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Clear Air Turbulence (CAT)
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Clear Air Turbulence (CAT) is defined as sudden severe turbulence occurring in cloudless regions that causes violent buffeting of aircraft. This term is commonly applied to higher altitude turbulence associated with wind shear. The most comprehensive definition is high-altitude turbulence encountered outside of convective clouds. This includes turbulence in cirrus clouds, within and in the vicinity of standing lenticular clouds and, in some cases, in clear air in the vicinity of thunderstorms. Generally, though, CAT definitions exclude turbulence caused by thunderstorms, low-altitude temperature inversions, thermals, strong surface winds, or local terrain features. [source:FAA AC 00-30C]
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. For further information, refer to the article entitled Mountain Waves.
- 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.
- Switch on Engine Ignition - Certain aircraft types recommend turning ignition on to prevent the turbulent airflow from flaming out engines.
- 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.
Accidents and Incidents
Turbulence Injury - Cabin Crew
- CL60 / A388, en-route, Arabian Sea, 2017 (On 7 January 2017, the crew of a Bombardier Challenger en route at FL340 over international waters between India and the Arabian Peninsula temporarily lost control of their aircraft approximately one minute after an Airbus A380 had passed 1,000 feet above them tracking in the opposite direction. The Investigation is ongoing but has noted that both aircraft were in compliance with their air traffic clearances, that a major height loss occurred during loss of control with some occupants sustaining serious injuries and that after successfully diverting, the structure of the aircraft was found to have been damaged beyond economic repair.)
- B733, en-route, Santa Barbara CA USA, 1999 (On 2 September 1999, a United Airlines Boeing Boeing 737-300 in the cruise at FL240, experienced severe turbulence due to an encounter with the wake vortex from a preceding MD11 on a similar track which had climbed through the level of the B737 with minimum lateral separation, 1.5 minutes earlier.)
- A333, en-route, Kota Kinabalu Malaysia, 2009 (On 22 June 2009, an Airbus A330-300 being operated by Qantas on a scheduled passenger flight from Hong Kong to Perth encountered an area of severe convective turbulence in night IMC in the cruise at FL380 and 10 of the 209 occupants sustained minor injuries and the aircraft suffered minor internal damage. The injuries were confined to passengers and crew who were not seated at the time of the incident. After consultations with ground medical experts, the aircraft commander determined that the best course of action was to complete the flight as planned, and this was uneventful.)
- B772, en-route, Northern Kanto Japan, 2014 (On 16 December 2014, a US-operated Boeing 777-200 encountered a significant period of severe clear air turbulence (CAT) which was unexpected by the flight crew when travelling eastbound over northern Japan at night between FL 270 and FL290. The decision to turn back to Tokyo to allow the nine seriously injured passengers and crew to be treated was made 90 minutes later. The Investigation concluded that the CAT encountered had been correctly forecasted but the Operator's dispatcher-based system for ensuring crew weather awareness was flawed in respect of international operations out of 'non hub' airports.)
- B741, en-route, Pacific Ocean, 1997 (On 28th December 1997, a Boeing 747-100 being operated by United Airlines, which had departed from Tokyo for Hawaii, encountered severe turbulence thought to have been associated with a Jet Stream over the Pacific Ocean.)
Pax Turbulence Injury - Seat Belt Signs on
- B773, en-route, east northeast of Anchorage AK USA, 2015 (On 30 December 2015, a Boeing 777-300 making an eastbound Pacific crossing en-route to Toronto encountered forecast moderate to severe clear air turbulence associated with a jet stream over mountainous terrain. Some passengers remained unsecured and were injured, one seriously and the flight diverted to Calgary. The Investigation found that crew action had mitigated the injury risk but that more could have been achieved. It was also found that the pilots had not been in possession of all relevant information and that failure of part of the air conditioning system during the turbulence was due to an improperly installed clamp.)
Pax Turbulence Injury - Seat Belt Signs off
- A332, en-route, near Bangka Island Indonesia, 2016 (On 4 May 2016, an Airbus A330-200 in the cruise in day VMC at FL390 in the vicinity of a highly active thunderstorm cell described by the crew afterwards as ‘cumulus cloud’ encountered a brief episode of severe clear air turbulence which injured 24 passengers and crew, seven of them seriously as well as causing some damage to cabin fittings and equipment. The Investigation was unable to determine how close to the cloud the aircraft had been but noted the absence of proactive risk management and that most of the injured occupants had not been secured in their seats.)
- FAA AC00-30C: Clear Air Turbulence Avoidance
- B741, en-route, Pacific Ocean, 1997: 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.
- Bureau d’Enquêtes et d’Analyses (BEA) Incidents in Air Transport No 5 - Wind Gradients and Turbulence