Before 2020, the North Atlantic was one of the busiest airspaces in the world. Every day more than 1,300 flights would cross the pond on their journey between North America and Europe. Despite the large volume of traffic, due to the remoteness of the open water, flights were not covered by radar like they are over land.
To ensure that flights remained safely separated, aircraft would fly prescribed tracks, like motorway lanes in the sky — called the organised track structure (OTS). However, these routes were not always the most direct, and extra carbon emissions were produced as a result.
However, in recent weeks, scientists at Reading University have found that considerable savings could be made on emissions if aircraft were able to fly routes that took advantage of the wind. Over the course of a typical winter, this would result in a 2.7% saving on carbon emission per passenger, totalling a saving of 6.7 million kilograms of CO2.
With the fall in oceanic traffic in recent months, NATS, the organisation responsible for the air traffic system over the U.K., and NAV Canada, responsible for the Canadian side, are going to trial disbanding the OTS on days when they are not required and allow aircraft to fly routes optimised for winds.
The results could be quite interesting. On days where there is no benefit from the wind, we are likely to see more direct flights. On days, or more precisely nights, when there is a strong jetstream blowing from North America to Europe, we could expect to see routes that look longer over the ground but are actually making the most of the strong winds.
Overall, it’s a great step forward in helping aviation become more efficient. Even though it only produces 2.4% of global carbon emissions, every little bit helps in tackling climate change.
The organised track structure
With more than 1,000 flights crossing the Atlantic Ocean each night from North America to Europe, it’s one of the busiest airspaces in the world. To complicate things further, for the vast majority of the crossing, there’s no radar coverage. This means that ATC is unable to see in real-time where aircraft are.
Aircraft need to go from A to B as safely as possible but also as commercially efficiently as possible. For the most part, traffic heading east from North America to Europe does so overnight. A few hours later, the flow is reversed as the aircraft make their way back to the U.S. and Canada.
To facilitate this flow, each day, airlines send NAV Canada their preferred routings for their eastbound flights during the night. Controllers collate this data and create a set of routes using GPS positions. This is known as the organised track system (OTS).
There are usually around six to seven of these tracks each night and they are then given an individual designator. Westbound flights during the day use tracks labelled Alpha, Bravo, Charlie, etc. So, to avoid any confusion, eastbound flights overnight start at Zulu and work backwards. Therefore Yankee, X-ray, Whisky, etc. Any flights planning to fly in this area of the Oceanic airspace must use the OTS.
Traditionally these tracks were spaced at one degree of latitude intervals, roughly 60 nautical miles apart. Aircraft are then separated by 10-minute intervals along the track and 1,000 feet vertically.
However, due to demands on the system, ATC has utilised the greater navigation accuracy of modern aircraft and reduced the lateral separation to 30 nautical miles. Not only does this allow more aircraft to cross the Atlantic in a given time, but it also enables more aircraft to fly at their optimum cruising level, reducing their carbon emissions as a result.
Jet streams
In simple terms, a jet stream is a concentrated area of very fast-moving wind.
The tropopause (the boundary in the earth’s atmosphere between the troposphere and the stratosphere) varies in height above the surface depending on where on the earth you are. Over the poles, it’s lower, and over the equator, it’s higher. However, in both hemispheres at approximately 30 degrees and 60 degrees, there are steps in the tropopause where large temperature differences occur.
In these locations, due to the rapid change of temperature in a localised area, the wind also changes rapidly. It’s these changes that create the jet streams. The greater the temperature difference, the stronger the wind. This is why jet streams tend to be faster in the winter when there is a greater range of temperatures between the colder polar air and the warmer air next to it.
Jet streams flow from west to east, which is why flights from Europe to North America take longer than the other way round. In the core of jet streams, winds regularly reach 100 knots to 150 knots. Occasionally, they can reach 150 knots to 200 knots. As a result, aircraft routes across the Atlantic vary according to the wind. When going westbound, the routes keep aircraft out of the strongest headwinds. When heading east to Europe, they try to make the most of these jet streams by flying as close to the core as possible.
These strong winds also explain why it’s often more bumpy on flights heading eastbound than heading west. As pilots, we try to fly in the very core of the jet stream as the air is much smoother. You can be rocketing along with a 150-knot tailwind and be in completely smooth air. Descend just a couple of thousand feet and you can have half the tailwind and be bumped around all night.
Wind-assisted routes
The OTS works fine but the main problem is that they force aircraft to fly longer routes to fit in with other traffic. Sometimes these can be hundreds of kilometres longer than the direct Great Circle route. As a result, airlines are obliged to create more carbon emissions than they need or want to. What would be better would be if flights could fly random routes to make the most of the tailwinds, reducing the emissions.
A recent study by Wells et al. found that by optimising routes for winds, savings of up to 16.4% could be made on the air distance, the product of airspeed and flight time. Multiplied up by thousands of flights over a single winter (when the winds are strongest), this could result in a reduction of 6.7 million tons of CO2.
Reducing carbon emissions and slowing the effects of climate change is in everyone’s interest, even for aviation. Warmer global temperatures will result in more turbulent flights as the conditions in jet streams change, longer flights as these jet streams shift position and also reduced payload capacity from increased temperatures.
Aircraft fly better in colder temperatures as the air is denser and creates more lift. When departing from hot destinations, we sometimes have to limit our takeoff weight to ensure that we have enough performance to get safely airborne. Warmer global temperatures will limit more flights, so it’s in our interest to do our part for the environment.
Looking at flights on a particular day, the scientists found that the eastbound routes prescribed by the air traffic services (“ATM” in the figure below) were pretty close to the route that would be optimised for wind (OFW).
However, the westbound flights differed quite significantly. They found that 54 km of air distance was saved by using the OFW route compared with the most efficient ATM track, but it was reduced by a massive 502 km compared with the least-efficient ATM track. The average saving across all routes was 173 km.
This doesn’t seem like a massive amount but multiplied over thousands of flights a year it could make a huge difference.
Interestingly, they also found that when an eastbound track is very far away from the route optimised for wind, flying faster will not make up for the lack of wind assistance. It will just use even more fuel and create even more carbon emissions.
However, the study stated that current routes are heavily restricted by the poor surveillance available to ATC when aircraft are over the water. Whilst flying more efficiently is high on the list of priorities for aviation, flight safety is, and will continue to remain, number one. As a result, ATC units have been able to make only small changes to the current OTS structure.
A new way of flying
That said, things are changing fast. In 2019, NATS and NAV CANADA become the first air traffic services in the world to use satellite-based surveillance, which gives a real-time view of aircraft over the Atlantic.
The benefits have already been seen, allowing them to reduce distances between flights to as little as 14 miles. This has meant that more flights have been able to fly routes closer to their preferred option, whilst still remaining within the OTS structure.
It has long been the air traffic services plan to simplify the airspace, but doing so with more than 1,300 flights a day would take time.
However, this year, that is going to change. Whilst the effects of the pandemic has decimated air travel, it has given NATS and NAV CANADA a unique opportunity to try something new. With only around 500 flights crossing the Atlantic at the moment, now is the time to trial a new system.
On days where aircraft numbers allow, no OTS tracks will be published. Instead, airlines will be asked to plan flights based on the optimum route, speed and altitude.
It’s hoped that by trialling the new system when the skies are quiet, as traffic starts to build up again, the new environmentally friendlier procedures can become the norm.
What does this mean for us, the pilots, though? In practice, very little. Instead of crossing the Atlantic in a procession of aircraft in straight lines, we will instead be flying more random routes direct towards our destination.
Close scrutiny of aircraft positions by ATC using the new surveillance methods will be able to detect if any aircraft are not flying their planned route. They can then send rapid messages via controller pilot data link communications (CPDLC) to ascertain if there is a problem and ensure that there is no loss of separation from other flights.
It will hopefully mean that we can fly in the best conditions for passenger comfort and not have to endure turbulence just because we are confined to a particular route.
Bottom line
Aviation is always evolving and changing with the latest advancements in science and technology. Making changes to systems that have been in place for decades is a significant part of this.
The OTS was created for a time when ATC was unable to see aircraft as they crossed the Atlantic. However, with advancements in surveillance technology, they now get a nearly real-time view of the entire ocean. This means that it’s time to shake up how we fly these routes, making the most of the latest scientific research.
Safety will always be the number one priority, but our impact on the environment must continue to be a focus. Personally, I’m looking forward to seeing how these changes pan out.
