09 December 2024
Bumpy Rides
Climatologists say warmer air means more turbulence
The atmosphere assaulted Singapore Airlines Flight 321. The Boeing 777 was cruising at 37,000 ft en route from London to Singapore on May 21 when it hit what the airline described as “sudden extreme turbulence”. The aircraft dropped to 31,000 ft, and news reports quoted passengers as saying that those not belted into their seats got launched toward the ceiling. A 73-year-old British man died from what may have been a heart attack, and authorities say seven other people suffered critical injuries.
Researchers say a warming atmosphere is becoming more turbulent, and airlines, meteorologists, and regulators are working to meet the threat. Various types of turbulence can affect air travel. They include convective turbulence, associated with thunderstorms; clear-air turbulence (CAT), often associated with the jet stream; and mountain wave turbulence, resulting from air currents flowing over ridges or bluffs.
A research letter published last year by the American Geophysical Union said CAT is expected to intensify due to future climate change. The study said severe or greater CAT became 55 per cent more frequent in 2020 than in 1979, and the increases were largest over the United States and the North Atlantic, which are both busy flight regions.
Jonathan Hutchinson, a meteorologist for Southwest Airlines, told AeroSafety World that temperature extremes create more CAT, partly due to a more dynamic jet stream. “More curvature and greater speeds mean more wind shear,” Hutchinson said.
Researchers say a warming atmosphere is becoming more turbulent, and airlines, meteorologists, and regulators are working to meet the threat.
Research also shows a rising trend for thunderstorms and related convective turbulence. Britain’s Royal Meteorological Society says, “Rising global temperatures due to climate change means warmer air, which allows it to hold more moisture, roughly 7 per cent more moisture per 1 degree C of warming. This boosts the chance of thunderstorms, leading to more violent storms and more lightning strikes.”
Hutchinson agreed that thunderstorms are growing more rapidly and becoming more severe. “We can have cumulus development happen much more quickly, and that can take pilots by surprise,” Hutchinson said. He added that stronger, fast-developing storms can roil the atmosphere at greater distances. Pilots are typically trained to avoid thunderstorms laterally by 20 nm (32 km). “I believe that standard may have to change,” Hutchinson said, “because we’re seeing turbulence happening much farther out.” He also said pilots may need to give thunderstorms greater vertical avoidance, as well.
More turbulence has not necessarily correlated to more injuries, perhaps due to better forecasting and increased flight crew awareness. According to U.S. Federal Aviation Administration (FAA) data, there were 184 serious turbulence injuries on commercial airlines between 2009 and 2023. The data shows no clear increasing trend. For example, there were 20 serious injuries in 2023, six in 2021, 13 in 2019, five in 2013, and 18 in 2009.
However, among all injuries reported on airliners, turbulence takes its share. A study by the U.S. National Transportation Safety Board (NTSB) found that between 2009 and 2018, turbulence was involved in more than one-third of all airline accidents. “The good news for travellers is that wearing a seat belt while flying virtually eliminates the risk of being injured if an aircraft experiences turbulence,” the NTSB said. Flight attendants, who cannot always remain seated and belted, take the brunt of the injuries. The NTSB study said flight attendants accounted for 79 per cent of the serious injuries in the study.
FAA Advisory Circular 120-88A, “Preventing Injuries Caused by Turbulence,” lists several recommendations, ranging from low-tech to high-tech. The recommendations include good crew resource management (CRM) so that pilots, dispatchers, and flight attendants share information on flight conditions and turbulence risk. Low-tech solutions include cabin modifications such as handrails and grab bars.
Between 2009 and 2018, turbulence was involved in more than one-third of all airline accidents.
High-tech tools include data delivery systems that can bring turbulence reporting and forecasting into the flight deck in near real-time, and research continues in that area. Airlines can now try out a beta version of a turbulence “nowcast,” developed by the FAA through a contract with the National Centre for Atmospheric Sciences. The “nowcast” provides rapidly updated 15-minute turbulence forecasts for pilots and dispatchers.
Stephanie Avey, science and operations officer for the National Weather Centre’s Aviation Weather Centre in Kansas City, Missouri, said that with airline flights back to pre-pandemic levels, there are more aircraft aloft to encounter turbulence — but those crews are better informed. She explained that forecasters now have improved data from the Geostationary Operational Environmental Satellites (GOES). “The models are getting better,” Avey said.
Methods of disseminating that information are improving, as well. “We just overhauled AviationWeather.gov to be more mobile-friendly,” Avey added. She said turbulence data on the website includes high-level and low-level versions of the Graphical Forecast for Aviation.
At the same time, traditional methods remain useful. Air traffic controllers encourage crews to offer pilot reports (PIREPS) when they encounter turbulence, based on standard phraseology from the FAA Aeronautical Information Manual:
Light chop — Slight, rapid, and somewhat rhythmic bumpiness without appreciable changes in altitude or attitude.
Light turbulence — Slight, erratic changes in altitude and/or attitude. Occupants may feel a slight strain against seatbelts. Unsecured objects may be displaced slightly. Food service may be conducted and little to no difficulty is encountered in walking.
Moderate chop — Rapid bumps or jolts without appreciable changes in aircraft altitude or attitude.
Moderate turbulence — Changes in altitude and/or attitude occur, but the aircraft remains in positive control at all times. Moderate turbulence usually causes variations in indicated airspeed. Occupants feel definite strain against seatbelts, unsecured objects are dislodged, and food service and walking are difficult.
Severe — Large, abrupt changes in altitude and/or attitude. Usually causes large variations in indicated airspeed. Aircraft may be momentarily out of control. Occupants are forced violently against seatbelts, unsecured objects are tossed about, and food service and walking are impossible.
Extreme — Aircraft is violently tossed about and is practically impossible to control. May cause structural damage.
For commercial airliners, encounters with severe or extreme turbulence normally require a logbook entry so that maintenance personnel can perform required safety inspections.
The Walbrook Building 25 Walbrook London, EC4N 8AW
Author
Thomas W. Young
Flight Safety Foundation
First published in AeroSafety World 20 August 2024
Thomas W. Young is a retired airline captain and a former instructor flight engineer with the West Virginia Air National Guard. Young has logged nearly 12,000 hours of pilot and flight engineer time.
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