Airlines plan their routes to take advantage of geophysical phenomena.
If you have traveled east to west and back again (or vice versa), you have most likely noted the discrepancy in flight times between the two directions. For instance, flying from London to New York takes just over eight hours, while the reverse journey is often under seven. Follow along as we take a look at the reason behind this occurrence.
It has nothing – directly – to do with the Earth spinning
Many might think it is the Earth’s eastward rotation that is the cause for the faster eastbound journeys; this is not so. As NASA’s Robert Frost explained in an interview with Forbes, the Earth’s rotation has nothing directly to do with the speed of a flight. Just as you do not walk faster when trotting from east to west, the direction does not affect how fast a plane flies.
The spinning of the Earth around its own axis might look like an intuitive answer. And while it isn’t directly affecting the aircraft, the truth is still to be found in the indirect impact it has on the weather. More specifically, the real answer concerns a geophysical phenomenon known as jet streams.
Photo: Vincenzo Pace | Simple Flying.
So what are jet streams?
The reason for quicker flights while flying eastwards is jet streams. Put simply, these are fast-flowing, narrow air currents in the atmosphere found at high altitudes. These currents are formed due to atmospheric heating from the sun’s radiation and the Earth’s Coriolis force (defined as a rotating object has a force perpendicular to the rotation axis). Combined, these factors produce streams of fast-flowing air that are responsible for flight times looking significantly different depending on which way they are traveling.
The most prominent jet streams are the polar stream (also called polar front jet or midlatitude jet stream) and the subtropical stream. These can be found at 60° and 30° north and south of the equator, respectively. The polar stream is the stronger of the two, and it causes much faster winds compared to the subtropical. Most airlines on transatlantic and transpacific routes make use of the polar stream while planning flight paths.
Jet streams can be as strong as 80 to 140 miles per hour, sometimes going all the way up to 275. These strong winds come with significant upsides and downsides for commercial air travel. What’s more, as the temperature of the Earth being affected by climate change, they could be about to shift, impacting far more than just flight times.
Recent research from the University of Southampton has shown that the winter jet stream over the North Atlantic and Eurasia (responsible for storm Eunice in the UK earlier this year) has increased its average speed by 8% to 132 miles per hour. It could also shift northwards and beyond its historical boundaries within the next few decades.
Hitching a ride on the wind
But let’s forget about the future for a moment and look at the historic relationship between aviation and jet streams. The first time these fast-flying air currents came to use in commercial aviation was in 1952 on a flight from Tokyo to Honolulu.
It was discovered that flying along jet streams cut the journey from 18 hours to a mere 11.5 hours when flying just under 25,000 feet. Airlines quickly realized the value of jet streams and began implementing them while planning routes.
Since the jet streams flow from west to east, they make one leg of the journey much faster (when flying with the stream) and one slower (against the stream). Imagine going downstream or upstream a river, or how it feels when you are cycling against the wind, as opposed to when you have it at your back. Returning to the example from New York to London, some flights even take a slightly longer route, specifically to benefit from the jet stream.
Even on shorter transcontinental flights between cities like New York and Los Angeles, jet streams can impact the flight time by nearly an hour. On long-haul transpacific routes, these streams can be extremely helpful for both passengers and airlines. By following the polar stream, flight time from Tokyo to Los Angeles is just nine hours and fifty-five minutes, versus the 11 hours and forty-five minutes in reverse.
The 747’s record 4h 55min New York to London
In February 2020, a British Airways 747 beat the transatlantic subsonic speed record thanks to strong jet streams. The aircraft made the JFK to LHR hop in just four hours and fifty-five minutes, a brand-new record, flying at over 800 miles per hour.
In short, jet streams can drastically cut down flight times and reduce fuel burn, both important revenue implications for airlines and reduction of emissions for the planet. While this may all sound like a win-win situation, there are a few things to consider.
Clear air turbulence
While jet streams can speed up flights, they do have a significant drawback: clear air turbulence. Clear air turbulence (CAT) is sudden severe turbulence that occurs in a cloudless sky, causing violent shaking of the aircraft. It happens when a slow jet stream interacts with a fast jet stream, creating a pocket of extreme disturbance. CAT is also impossible to detect visually or by aircraft radar, unlike other forms of turbulence.
Studies have concluded that CAT is set to rise in frequency by as much as 170% in the coming decades as a result of global warming. This means flying through jet streams will only become riskier in the coming years. Some areas could experience several hundred percent more turbulence. Estimates state that by 2050, the rate of injuries will have almost tripled.
Not so simple
Jet streams are a natural phenomenon and, just like all natural things, is subject to change. While on regular routes this only means a few more minutes or maybe an hour of extra flying, on ultra-long-haul routes, it can become an issue. The past few years have seen an extraordinary rise in demand and supply of 15+ hour flights, connecting North America and Europe with Asia and Oceania. However, they haven’t come without their bumps.
Air New Zealand’s Auckland-New York service was heavily affected by headwinds, for instance, in its infancy late last year. Clocking in at 17.5 hours, the flight pushes the Boeing 787-9 to its range limit, meaning even the smallest change can result in cancelation or, worse yet, unloading passengers and luggage.
However, the hope is that newer aircraft like Qantas’ specially modified A350-1000 for Project Sunrise will have the extra capacity needed to offset these slight changes. Singapore Airlines’ A350-900ULR has avoided these issues, but as airlines try to push their current planes to the limit, expect to read a lot more about variable weather conditions. Indeed, the Dreamliner, A321neo, and A321LR have all proven that long-haul flights can be made by medium-haul aircraft.
The jet lag price
While the flight time might be shorter when flying from west to east due to jet streams, that is not necessarily beneficial for passengers. Studies seen in Travel and Leisure have shown that passengers suffer from more jet lag on eastbound flights. While there are other factors at play, it could also mean that shorter flights leave lesser time to adjust and grab some shut-eye on such long-haul routes.
London to New York is oft-cited as the prime example of lost sleep. The evening return flights from JFK and Newark land in the early hours of London, but only take 6-6.5 hours, leaving travelers with five hours of good sleep (at best). However, faster travel is always the preferred choice, and airlines are unlikely to change routes just to give passengers a bit more shut-eye.
In summary, jet streams are the reason why flights take shorter when flying from the west to the east. While they do help to shave off up to a few hours from long flights in some cases, they are not entirely without drawbacks.
Source: Simple Flying