vol 8, issue 7

Climate-change turbulence risk for air carriers

15 July 2013
By Anne Harris
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People's belongings litter the inside of flight SQ308

The SQ308 flight from Singapore to London reportedly fell 65 feet during turbulence

Coffee dripping from the plane's ceiling

The 26 May flight was buffetted enough to fling coffee to the ceiling and possessions to the floor

Passenger's belongings litter the floor

A reported 11 passengers and one crew sustained minor injuries during the episode

Blankets are placed on the floor of the plane to soak up spills and cover glass

Perhaps mindful of ‘citizen journalists’, the crew set about a hasty clean-up operation

A map of the US plotted with pilot's reports of turbulance

Pilot reports of turbulence in the US

A plane undergoing tests on its wings

Plane wings undergo vigorous testing to ensure they can withstand the most severe of turbulence stress

Researchers at two UK universities have identified the likelihood of increased turbulence in aircraft over the coming half-century. But can we doing anything to stop it?

Climate change has been blamed for a dizzying array of problems – melting ice caps, severe storms, species migration, death and destruction – but one of the more specific phenomena to be attributed to it is bumpier flights.

Turbulence is the dread of most air passengers. Despite endless reassurances that modern planes can withstand the wildest turbulence the atmosphere has to offer, even the gentlest of bumps has some passengers grabbing their armrests in alarm.

The bad news for these nervy travellers is that by the middle of this century, according to research from the Universities of Reading and East Anglia, climate change will lead to bumpier transatlantic flights. Consequently, journey times could lengthen while fuel consumption and emissions look set to increase.

The modelling produced by the researchers suggests the average strength of transatlantic turbulence could increase by between 10 and 40 per cent, and the amount of airspace likely to contain significant turbulence by between 40 and 170 per cent, where the most likely outcome is around 100 per cent.

The research focused on clear-air turbulence in winter, which is particularly problematic to airlines, because clear-air turbulence is invisible to pilots and satellites, and winter is when it peaks. "It can be a problem because usually when you are flying through clouds it will be during take-off and descent so all the passengers should have their seatbelts on at that time," Dr Paul Williams from the University of Reading and an author of the report says. "While it may not be pleasant for the passengers it is not particularly dangerous. However, when you are flying at 11km up through clear blue skies, the seatbelt sign is off and people are moving around the cabin when you suddenly hit turbulence and that is when people can suffer injuries."

Refining predictions

Williams has been researching the effects of turbulence for five years, refining an algorithm to make aviation turbulence predictions that are valid for a period from an hour to over a day. Williams admits that these predictions are certainly not perfect. "They do not get it right 100 per cent of the time, I think the success rate is about 70 per cent, and so I worked on developing a new method for this a few years ago," he said.

Williams has also worked on climate-change problems and he realised there was an opportunity to bring together these two independent strands of research. "The climate science community and the aeroplane turbulence community are two very different groups of people. They usually do not talk to each other or attend the same conferences. I think this is why no one has ever looked at this before.

"It is shocking, really, that this is the first ever study to look at how climate change will affect the future of aviation turbulence. I was astonished when I went in search of literature and could find no previous work on it. I had worked with both communities before and because I had one foot in each camp I think I could see how to link them and bring them together."

There is anecdotal evidence from regular fliers that incidences of turbulence are increasing but Williams warns that we need to be wary of a psychological bias. He explains that when you hear something is getting worse you are highly vigilant towards it and notice it a lot more than you may have previously. "However, the independent data seems to back-up an increase," he says. "The two studies that have come out so far that claim to have some evidence of an increase have both removed themselves from their findings to some extent and come up with other explanations such as changes in the data, or the statistical significance perhaps not quite being there. I notice it too and other people I talk to have mostly suggested that they have noticed an increase as well."

Williams's original algorithm takes gridded atmospheric data from a computer model output; the same sort of model that would predict the weather. "That is used as a data source," Williams adds. "There are ways of taking that data set and estimating from it where and when the turbulence would be. I should say the resolution and grid spacing of the data is really far too coarse to explicitly predict if there is any turbulence in it."

Using a grid of tens of kilometres means that the data points are tens of kilometres apart, but now scientists have developed methods of taking data of this resolution and estimating where the turbulence will be using dozens of different measures to do these calculations. "In the paper we have used those exact same mathematical algorithms and instead of using data that corresponds to a weather forecast for tomorrow's weather as being our input, we take two different climates in relations and have looked at different turbulence statistics from the pre-industrial to the double CO2."

Trouble spots

Any location in which the wind speed changes with altitude – a wind shear effect – will always tend to be an unstable situation. There is a compensating, stabilising mechanism, which suppresses the instability – that is, the higher up you go in the atmosphere the less dense it gets – called stratification. In the same way that oil will rest on top of water in a stable situation, the stratification of the atmosphere is a stabilising effect that will try to inhibit the turbulence that is trying to be produced by the wind speed. The jet stream is a high wind-shear region, the wind is changing rapidly with the height, and that is why you tend to get this instability in the jet stream itself.

"Certainly, if you fly over a mountain range there are 'mountain waves', which result from the interaction of the airflow of the mountain itself that creates a 'gravity wave' that propagates up into the atmosphere and eventually breaks up," Williams adds. "It increases the shear locally; that is one reason you can get a lot of turbulence when you're flying over a mountain."

The effect that climate change is having on flying conditions boils down to warming by different amounts in different places. It is the difference the temperature between the North Pole and the equator at flight altitudes that drives the jet stream, and the difference is getting larger due to climate change. "At 10km up it is getting warmer more at the equator than it is at the North Pole," Williams explains. "That makes the jet stream flow faster, which is a well-known climate phenomenon. What we have looked at is the particular consequences of that for clear-air turbulence because a stronger jet stream will give you more turbulence.

"We have looked at these two computer simulations and tried to estimate the turbulence in both cases and indeed we have found that the stronger jet stream does mean more turbulence and we have been able to put some numbers to that to show exactly what the percentage increase is."

The concept seems self-evident, but someone is still needed to do the actual work. "I think it is consistent with our understanding of climate science and changes to the jet stream, but the increases are not small," Williams says. "We are looking at the most likely outcome as being a doubling of the amount of airspace containing significant clear-air turbulence – an enormous increase."

Levels of turbulence (light, moderate, severe and extreme) are all measured in terms of G-force that is felt by the plane. Light turbulence is anything up to 0.5 g, moderate is 0.5 g to 1 g, severe is 1-2 g and extreme is anything above 2 g.

Whenever a pilot passes through turbulence he or she is supposed to log it. The PIREP database hold records of not only turbulence but other hazards such as icing of the wings.

Advanced algorithms

The research itself hasn't helped the formulation of more accurate algorithms but has given extra motivation to the need to develop them. "If the atmosphere is going to contain twice as much airspace containing turbulence, the only way planes are going to be able to avoid a 100 per cent increase in the amount of time with the seatbelt sign on is if they know where the turbulence will be," Williams says. "In order to get that we need to improve these algorithms that are used and so I think one consequence of this paper is to make it even more clear that we have got to develop better algorithms pretty urgently."

The major airlines are wary about discussing the subject, and Williams himself has had no formal contact with any of the airlines although he has given a talk at an aviation turbulence symposium in the US where pilots and aviation industry representatives were present and he had "informal chats" with them.

"I think from a pilot's perspective it is not an issue of safety, but an issue of comfort," Williams says. "It makes the passengers' flight more of a nuisance and one in which drinks will get spilt, but that is not a huge hazard. There are hundreds of injuries each year, mainly to crew as they are out of their seats trying to serve passengers as the turbulence hits. So, while this is not a safety issue primarily, it does stand to reason that an increase in turbulence will lead to an increase in injuries."

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Boeing aircraft testing

Engineers at Boeing put the aircraft through its paces to ensure that it can survive even the most severe turbulence.

No aeroplane has been damaged by normal turbulence and the simple reason for that is that they are designed to perform under the worst that nature can throw at them. Planes are designed to take stresses of up to 3.5 g before major damage occurs.

A team of 160 test and evaluation structural test engineers at Boeing's Seattle base have a hand in making sure that all Boeing products can be operated safely. The team pushes structures to their physical limits, finding out where the breaking point is, often with an audible pop, snap or crack. Their efforts help ensure the safety of the company's jets by verifying that the breaking point lies exceptionally far away from what a pilot may experience, even in extreme circumstances.

"Our job is to make sure that passengers and crew can trust the plane they're in," Marshall Short, lab test operations vice president, said. "We test and sometimes break things so people know they can trust the planes. It's too bad the average traveller has no idea about all the work these teams do to keep them safe."

Structural tests fall into two main categories: static and fatigue.

Static testing determines an airframe's ability to carry loads. Loads applied during the final phase of static testing are 50 per cent greater than loads that may be encountered in service. Photos and videos of static testing, with airplanes encased in large scaffold-like structures, show dramatic images of planes surviving seemingly impossible stresses, such as having their wings bent almost vertical.

Fatigue testing subjects airframes to the equivalent of up to three lifetimes of in-service wear and tear to help determine durability. This also helps set operator maintenance and repair schedules.

The basics behind these tests haven't changed for almost a century, but the execution has, and in dramatic fashion. For instance, data-collection techniques have advanced significantly since the company started structural testing.

Originally, there were several 'deflection men' responsible for manually recording data points. Today the static tests use a system that's precise, sophisticated and the largest of its kind. Devices that capture any change in position to within 0.06mm allow more than 50 design engineers and stress analysts to remotely monitor airframe health, comparing their predictions to test data in real time.

The way loads are applied to the airframe has also evolved. Each structural test once required 29 employees, including 11 pump men who manually operated hydraulic controls to apply flight loads to a test article. Today, two engineers operate one computer that controls in excess of 150 servo-hydraulic load systems.

"Although the tools and instruments have advanced, our job still is as exciting as it ever was," says Michelle Fitzgerald, deputy capability leader for structural testing. "We push every piece of every product to its limits – and beyond – to ensure the planes can easily handle any situation they are likely to encounter. I don't stress when I fly. I've seen those wings bend 26 feet – I have supreme confidence in our products and I know that they will withstand anything they come across in flight."

BA pilot talks turbulence

British Airways Captain Steve Allright, who runs the BA Flying with Confidence Course, talks about his experiences of clear-air turbulence

"Many different things may cause turbulence, but each and every one of them is known and understood by pilots. Every day I fly, I expect a small amount of turbulence, just as I'd expect the odd bump in the road on the drive to work.

"Different aspects of the weather cause different types of turbulence. Clear-Air Turbulence (CAT) is the most common form you are likely to experience.

"Air tends to flow as a horizontal snaking river called a jet stream. A jet stream can sometimes be thousands of miles long but is usually only a few miles wide and deep. Depending on the direction of travel, our flight planners either avoid (into a headwind) or use (into a tailwind) these jet streams to cut fuel costs, as they can flow up to 250mph. Just like a fast-flowing river swirling against the riverbank, where the edge of the jet stream interacts with slower moving air, there may be some mixing of the air which causes turbulence.

"You cannot see CAT, you cannot detect it on radar and you cannot accurately forecast it, but there are other ways of avoiding it. In the main we rely on reports from other aircraft, which we hear either directly or which are passed on by air traffic control. We then consider the options available to us. Our endeavours to fly at an altitude that has been reported as smooth may be prevented by several constraints such another aircraft occupying that level, or the weight of the aircraft at that time. Whatever the circumstances, a pilot will find the most comfortable path to a destination without compromising safety.

"Flight crews around the world share a common classification of turbulence: light, moderate and severe. The definitions are laid down in our manuals and help us to make an assessment as to what our course of action should be. For the fearful flyer, even light turbulence can be upsetting."

The Richter scale of air turbulence

British Airways Captain Steve Allright, who runs the BA 'Flying with Confidence' course, explains the three levels of turbulence

Light turbulence: "For pilots, light turbulence is no different to a bumpy road for a taxi driver or a slightly uneven section of track for a train driver – a small, but totally safe, inconvenience and very much part of our daily lives. In light turbulence, the aircraft may be deviating by just a few feet in altitude.

Moderate turbulence: "This strikes no fear into pilots, as they will experience this level of turbulence for a few hours in every thousand hours they fly. It usually lasts for no more than 10 or 15 minutes, but occasionally may last for several hours, on and off. This sort of turbulence will unsettle even some regular travellers and will cause drinks to spill. The aircraft may be deviating in altitude by 10 or 20 feet. No action is required by the pilot to control the aircraft, but the flight crew may decide to try a different altitude if the turbulence persists.

Severe turbulence: "In a flying career of over 10,000 hours, I have experienced severe turbulence for about five minutes in total. It is extremely uncomfortable but not dangerous. The aircraft may be deviating in altitude by up to 100 feet, but nothing like the thousands of feet you hear some people talking about when it comes to turbulence."

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