thunderstorms. 

• The basic causes of turbulence are wind shear (change in wind strength and direction horizontally and/or vertically) and thermal instability (buoyancy of air) which, working together or independently, produce a local random variation in wind velocity.

• Turbulence can even occur when the sky appears to be clear.

• While turbulence is normal and happens often, it can be dangerous. Its bumpy ride can cause passengers who are not wearing their seat belts to be thrown from their seats without warning.

The intensity of turbulence is categorised as follows:

1) Light: Effects are less than those of moderate intensity.

2) Moderate: There may be moderate changes in aircraft attitude and/or height but the aircraft remains in control at all times. Air speed variations are usually small. Occupants feel strain against seat belts. There is difficulty in walking. Loose objects move about.

3) Severe: Abrupt changes in aircraft attitude and/or height. The aircraft may be out of control for short periods. Air speed variations are usually large. Objects are forced violently against seat belts. Loose objects are tossed about.

4) Extreme: Effects are more pronounced than for severe intensity.

Types of turbulence:

1) Mechanical turbulence: When the air near the surface of the Earth flows over obstructions such as bluffs, hills, mountains, or buildings, the normal horizontal wind flow is disturbed and transformed into a complicated pattern of eddies and other irregular movements. The intensity of the turbulence generally increases with the wind speed and surface roughness.

2) Mountain wave turbulence: Can occur above and downwind of topographic barriers in strong winds and a stable environment. Air will oscillate in a series of waves and rotors as it moves downstream. 

3) Clear air turbulence (CAT): High-level turbulence (above 15,000 ft) not normally associated with cumuliform clouds. More prevalent in regions of wind shear, converging winds, horizontal wind deformation, temperature lapse rate discontinuities or strong thermal gradients. The main synoptic features associated with CAT are jet streams, high altitude lows or troughs, or the tropopause.

4) Thunderstorm turbulence: Turbulence associated within and in the vicinity of thunderstorms or cumulonimbus clouds. The most turbulent areas in towering cumulus and cumulonimbus cloud are the updraft/downdraft boundaries within the cloud, the leading edge of the gust front, above the cloud tops, upper parts of the updraft within the cloud, and in any funnel cloud (tornadoes and waterspouts).

5) Thermal (convective) turbulence: Localised columns of convective current (a rising column of warm air) resulting from surface heating or cold air moving over warmer ground. For every rising current there is a compensating downward current, usually slower in speed since it covers a broader area, causing turbulence. Sometimes observed as a “dust devil”.