When the long way home is faster

Why do some flights take longer in one direction than the other – sometimes an hour or two more? The simple answer is wind, and an upper atmosphere effect known – appropriately – as the jet stream. 

How does it work? Essentially, the Earth’s rotation causes winds in the northern hemisphere to blow from west to east. It also causes air and water currents to circulate from warmer to colder regions and back. Take the Pacific Ocean: the warm air and water circulates up the coast of Asia and Russia, drops down from the poles along the west coast of the American landmass, and crosses west back above the equator. For centuries, sailors have known these flows as trade winds.

A similar phenomenon takes place far above the surface of the earth. As cold and warm air masses meet, they create jet streams: tunnels of faster-moving westerly air that move further north or south, depending on the changing seasons. 

This presents challenges and opportunities for flight planners. Take flights to Los Angeles. The outbound route from Hong Kong – coincidentally pretty similar to those taken by Spanish galleons riding the trade winds 500 years ago – is always quicker than the return, because of the wind direction and the jet stream. Currently, the flight time averages out at 13 hours and 10 minutes outbound, and 15 hours and 30 minutes back. 

But this varies daily and by season, as Head of Line Operations Captain James Toye explains. He says that last year, the longest outbound flight on this route was 13 hours and 38 minutes in July, while the shortest was 12 hours and 12 minutes in December, aided by massive tailwinds in the jet stream. 

On the return leg, the flight follows a similar route, because aircraft can use the brute force of jet propulsion rather than rely on the wind that 16th-century galleons required. But even with clever routing, the longest flight time was 16 hours and seven minutes in February, while the shortest was 14 hours and seven minutes in August. 

You can see the jet stream in an app called Windy , which Toye’s team uses as an overlay when they plan routes or track typhoons. It shows wind speed and direction in a visually clear way. “At sea level, the wind is pointing in myriad directions, and there’s nothing around here at more than 20 knots today,” says Toye. But as he scrolls up the altitude, wind strength increases and the direction becomes more unified. 

At 34,000 feet, the jet stream is clearly visible. “It’s a completely different picture at this height, and often the wind can be totally different to that at sea level,” he adds. Today’s LA flight is sitting in the jet stream. “We cruise at 480 knots [500mph] but they’ve got about a 100-knot advantage today,” he observes. That’s a significant advantage and an excellent reason to plan routes with the strongest winds on the way over and the gentlest for the return. 

But because jet streams can vary in size, speed and direction, there can be surprising outcomes. Last winter, Cathay Pacific routed some New York flights back to Hong Kong via the Atlantic – the long way around. The physical distance over the Pacific was shorter, but strong winds made it quicker to hitch a lift with the Atlantic jet stream. “We saved 45 minutes,” says Toye. “I sent the operating crew an email to remind them to turn right, not left on departure!”

One of these routes clocked in as Cathay Pacific’s longest-ever commercial flight, at just over 10,300 miles. “The flight times may change,” says Toye, “but we’ll always make sure to get you home as quickly as possible.”