Picture Heathrow's two runways, side by side, both running east to west. Most days the wind is from the west, so planes land heading west over London. On those days one runway takes the landings and the other the take-offs. At 3pm they swap. The homes under the landing runway get their afternoon back.
Week 1: north lands till 3pm→Swap at 3pm→Week 2: south lands till 3pm→Swap at 3pm
People call this runway alternation. The word just means taking turns. Landings are the louder half, because arriving planes are low over houses for miles. So the airport moves the landings between the two runways at 3pm, and then flips the whole pattern the following week, so each side gets quiet mornings one week and quiet afternoons the next.
3pm
↓→
Landings move runway
It is 10am on a westerly day and the north runway has the landings. What happens at 3pm?
Flights stop for an hour.
Not yet. Nothing stops. The same number of planes arrives; they arrive over a different set of streets.
Nothing. The runways are fixed for the day.
Not yet. They are fixed until 3pm. The swap is the whole point of alternation, and it happens every westerly day.
The landings move to the south runway, and the north side gets a quieter afternoon.
Right. That is alternation: landings, the louder half, change runway at 3pm. The homes that had the morning's arrivals get the afternoon off.
Set the week and the time of day on a westerly day. Picture a home under the north runway's approach. The figure shows what it hears.
Landings over this home, an hour40
Over the other runway's homes0
Week: Week 1 · Time: Morning
Week 1, morning: about 40 landings an hour over this home. Loud here. The 40 is made up and round; the pattern is the real one.
Landings over this home, an hour0
Over the other runway's homes40
Week: Week 1 · Time: Afternoon
Week 1, afternoon: about 40 landings an hour over the other runway's homes. Quiet here. The 40 is made up and round; the pattern is the real one.
Landings over this home, an hour0
Over the other runway's homes0
Week: Week 1 · Time: After 23:30
Week 1, after 23:30: no landings scheduled over anyone. 207 nights in 2025 had none at all between 23:30 and 04:30; the rest had a few late runners.
Landings over this home, an hour0
Over the other runway's homes40
Week: Week 2 · Time: Morning
Week 2, morning: about 40 landings an hour over the other runway's homes. Quiet here. The 40 is made up and round; the pattern is the real one.
Landings over this home, an hour40
Over the other runway's homes0
Week: Week 2 · Time: Afternoon
Week 2, afternoon: about 40 landings an hour over this home. Loud here. The 40 is made up and round; the pattern is the real one.
Landings over this home, an hour0
Over the other runway's homes0
Week: Week 2 · Time: After 23:30
Week 2, after 23:30: no landings scheduled over anyone. 207 nights in 2025 had none at all between 23:30 and 04:30; the rest had a few late runners.
Landings over this home, an hour40
Over the other runway's homes0
Week: Week 3 · Time: Morning
Week 3, morning: about 40 landings an hour over this home. Loud here. The 40 is made up and round; the pattern is the real one.
Landings over this home, an hour0
Over the other runway's homes40
Week: Week 3 · Time: Afternoon
Week 3, afternoon: about 40 landings an hour over the other runway's homes. Quiet here. The 40 is made up and round; the pattern is the real one.
Landings over this home, an hour0
Over the other runway's homes0
Week: Week 3 · Time: After 23:30
Week 3, after 23:30: no landings scheduled over anyone. 207 nights in 2025 had none at all between 23:30 and 04:30; the rest had a few late runners.
Week 1 morning is loud for this home. Which other frame is loud for it?
Every frame. A home under an approach is always loud.
Not yet. Half the frames are quiet for it. That is the point of alternation: the same noise, taken in turns.
Week 2 afternoon. The pattern flips each week, so its loud half moves from morning to afternoon.
Right. Week 1: landings over this home till 3pm. Week 2: landings over the other side till 3pm, then over this home. Every home gets a quiet morning one week and a quiet afternoon the next.
Week 2 morning. Mornings are always loud under an approach.
Not yet. Week 2 morning is the other side's turn. The weekly flip is what stops one side having every loud morning.
Move both controls.
promise of 1952Cranford
↓→
by 2028Easterly alternation
There is a catch from 1952. The government promised Cranford, a village at the north runway's end, that planes would not take off east over it. So on easterly days, about 3 in 10, there is no taking turns. The promise ended in 2009; the council backed the works in December 2025, subject to a legal agreement, for 2028.
Put Cranford's story in the order it happened.
Tap them in the order they happen, first to last. Each tap gets a number.
1952 promise→2009 ended→2017 lapsed→2025 backed→2028 due
Right. Promise, end of promise, an approval that lapsed, a second approval, start. Nineteen years from ending the promise to the swap, because changing where the noise lands means someone new gets it.
Summer season3,250 night flights allowed
Winter season2,550 night flights allowed
Night is rationed by law, not by turns. Between 23:30 and 6am the government caps Heathrow at 5,800 take-offs and landings a year, 3,250 in summer and 2,550 in winter. Each night flight also spends points from a noise budget, so a louder plane uses up more. Heathrow's own target adds the 150 quiet nights.
What share of the year does Heathrow run westerly, landing over London?
Pick one. Nothing is scored.
7 in 10Westerly days
vsvs
3 in 10Easterly days
About 7 in 10.About 7 in 10. The wind in southern England blows from the west most of the time, and planes land into the wind. Guesses tend to land on half. The 3 in 10 easterly days are the Cranford days, with no turns until 2028.
Two tracks
↓→
Houses on one side
A railway runs freight trains at night through a town on two parallel tracks, always on the track nearest the houses. Which of Heathrow's tools fits?
Alternation: use the far track half the nights, so the noise is taken in turns.
Right. Two tracks and one set of houses is the runway problem exactly. Taking turns does not reduce the noise; it shares it, which is what Heathrow does at 3pm.
A mandate for cleaner fuel.
Not yet. A fuel mandate is about carbon, and carbon spreads over everyone. Noise lands on the near houses, and the tool for that is turns or caps.
A contour count.
Not yet. A contour measures who hears it; it does not move the trains. It would tell the railway the size of the problem, not solve it.
Lesson complete
Heathrow shares its noise by the clock: one runway lands till 3pm, then they swap, and flip weekly.