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Clear Air Turbulence
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Clear Air Turbulence is defined as turbulence which is not associated with cloud and therefore cannot be detected visually or by conventional weather radar.
There are two types of CAT:
- Mechanical. Disruption to the smooth horizontal flow of air.
- Thermal. Turbulence caused by vertical currents of air in an unstable atmosphere.
Common causes and sources of CAT are:
- Jet Stream. A Jet Stream is a narrow, fast moving current of air, normally close to the Tropopause and generated as a result of the temperature gradient between air masses. Although not all jet streams have CAT associated with them, there can be significant vertical and horizontal Wind Shear on the edges of the jet stream giving rise to sometimes severe clear air turbulence. Any CAT is strongest on the cold side of the jet stream where the wind shear is greatest. In the vicinity of a jet stream, CAT can be encountered anywhere from 7,000 feet below to about 3,000 feet above the tropopause. Because the strong vertical and horizontal wind shear occurs over short distances, this jet stream related CAT tends to be shallow and patchy so a descent or climb of as little as 2,000 feet is often enough to exit the turbulence.
- Terrain. High ground disturbs the horizontal flow of air over it, causing turbulence. The severity of the turbulence depends on the strength of the air flow, the roughness of the terrain, the rate of change and curvature of contours, and the elevation of the high ground above surrounding terrain.
- Thunderstorm Complexes. Cumulonimbus cells have strong vertical currents. Aircraft passing within 20 nautical miles horizontally, or less than 5,000 feet above the top, of a Cb may encounter CAT.
- Structural Damage. Aircraft can suffer structural damage as a result of encountering severe clear air turbulence. In extreme cases this can lead to the break-up of the aircraft. In even moderate turbulence, damage can occur to fittings within the aircraft, especially as a result of collision with unrestrained items of cargo or passenger luggage. Prolonged exposure to turbulence will shorten the fatigue life of the aircraft.
- Physical Injury to Crew/Passengers. If caught unaware, passengers and crew moving around in the aircraft cabin can be injured. In one case, where a B747 encountered CAT over the Pacific ocean, several passengers and crew were severely injured and one passenger subsequently died (see Further Reading).
- Impaired Flight Crew Performance. Moderate or Severe turbulence can make simple tasks, including reading instruments, near impossible.
- Awareness. SIGMET charts give forecasts of the location and level of clear air turbulence. Information on local terrain induced CAT may be contained in appropriate AIPs e.g. Approach plates for Gibraltar contain information on turbulence to be expected for given wind directions.
- Restraint Systems. Passengers and crew should fit seat belts and harnesses when seated to protect them in the event of unforeseen turbulence.
An aircraft descending for an approach into Milan encounters moderate turbulence associated with a southerly airflow over the Alps. A member of the cabin crew checking the security of the cabin falls breaking an arm.
- Slow down. Reducing the aircraft speed reduces the risk of structural damage and reduces vibration making instruments easier to read.
- Strap in. Notify the crew/illuminate seat belt sign. All passsangers and crew should immediately sit down and fit seat belts/harnesses.
- Inform ATC. Notify ATC/warn other aircraft on chat or guard/emergency frequency (121.5 or 243.0). Request clearance to climb/descend or diverge from track to escape turbulence.
- Assess Damage/Injuries. Carry out a damage assessment and ascertain condition of any injured passengers. Consider precautionary diversion.
- Suspend Cabin Service. Obviously the serving of hot drinks and meals during turbulent conditions puts both cabin crew and passengers at risk.
- B741, en-route, Pacific Ocean, 1997 (WX): On 28th December 1997, a 747-100, operated by United Airlines, which had departed from Tokyo heading for Hawaii, encountered severe turbulence thought to have been associated with a jet stream over the Pacific Ocean.
- A321, en-route, Vienna Austria, 2003 (WX LB LOC): On 26th May 2003, a British Midland A321 suffered severe damage from Hail en route near Vienna.
- Bureau d’Enquêtes et d’Analyses (BEA) Incidents in Air Transport No 5 - Wind Gradients and Turbulence