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Sustainable Planet

Big idea · a national argument

Circling for a Slot

The hidden pollution of planes stacking over London.

This is a case we're making — about awareness and advocacy. Aircraft waiting to land at Heathrow don't hover — they fly in circles, burning fuel and emitting CO₂ for no journey at all. It's avoidable waste, and worth putting in front of the people who can fix it.

When Heathrow can't land arrivals straight away, they're sent to one of four holding “stacks” — giant aerial roundabouts where planes circle and slowly descend, waiting their turn. Every minute in the stack is fuel burned and carbon emitted to travel nowhere.

The four stacks over London

Heathrow's arrivals hold over Bovingdon (north‑west), Lambourne (north‑east), Ockham (south‑west) and Biggin (south‑east), descending in 1,000 ft steps from 16,000 ft down to 8,000 ft before being released onto final approach.

Real aircraft holding, collected from live ADS-B (OpenSky). The red heat map shows where aircraft loiter and burn fuel circling — the four stacks light up as CO₂ hotspots. Press ▶ to replay the collected window and watch the loops draw themselves, or use the layer control to switch on the modelled per-stack estimate. Indicative CO₂ (~25 kg fuel/min × 3.16); ADS-B coverage varies. © OpenStreetMap contributors.

The scale of the waste

  • Around 311 aircraft a day enter Heathrow's holding stacks — over 110,000 a year (Heathrow's 2024 noise & movements report).
  • They hold for an average of about 7 minutes each — sometimes far longer at busy times or in poor weather, circling repeatedly.
  • That's fuel burned and CO₂ emitted purely to wait — on top of the noise for communities directly beneath the stacks.
  • NATS — the UK's air‑traffic service — estimates smarter arrival sequencing could cut Heathrow's airborne‑holding emissions by up to ~30,000 tonnes of CO₂ a year. That's the scale of the waste.
~30,000 tCO₂ a year from airborne holding that better sequencing could avoid at Heathrow (NATS)
~26 t / dayjet fuel burned circling — ≈£5–6m a year at recent prices (derived from the CO₂ figure)
4.4 MtCO₂ a year a growth runway could add — ~150× the holding saving (CIEH). Why our support needs a flights cap.

Holding isn't the pilots' choice or the airlines' — it's a capacity-and-sequencing problem. The planes are ready to land; the system makes them queue in mid-air.

The counter‑intuitive fix: capacity to land, not to grow

Our argument is deliberately unusual for an environmental charity. We'd back freeing up landing capacity — including a further runway — but strictly to end the holding, not to add flights. The prize isn't more planes; it's letting the flights that already come land on arrival instead of circling. The fuel saved, and the emissions avoided, would be huge.

The catch that makes or breaks it: new capacity only cuts pollution if total flights are firmly capped. Used for growth, a runway means more flights and more carbon overall — the opposite of the point. So our support is conditional: capacity ring‑fenced to remove holding, with a binding cap on movements. We say that plainly, because it's the honest heart of the case — and the fair challenge critics will rightly put to it.

Alongside that, near‑term arrival management helps now — slowing aircraft earlier and further out so they arrive “just in time”, and flying continuous descents instead of stepping down a stack.

A holding pollution levy — polluter pays

If a flight still ends up circling, that avoidable pollution should carry a price: a holding pollution levy, charged per minute spent in the stack — to put the cost of the waste on the record and drive the system to design it out.

Who pays — and is it fair? Honestly, crews don't choose to hold; it's a capacity‑and‑ sequencing decision. So a fair levy targets the system, not the pilot — a charge on the airline /airport that only bites on avoidable holding, with the money ring‑fenced to fund the very capacity and cleaner operations that reduce it (and benefit for the communities under the stacks). Designed well, it rewards landing on time and prices the circling.

What we're asking for

  • Capacity to end holding, not to grow — support a further runway only if paired with a firm cap on total flights, so the gain is less circling, not more traffic.
  • Better arrival management now — sequencing and continuous descents to cut holding today.
  • A holding pollution levy — polluter‑pays, on avoidable holding, ring‑fenced to fund the fix.
  • Count it honestly — publish the fuel and carbon burned in holding, every year.

We keep this evidence‑led and non‑partisan, within charity campaigning rules — the aim is less avoidable pollution. We'd back new capacity strictly to end holding and only with a binding flights cap; we do not advocate aviation growth.

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Sources: Heathrow Annual Noise & Air Transport Movement reporting (≈311 aircraft/day entering the holding stacks; average hold ≈7 minutes; stacks descend 16,000→8,000 ft); the four stacks — Bovingdon, Lambourne, Ockham, Biggin — are long-established published arrival holds. Stack positions on the map are approximate (VOR/hold fixes). Holding‑emissions figure from NATS (its Demand‑Capacity‑ Balancer “digital twin”, ~30,000 t CO₂/yr avoidable at Heathrow); the fuel/cost line is derived from that (CO₂ ≈ 3.16 × fuel; jet fuel ≈ £600/t) and is indicative. Third‑runway growth figure (~4.4 Mt CO₂/yr) from CIEH reporting, 2025. Figures indicative, from public reporting, September 2026. The additional‑runway (capped) and holding‑levy ideas are Sustainable Planet's own advocacy proposals, not current policy. Flight‑track data (e.g. OpenSky Network, non‑commercial) could be added to visualise real holding patterns. On the map, the ~30,000 t/yr NATS figure is apportioned equally across the four stacks (real per‑stack shares aren't published, and vary with runway direction and weather); the “~11,000 cars” comparison uses ≈2.7 t CO₂/yr for an average UK car (DfT).