Clear Air Turbulence (CAT)

Clear Air Turbulence (CAT)

Definition

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].

Description

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 Low Level 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 (Cb) 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.

Effects

  • 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.
  • Impaired Flight Crew Performance. Moderate or Severe turbulence can make simple tasks, including reading instruments, near impossible.

Defences

  • 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 Aeronautical Information Publications (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.

Scenarios

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.

Solutions

  • 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 passengers 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

Clear Air Turbulence encounter

On 5 December 2021 an Airbus A359-900 crew encountered a very brief episode of unexpected clear air turbulence associated with visible signs of convective weather in the vicinity and not having had prior warning, the senior cabin crew fell and was seriously injured. The Investigation concluded that the actual risk of turbulence prevailing and typical for the location and season as the end of daylight approached was greater than that perceived by the pilots despite their familiarity with the local area and its weather and that releasing the cabin crew from their previously secured positions had been inappropriate.

On 16 January 2022, an Airbus A320 in the cruise unexpectedly and very briefly encountered light clear air turbulence. Despite being secured in their seat, one passenger sustained a serious injury not assessed by the passenger concerned or the cabin crew as such at the time but which subsequently resulted in hospitalisation with a broken rib. The minor turbulence encountered had included a lateral movement which caused firm impact with the seat armrest. The operators’ comprehensive response included amending the safety briefing and related procedures and introducing a new video on turbulence awareness to be shown immediately after the briefing.

On 12 October 2019, an ATR 42-500 on which Captain upgrade line training was being conducted encountered mild clear air turbulence soon after descent began and despite setting flight idle power, a concurrent speed increase led to concern at a possible VMO exceedence. An abrupt and ultimately simultaneous manual increase in pitch attitude followed leading to serious injury to the unsecured cabin crew which rendered them unfit to work. The Investigation found that the upset - a change in pitch from -2.3° to +6.3°in one second - was almost entirely due to pitch input from both pilots rather than turbulence.

On 17 January 2021, a Boeing 777-300 which had just begun descent into Beirut encountered unexpected moderate to severe clear air turbulence which resulted in one major and several minor injuries to unsecured occupants including cabin crew. The Investigation found that the flight crew had acted in accordance with all applicable procedures on the basis of information available to them but noted that the operator’s flight watch system had failed to generate and communicate a message about a relevant SIGMET until after the severe turbulence episode due to a data processing issue not identified as representing an operational safety risk.

On 13 September 2017, the airspeed of a Boeing 737-800 unexpectedly increased during an intentionally high speed descent and the Captain’s overspeed prevention response, which followed his taking over control without following the applicable procedure, was inappropriate and led directly to cabin crew injuries, one of which was serious. The Investigation found that the speed increase had been the result of a sudden decrease in tailwind component associated with windshear and noted that despite moderate clear air turbulence being forecast for the area, this had not resulted in the seat belt signs being on or any consequent cabin crew briefing.

Turbulence Injury - Cabin Crew

On 5 December 2021 an Airbus A359-900 crew encountered a very brief episode of unexpected clear air turbulence associated with visible signs of convective weather in the vicinity and not having had prior warning, the senior cabin crew fell and was seriously injured. The Investigation concluded that the actual risk of turbulence prevailing and typical for the location and season as the end of daylight approached was greater than that perceived by the pilots despite their familiarity with the local area and its weather and that releasing the cabin crew from their previously secured positions had been inappropriate.

On 31 July 2021, a Boeing 737-800 descending through an area of convective activity which was subject to a current SIGMET encountered some anticipated moderate turbulence whilst visually deviating around storm cells without reducing speed. When it appeared possible that the maximum speed may be exceeded because of turbulence, the autopilot was disconnected and a severe pitch up and then down immediately followed resulting in serious injuries to two of the four cabin crew and a passenger. This disconnection was contrary to the aircraft operator’s procedures and to the explicit training received by the pilot involved who was in command.

On 25 July 2021, a Boeing 737-800 which had previously been manoeuvring visually around storm cells over the Alps during the initial descent into Nice turned back on track believing the avoidance action was complete but was then unable to avoid penetrating a further cell during which severe turbulence caused a serious injury to one of the cabin crew and a lesser injury to another. Multiple aircraft in the area had been simultaneously requesting track deviations at the time with ATC displays not showing weather returns. In the absence of plans to introduce this, a corresponding safety recommendation was made.

On 17 January 2021, a Boeing 777-300 which had just begun descent into Beirut encountered unexpected moderate to severe clear air turbulence which resulted in one major and several minor injuries to unsecured occupants including cabin crew. The Investigation found that the flight crew had acted in accordance with all applicable procedures on the basis of information available to them but noted that the operator’s flight watch system had failed to generate and communicate a message about a relevant SIGMET until after the severe turbulence episode due to a data processing issue not identified as representing an operational safety risk.

On 13 September 2017, the airspeed of a Boeing 737-800 unexpectedly increased during an intentionally high speed descent and the Captain’s overspeed prevention response, which followed his taking over control without following the applicable procedure, was inappropriate and led directly to cabin crew injuries, one of which was serious. The Investigation found that the speed increase had been the result of a sudden decrease in tailwind component associated with windshear and noted that despite moderate clear air turbulence being forecast for the area, this had not resulted in the seat belt signs being on or any consequent cabin crew briefing.

Pax Turbulence Injury - Seat Belt Signs on

On 31 July 2021, a Boeing 737-800 descending through an area of convective activity which was subject to a current SIGMET encountered some anticipated moderate turbulence whilst visually deviating around storm cells without reducing speed. When it appeared possible that the maximum speed may be exceeded because of turbulence, the autopilot was disconnected and a severe pitch up and then down immediately followed resulting in serious injuries to two of the four cabin crew and a passenger. This disconnection was contrary to the aircraft operator’s procedures and to the explicit training received by the pilot involved who was in command.

On 17 January 2021, a Boeing 777-300 which had just begun descent into Beirut encountered unexpected moderate to severe clear air turbulence which resulted in one major and several minor injuries to unsecured occupants including cabin crew. The Investigation found that the flight crew had acted in accordance with all applicable procedures on the basis of information available to them but noted that the operator’s flight watch system had failed to generate and communicate a message about a relevant SIGMET until after the severe turbulence episode due to a data processing issue not identified as representing an operational safety risk.

On 10 July 2019 an Airbus A380 in the cruise at night at FL 400 encountered unexpectedly severe turbulence approximately 13 hours into the 17 hour flight and 27 occupants were injured as a result, one seriously. The detailed Investigation concluded that the turbulence had occurred in clear air in the vicinity of a significant area of convective turbulence and a jet stream. A series of findings were related to both better detection of turbulence risks and ways to minimise injuries if unexpectedly encountered with particular reference to the aircraft type and operator but with wider relevance.

On 13 February 2019, a Boeing 737-800 en-route over the southern Adriatic Sea unexpectedly encountered severe clear air turbulence and two unsecured cabin crew and some unsecured passengers were thrown against the cabin structure and sustained minor injuries. The Investigation found that the Captain had conducted the crew pre-flight briefing prior to issue of the significant weather chart applicable to their flight by which time severe turbulence due to mountain waves at right angles to an established jetstream not shown on the earlier chart used for the briefing was expected at a particular point on their route.

On 19 April 2018, a Boeing 787-8 suddenly encountered a short period of severe turbulence as it climbed from FL160 towards clearance limit FL 190 during a weather avoidance manoeuvre which had taken it close to the Amritsar overhead and resulted in a level bust of 600 feet, passenger injuries and minor damage to cabin fittings. The Investigation found that the flight had departed during a period of adverse convective weather after the crew had failed to download a pre flight met briefing or obtain and review available weather updates.

Pax Turbulence Injury - Seat Belt Signs off

On 16 January 2022, an Airbus A320 in the cruise unexpectedly and very briefly encountered light clear air turbulence. Despite being secured in their seat, one passenger sustained a serious injury not assessed by the passenger concerned or the cabin crew as such at the time but which subsequently resulted in hospitalisation with a broken rib. The minor turbulence encountered had included a lateral movement which caused firm impact with the seat armrest. The operators’ comprehensive response included amending the safety briefing and related procedures and introducing a new video on turbulence awareness to be shown immediately after the briefing.

On 16 January 2020 an Airbus A380 in the cruise at FL 400 in an area of correctly forecast convective turbulence encountered severe turbulence not anticipated by the crew who had not put on the seatbelt signs or alerted the cabin crew in time for the cabin to be secured. An unsecured passenger was seriously injured and several other passengers and an unsecured member of cabin crew were lifted off their feet but managed to avoid injury. The Investigation concluded that the flight crew had not made full use of the capabilities of the available on board weather radar equipment.

On 15 August 2019, a Boeing 787 descending towards destination Beijing received ATC approval for convective weather avoidance but this was then modified with both a new track requirement and a request to descend which diminished its effectiveness. A very brief encounter with violent turbulence followed but as the seat belt signs had not been proactively switched on, the cabin was not secured and two passengers sustained serious injuries and two cabin crew sustained minor injuries. The Investigation noted that weather deviation requests could usefully be accompanied by an indication of how long they were required for.

On 2 February 2020, an Airbus A380 in the cruise at night at FL 330 encountered unforecast clear air turbulence with the seatbelt signs off and one unsecured passenger in a standard toilet compartment sustained a serious injury as a result. The Investigation noted that relevant airline policies and crew training had been in place but also observed a marked difference in the availability of handholds in toilet compartments provided for passengers with disabilities or other special needs and those in all other such compartments and made a corresponding safety recommendation to standardise and placard handhold provision in all toilet compartments.

On 21 August 2019, an Airbus A340-600 encountered sudden-onset moderate to severe clear air turbulence whilst in the cruise at FL 360 over northern Turkey which resulted in a serious passenger injury. The Investigation found that the flight was above and in the vicinity of convective clouds exhibiting considerable vertical development but noted that neither the en-route forecast nor current alerting had given any indication that significant turbulence was likely to be encountered. It was noted the operator s flight crew had not been permitted to upload weather data in flight but since this event, that restriction had been removed.

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