Shortest route vs. quickest route

Before we dive into this topic, it’s important to note the difference between the shortest route and the quickest route and why the two may not necessarily be the same.

Winds

One of the biggest variables in this equation is the winds. For the most part, global winds move around the earth from west to east due to the fact that the earth is spinning. This means that when flying westbound, aircraft tend to travel into head winds and when flying eastbound, they tend to encounter tail winds. So what does this mean for aircraft?

I’m sure you’ve been out walking on a particularly windy day and as you turn a corner, you suddenly find yourself battling a stiff head wind. Now, instead of walking easily, you have to put extra effort into keeping up the pace, using more energy in the process.

Conversely, if you turn in the opposite direction, you’ll suddenly find that your pace picks up. It’s the same for aircraft.

When flying west, we are normally flying against the wind, so our speed slows down and we use more fuel. When flying east, the wind tends to be behind us, increasing our groundspeed. This reduces our flight time and the amount of fuel we use.

This is why the eastbound part of a particular city pairing is invariably quicker than the westbound trip.

These winds are most noticeable in the areas of jet streams — fairly narrow bands of extremely fast-moving air. They are depicted by the green lines in the graphic below.

Even though the course of these jet streams isn’t exactly west to east, it’s clear to see that their overall path does follow that rule. By virtue of weather systems, these jet streams will change in strength and location from day to day, which is why flight routes vary.

When flying westbound, airline flight planning departments do their best to avoid the strongest of these jet streams. This may mean flying a route that takes the aircraft quite a long way off the Great Circle track but ultimately results in a quicker flight time than the route on the Great Circle line.

Traffic volumes

This clip also demonstrates another variable in the route a flight may take on a particular day: the amount of traffic.

It may seem ludicrous that there could be traffic jams in the sky. However, because there is a minimum distance required between aircraft, traffic jams do indeed occur when too many aircraft are trying to get through a certain area at the same time.

However, the beauty of internationally coordinated flight planning systems is that air traffic controllers can see where these issues may occur. Due to the geopolitical situation in a number of countries around the world at the moment, the routes that aircraft can take — particularly between Europe and southern and east Asia — are limited.

If air traffic control agencies notice that there will be a backup in a particular area at a certain time, they will place a departure time restriction on some aircraft even before they leave the gate. In the industry, this is known as a slot.

If this happens, the flight planners have two options: They can decide to wait for the slot restriction and take the originally planned route, or they can look to reroute the aircraft through airspace that is less congested.

The option they choose will ultimately come down to a number of variables. These variables include the cost difference in fuel between the two routes, the difference in time, effects on the on-time performance of the flight and subsequent flights, and the cost of any passengers missing connections.

If the slot time isn’t too far away, it may be better to wait for it to come around. If it’s hours away, as is sometimes the case during a busy summer, a reroute may be the best option.

Geopolitical issues

Thunderstorms and turbulence

Not only do jet streams bring strong winds, but they can also generate turbulence — as indicated by the dotted yellow line in the chart at the top. While it can be quite smooth in the core of the jet stream, the turbulence can be quite rough on the edges. This is particularly true when crossing a jet stream at 90 degrees.

If the planned route of a flight will pass through an area with a severe forecast that could produce turbulence, pilots will often spot it during a preflight briefing. In this scenario, we would make a quick call to flight planning to alert them to this threat and ask if there is another route that we can take.

Depending on other airspace restrictions, such as those listed above, flight planners may then be able to find a new route that keeps us away from the worst of the turbulence.

It’s the same for forecast thunderstorms. Crossing the equator we fly through an area called the Inter Tropical Convergence Zone. Here, rotating air masses converge and the only way for them to go is up. These upward-moving winds take moisture with them and form thunderstorms that can reach 60,000 feet and beyond.

We can end up flying hundreds of miles off-course to avoid these areas. As a result, if storms are in the forecast for a particular area, the flight route will avoid this area as much as possible.

Of course, there’s no guarantee that storms won’t also develop along the route we are flying. However, by planning ahead and using accurate forecasts, we can avoid the worst areas.

Bottom line

In the life of an airline pilot, no two days are ever the same nor are the routes that we fly. The biggest reason for a difference in routing tends to be the winds. When flying between London and Miami, the route can vary massively depending on where the jet streams are located.

With strong jet streams in the middle of the Atlantic, the flight will most likely route via the northern Atlantic and then down the Eastern Seaboard. The return flight will then make the most of those winds by flying straight across the pond.

However, if the jet streams are located farther north, it may be a better option to fly the London-to-Miami route straight out across the middle of the water.

We know that each time we fly to a destination, the route will always be different in some way from the last time we visited due to high traffic density, geopolitical issues or weather.