What is turbulence?
The term derives from the Latin word "turbulentus" meaning unruly, riotous or violently disturbed. The US Federal Aviation Administration (FAA) defines turbulence as "air movement that normally cannot be seen and often occurs unexpectedly. It can be created by many different conditions, including atmospheric pressure, jet streams, air around mountains, cold or warm weather fronts or thunderstorms". They further define Clear-Air Turbulence (CAT) as "sudden severe turbulence occurring in cloudless regions that causes violent buffeting of aircraft".
The FAA offer an analysis based upon National Transportation Safety Board (NTSB) evidence of turbulence statistics from 2009 to 2022. In that 14 year period, a total of 163 serious injuries were recorded as a result of turbulence incidents – 34 impacting passengers and the balance of 129 involving crew members (less likely to be seat-belted when unexpected turbulence occurs by the nature of their duties). Serious injury is considered to comprise any injury requiring more than 48 hours hospitalisation within 7 days of the incident, a bone fracture, severe haemorrhage, impact on internal organs or any burn affecting more than 5% of the body surface. There were more injuries recorded in 2022 than any year since 2011, although injury levels fell in 2020 / 2021 as a result of reduced aviation activity during the Covid-19 pandemic. That aside, it is difficult to see a trend in the NTSB data which suggest (in the US at least) a significant increase in injuries resulting from turbulence in the past 14 years.
The University of Reading study
Focusing on CAT, the study concludes "we find clear evidence of large CAT increases in various places around the world at aircraft cruising altitudes since satellites began observing the atmosphere. For example, at a typical point over the North Atlantic, the upward trend is such that the strongest category of CAT was 55% more frequent in 2020 than 1979". The study highlights its findings as "the best evidence yet that CAT has increased over the past four decades, consistent with the expected effects of climate change".
Drilling down into the data, the annual duration of CAT is referenced as having increased from 466.5 hours in 1979 to 546.8 hours in 2020 for "light or greater CAT", but with "severe or greater CAT" notably increasing from 17.7 to 27.4 hours (55%). Put in context, a typical North Atlantic location will over a year now experience CAT for a cumulative period of more than 3 weeks, at least a day of which would be deemed "severe". The study notes that "similar increases are also found over the continental USA".
What does severe CAT feel like? The NTSB describe the reaction inside the aircraft as "occupants are forced violently against seatbelts or shoulder straps. Unsecured objects are tossed about. Food service and walking are impossible". The worst cases are categorised as "extreme" in which "the aircraft is violently tossed about and is practically impossible to control. It may cause structural damage".
In essence, stronger vertical wind shear as a result of warming trends is expected to continue to increase the amount of turbulence, although the hope is that "long-term improvements in operational CAT forecast skills should be acting to reduce the probability of encountering turbulence, even if the amount of turbulence in the atmosphere is increasing".
What is the result of a turbulence incident?
As the FAA has noted, turbulence can result in serious injury and even fatality. A 2021 report by the NTSB  concluded that "turbulence-related accidents are the most common type of accident involving air carriers" noting that from 2009 to 2018 "turbulence-related accidents accounted for more than a third of all … accidents; most of these accidents resulted in one or more serious injuries but no aircraft damage". Whilst turbulence encounters represented 36.9% of accidents, by contrast ground handling incidents constituted only 8.1% and ground collisions a slightly higher 10.5%. Put simply, on this data, a passenger would be twice as likely to experience an accident as a result of turbulence than from a handling or collision incident on departure or arrival.
Interestingly, the NTSB report highlighted key safety issues including insufficient dissemination of turbulence observations, lack of awareness of turbulence risks, and a need for mitigation of common turbulence-related injury circumstances (passengers wearing seatbelts mean the most commonly injured party is a flight attendant). The findings focused on a need for air traffic control procedures around turbulence observations and reporting to be simplified, for air carriers to share their observations of turbulence more widely within industry, and the development of a "frequently updated, short-term forecasting product known as a turbulence nowcast".
Helpfully, the NTSB also highlight specific case studies:
- Compass Airlines flight 5763 – operating an Embraer 175 aircraft in February 2019 en route from Santa Ana, California to Seattle, Washington. About an hour into the flight "the aircraft encountered severe turbulence causing a sharp increase in vertical speed, which triggered a "Descend" traffic collision avoidance system (TCAS) resolution advisory (RA) command due to another aircraft passing above. The pilot flying disconnected the autopilot and responded to the RA by pushing the yoke forward; the severe turbulence (uplift), however, prevented the pilot flying from complying with the RA … then the uplift subsided and resulted in a sharp drop of the aircraft". One flight attendant suffered serious injuries whilst two passengers and two flight attendants sustained minor injuries. The aircraft also sustained some damage to the overhead luggage bins and lavatory.
- Delta Airlines flight 957 – operating a Boeing 737-800 aircraft in February 2019 – turbulence was encountered en route from Yellowstone International Airport, Montana, to Atlanta, Georgia. ATC had advised the crew to expect light and occasional moderate turbulence during descent. About ten minutes before beginning their descent, moderate turbulence conditions had been warned. Whilst the aircraft sustained no damage, the report notes "the two aft flight attendants reported they were thrown up to the ceiling and then came "crashing back down" onto the galley floor. One of these two sustained serious injuries including a fractured vertebra.
More recently, in March 2023, Lufthansa flight 469 from Austin, Texas to Frankfurt, Germany was diverted to Washington following an incident of severe turbulence during the meal service. It was reported that one flight attendant was thrown up to the ceiling, puncturing it, and a total of 7 passengers and crew were hospitalised post-incident. The actor Matthew McConaughey was on board and wrote on social media – "On Flight last night, I was told plane dropped almost 4,000 feet, 7 people went to the hospital, Everything was flying everywhere".
How does the law govern the impacts of turbulence?
The Montreal Convention, a universal treaty governing civil airline liability, sets forth provisions for the compensation of passengers injured from accidents. It establishes airlines' responsibility to passengers on international flights through the concept of quasi-strict liability. No foreseeability of the accident is required. Compared to the situation under domestic law, this can greatly lower the burden on claimants to prove the liability of the airline, as the burden of proof is on the airline to show they took all necessary steps to avoid the accident.
To establish liability, a passenger must meet three requirements under Article 17(1) of the Montreal Convention:
- The passenger must suffer a bodily injury;
- The bodily injury must be caused by the accident;
- The accident must take place on board the aircraft or in the process of embarking or disembarking.
The application of Article 17(1) was confirmed on the 19 December 2019 by the Court of Justice of the European Union, which stated that an accident included "all situations occurring on board an aircraft in which an object used when serving passengers had caused bodily injury to a passenger, without it being necessary to examine whether those situations stem from a hazard typically associated with aviation". Whilst mild and moderate turbulence may not always be classed as an accident, an injury sustained by a passenger onboard an aircraft as a consequence of severe turbulence, more specifically, CAT, would more naturally fit within this application.
The more severe or extreme the turbulence, the more likely it would be considered not to be in the normal course of operation of a flight and therefore, the more likely it could constitute an accident for which the airline could be liable. CAT, as demonstrated in the case studies above, is increasingly classed as severe to extreme, as it often causes passengers and objects to be thrown about, as well as causing structural damage to the aircraft. The data shows increased CAT, which means an increase in the likelihood of passenger injuries and therefore, an increase in airline liability, subject to a claim meeting the requirements of Article 17(1).
Furthermore, case law has shown that injuries passengers sustain can ultimately result from a reckless act or omission by a pilot, if the accident could have been foreseen and if the pilot does not take the precautions specified against severe CAT. There are steps airlines can take to avoid this, such as pilots addressing the passengers in the first instance and throughout the flight, and advising passengers to keep their seatbelts on due to CAT concerns. Additionally, health and safety checks, such as checking overhead bins are secured, are now even more vital.
79% of serious turbulence injuries are suffered by flight attendants rather than passengers. Moreover, turbulence accounts for 75% of flight attendant injuries. Therefore, there could be a claim through employment contracts in the first instance, rather than the Montreal Convention. For CAT cases under domestic law, the most challenging aspect of the claim for the passengers to prove will be the foreseeability element. As stated above, CAT is often undetectable prior to its encounter therefore, if no warning could have been given, such as the pilot alerting the passengers by turning on the seatbelt fasten sign, there is unlikely to be liability. The more predictable the turbulence, and the more avoidable, the more likely a claim would be successful.
The courts recognise that ultimately, they "cannot require defendants to be clairvoyant and warn of unforeseeable severe turbulence". Airlines can take proactive steps to reduce their exposure to turbulence related liability: proper warning should be given when it can be anticipated and airlines should ensure that they can evidence they have taken all necessary steps to minimise turbulence related accidents.
As the singer P!nk once said of turbulence in her song of the same name – "you can't help when your stomach sinks, see your life happen in a flash … so when it hits, don't forget, as scary as it gets, it's just turbulence." 
 Evidence for Large Increases in Clear-Air Turbulence Over the Past Four Decades – Mark C. Prosser, Paul D. Williams, Graeme J. Marlton, R. Giles Harrison – https:/doi.org/10.1029/2023GL103814 – 8th June 2023
 Preventing Turbulence-Related Injuries in Air Carrier Operations Conducted Under Title 14 – Safety Research Report NTSB/SS-21/01 – www.ntsb.gov/safety/safety-studies/Documents/SS2101.pdf
 Accident has been defined in Air France v SAKS as an “unexpected or unusual event or happening that is external to the passenger” 470 U.S. 392 (1985), affirmed by Deep Vein Thrombosis And Air Travel Group Litigation  EWCA Civ 1005
 GN v ZU (C-532/18)
 Quinn v Canadian Airlines International Ltd (1994) 18 OR (3d) 326 followed in Koor v Air Canada (2001) 106 ACWS (3d) 6 (Ont Sup Ct)
 Goldman v Thai Airways International Ltd  3 WLUK 244 (1981) 125 S.J. 413. The case predates the Montreal Convention therefore, the Warsaw Convention on International Carriage by Air 1929 applied however, this Convention was unified by the Montreal Convention. Hence, its application is still relevant.
 National Transportation Safety Board
 Federal Aviation Administration Safety Summit in March 2023, NTSB Chair Jennifer Homendy
 Kohler v Aspen Airways Inc 214 Cal Rptr 720 (Cal App, 1985), 19 Avi 18,051