Why the Same Flight Can Take an Hour Longer Going One Way: The Jet Stream, Explained
Daily Explainer: one everyday bit of aviation, in plain English.
You book a return trip across the Atlantic, or from the US west coast to the east coast, and notice something odd. The flight out says seven hours. The flight back says six. Nobody changed the aircraft, and the pilots are not flying any faster. So what happened? In most cases, the answer is wind, and in particular a fast river of air high in the atmosphere called the jet stream.
Two speeds, not one
An aircraft has an airspeed and a ground speed, and they are not the same thing.
Airspeed is how fast the aircraft is moving through the air around it. This is what the pilots fly and what keeps the wings producing lift.
Ground speed is how fast the aircraft is moving over the ground below it. This is what decides how long the trip takes.
If the air itself is moving, the two numbers split apart. Fly into a headwind and your ground speed is lower than your airspeed. Fly with a tailwind and your ground speed is higher. It works the way it does when you walk on a moving walkway: step with it and you cover ground faster, walk against it and you crawl.
So what is the jet stream?
The jet stream is a narrow band of very fast winds high in the atmosphere, at roughly the altitudes where airliners cruise. It blows mostly from west to east in the mid-latitudes, which is where most of the world's busiest long-haul routes sit. It is not a fixed tube. It shifts, bends and strengthens from day to day and season to season, and it is generally stronger in winter than in summer.
Its speeds can be well over 100 miles per hour, and sometimes much more. A tailwind of that size adds a very large amount to an aircraft's ground speed, and a headwind of that size takes the same amount away.
What this means for your trip
Because the jet stream flows west to east, flights heading east often get a helping tailwind, and flights heading west often face a headwind. That is why:
- A flight from North America to Europe is often noticeably shorter than the same route in the opposite direction.
- A flight across the United States from the west coast to the east coast usually takes less time than the return.
- On long-haul routes the difference can add up to an hour or more, depending on the day.
The effect is not identical every day. On some days the jet stream sits further north or south, and the route can be planned to catch more of it or avoid more of it. That is also why the same flight number can show a slightly different scheduled or actual time from one week to the next.
Do pilots choose a route to use it?
Yes, within limits. Airlines plan each long-haul flight with the forecast winds in mind. Flight planning systems compare possible routes, altitudes and speeds, and weigh how much fuel each option would burn and how long each would take. On an eastbound flight the plan may move toward the strongest tailwind. On a westbound flight it may steer toward weaker winds to cut the headwind.
The route is also shaped by other things: air traffic control rules, restricted airspace, weather such as thunderstorms, and the need to stay within reach of suitable diversion airports. So the shortest line on a map is not always the route actually flown, and the route that is best for wind is not always the one that is allowed.
Why do schedules not change every day?
Timetables have to be published long in advance, so airlines build in typical times for each direction. They use typical winds to set a scheduled block time, which is the time from leaving the gate to arriving at the gate. On a good tailwind day the flight can land early. On a headwind day it can arrive close to, or later than, the scheduled time. Over a season it averages out.
A quick note on turbulence
Strong winds high up are not the same thing as turbulence. A steady jet-stream wind is just air moving in one direction, and the aircraft moves with it without any bumps. Turbulence tends to appear where the wind speed or direction changes quickly over a short distance, which can happen at the edges of the jet stream. Pilots and dispatchers watch for those areas and can change altitude or route to avoid them.
The takeaway for passengers
- A longer flight in one direction is normal, and most of the time it is just the wind.
- The airline is not slowing down or speeding up the aircraft to match. The air is doing the work.
- If you want to know why a given flight ran early or late, the answer is often "the winds aloft were different from the forecast".
Next time your return flight lands early and everyone claps, you will know the real thanks go to a river of air several miles above the ocean.