Wed 12 Apr 2017 – Incidents of severe aircraft turbulence on transatlantic routes are likely to become twice or even three times more common as a result of climate change, finds a new study from the Department of Meteorology at the University of Reading in the UK. The study used supercomputer simulations of the atmosphere to calculate how wintertime transatlantic clear-air turbulence would change at an aircraft’s cruising altitude of around 39,000 feet (12 km) in response to a doubling in the concentration of CO2 levels in the atmosphere, which scientists predict will occur later this century. The results show the average amount of light turbulence increasing by 59%, rising to 149% for severe turbulence. A significant number of injuries to passengers and crew already take place each year, as well as damage to aircraft, and the study concludes an intensification of clear-air turbulence could have important consequences for aviation.
Climate scientists predict the midlatitude jet streams in both the northern and southern hemispheres are expected to strengthen at aircraft cruising altitudes as the climate changes, leading to stronger vertical windshears within the jet stream. The windshears can become unstable and are a major cause of turbulence.
Previous climate modelling studies have indicated that the amount of moderate-or-greater clear-air turbulence on transatlantic flight routes in winter will increase significantly in future as the climate changes. This new research examined five different turbulence strength levels, to investigate how they will each change in future. The results show that the average amount of light turbulence in the atmosphere will increase by 59% (a range of 43%-68%), with light-to-moderate turbulence increasing by 75% (39%-96%), moderate by 94% (37%-118%), moderate-to-severe by 127% (30%-170%) and severe by 149% (36%-188%).
“Our new study paints the most detailed picture yet of how aircraft turbulence will respond to climate change,” said Dr Paul Williams, who conducted the research. “For most passengers, light turbulence is nothing more than an annoying inconvenience that reduces their comfort levels, but for nervous fliers even light turbulence can be distressing.
“However, even the most seasoned frequent fliers may be alarmed at the prospect of a 149% increase in severe turbulence, which frequently hospitalises air travellers and flight attendants around the world.”
A paper published in 2002 indicated 790 turbulence encounters by scheduled US carriers over the course of a year that resulted in 687 minor injuries and 38 serious injuries to flight attendants, along with 120 minor injuries and 17 serious injuries to passengers. However, Williams says typical rates are likely to be much higher because of under-reporting and other estimates show there are over 63,000 encounters with moderate-or-greater turbulence and 5,000 encounters with severe-or-greater turbulence annually.
Unlike turbulence in the weaker categories, severe turbulence does pose a safety risk to passengers and crew because it causes aircraft to execute random motions with vertical accelerations that exceed the gravitational acceleration and therefore arguably warrants separate consideration, says this new study, which is published in the journal Advances in Atmospheric Sciences.
There are also the economic costs from turbulence that arise from injuries, damage to airframes and cabins, flight delays, inspections, repairs and post-incident investigations. These costs have been estimated by Williams to be as high as $200 million annually for US carriers alone.
Currently, the median length of a patch of turbulence is around 60 km, which equates to about five minutes of flying time, and the median thickness is about 1 km, and this limits the effectiveness of altitude change as an evasive manoeuvre.
However, points out Williams in his paper, the projected increases in the prevalence of clear-air turbulence do not necessarily imply more in-flight injuries or greater levels of passenger discomfort. Future improvements in clear-air turbulence forecasts, such as the Graphical Turbulence Guidance system, would improve the ability of pilots to divert around patches instead of unexpectedly encountering them.
Some modern aircraft are also fitted with an accelerometer in their nose cone. If the accelerometer registers a sudden change in altitude, which is large enough to be indicative of turbulence, the wing flaps are rapidly adjusted in an attempt to damp the vertical motion and reduce the acceleration. The risk could also be mitigated by equipping aircraft with Light Detection and Ranging (LIDAR) ultra-violet laser systems which can warn pilots of any invisible density perturbations indicative of clear-air turbulence up to 10-15 km ahead, potentially with enough time to alert passengers and crew, or possibly take an evasive manoeuvre. Although a 2002 study found a negative business case for installing such technology, Williams believes this could change as the technology becomes less expensive and clear-air turbulence becomes more prevalent.
In the meantime, he would like to extend the study to examining other seasons, flight levels and geographic regions.
“My top priority for the future is to investigate other flight routes around the world. We also need to investigate the altitude and seasonal dependence of the changes, and to analyse different climate models and warming scenarios to quantify the uncertainties,” he said.
Early last year, a study by Williams and a research team was published that looked at the impact on transatlantic flight times of a faster jet stream caused by climate change (see article).
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