Why flight paths change with the wind

Flight paths near an airport are not fixed lines that planes follow in all weathers. They come in pairs, one for each direction the runway can be used, and the wind decides which pair is in use. That is why a street can have a plane every couple of minutes one week and silence the next.

Why planes take off and land into the wind

A wing only produces lift when air flows over it fast enough. What counts is the plane's speed through the air, not over the ground. Taking off into a headwind, the plane reaches flying speed while it is still moving relatively slowly along the runway; with a tailwind, it has to go faster over the ground to reach the same airspeed. Landing works the same way: into the wind, the plane touches down at a lower ground speed and stops in a shorter distance.

The difference is large. The FAA's handbook for pilots says a headwind of a tenth of the take-off speed cuts the take-off distance by about 19%, and a tailwind of the same strength adds about 21%, with the same effect on landing. A tailwind also makes the approach harder to fly: to stay on the glide path, the plane has to descend faster, with the engines close to idle.

So airports use their runways into the wind whenever it is more than light. When the wind swings round, they turn round with it.

How much tailwind is allowed

Airliners can cope with some wind from behind. For most modern types, the manufacturers' published tailwind limits are around 10 to 15 knots, according to the international airline pilots' federation, IFALPA. Airports allow much less than that when they choose a runway, so that there is a margin for gusts and changes.

Five knots is about 6 mph or 9 km/h: a light breeze. Below it, planes can use either end of the runway, and the airport can choose. Above it, safety comes first. The FAA's rules say so in as many words: tailwind and crosswind take precedence over delays, capacity and noise abatement.

Preferential runways: how airports pick a direction

Runways are built to line up with the local winds. ICAO recommends orienting them so that the airport can be used at least 95% of the time without excessive crosswind. That is why most airports have one main direction, and why, in most weather, the real question is only which end the planes use.

When the wind is light, many airports use the end that disturbs fewer people. This is a preferential runway system:

• Heathrow has run a daytime westerly preference since the 1960s, to reduce the number of planes taking off towards the east over London. Winds there are light about 20% of the time, so the preference makes a real difference. After a consultation in 2001, the government removed it at night to share the noise more fairly between east and west.
• Frankfurt treats the westerly direction as the lower-noise one for its neighbours, and changes only when the tailwind goes above 5 knots.
• In the US, airports can agree a runway use programme with the FAA, which lets controllers pick runways for noise reasons within set wind limits.

Air traffic control makes the decision, not the airport's noise team. At Heathrow, NATS controllers watch the wind at different heights up to 3,000 ft. Gatwick's controllers look at the wind on the runway and at 1,000 and 2,000 ft, at the first stages of take-off and final approach, and at the forecast for the next four to six hours. At Frankfurt, the German air navigation service decides on the upper winds and the German Weather Service's forecasts. The wind above the airport matters as much as the wind on the ground, because planes fly through it on the final approach and the first part of the climb.

How often it changes

The wind follows the weather, not a timetable, so there is no fixed pattern. Over a year it evens out into a familiar split. Heathrow, Gatwick and Frankfurt each report about 70% of their flights in the westerly direction and 30% easterly, because the wind at all three usually comes from the west.

Day to day, it is less tidy. Over the last year at Heathrow we recorded 62% of days that were mainly westerly, 26% mainly easterly, and 12% on which the direction changed during the day. Heathrow says the direction can change more than once in a day. Easterly days also come in spells: April, July and January had the most, with around four days in ten each. The split for each airport and runway direction is in easterly and westerly operations.

What it means for your street

Landing planes line up with the runway many kilometres out and descend on a shallow, straight slope, so under the approach they stay low for longest, on the same line every time. Take-offs climb far more steeply and spread onto several routes. When the airport turns round, the streets under the low landing line get climbing take-offs instead, or nothing.

Two of our sample reports show the size of the switch:

Some airports also rotate runways within the same direction, which moves the planes again on a timetable; see runway alternation explained.

Checking what tomorrow will bring

The wind forecast is the best guide you have. For an airport with east-west runways, a westerly or south-westerly wind means landings from the east; an easterly wind above about 5 knots means landings from the west. In light or changeable winds, expect the airport's preferred direction if it has one, with a chance of a switch during the day.

To see what a direction means for a particular street, look at a past day with the same wind. Our free flight map replays a whole day of landings and take-offs, so you can pick a westerly day and an easterly day and compare them.