Will electric planes and sustainable fuel make Heathrow's third runway green?

The UK government is expected to formally support the construction of a third runway at London Heathrow Airport on Wednesday. Chancellor Rachel Reeves is expected to announce the decision in a major speech about achieving economic growth. The expansion of Heathrow has long been opposed by environmental groups, and her announcement will be highly controversial, especially given its environmental impact.

Reeves claimed over the weekend that "the aviation industry has changed a lot" since plans for a new runway at Heathrow were first discussed decades ago. She stated that "sustainable aviation fuels" will reduce emissions, there is huge investment in the electric aircraft sector, and a third runway would mean fewer planes circling over London while pilots wait to land. The BBC's fact-checking unit has assessed whether the Labour government's arguments truly hold water.

"Sustainable" fuels are alternatives to fossil fuels made from renewable sources. They can come from agricultural waste and used cooking oil, and the government argues these fuels emit 70% less carbon over their lifecycle. This is because the plants the fuel is typically made from absorb carbon dioxide from the atmosphere as they grow. Previous governments have declared that by 2030, 10% of all jet fuel used by flights departing from the UK must be sustainable, in an attempt to reduce aviation's impact on emissions. Labour has retained this target.

However, there are many issues with using the promotion of sustainable fuels as a justification for airport expansion. Currently, sustainable fuels only make up a tiny fraction of jet fuel – the government's target for 2025 is 2% – and scaling this up will be a major challenge. They are not completely carbon neutral due to the energy used in production, refining, and transportation, and there is a wide variation between different fuel types. Many environmentalists argue that expanding UK airports is incompatible with the UK's net-zero goals, as there are currently no viable, widely available alternatives to fossil fuels to power planes.

There are a small number of small, battery-powered electric planes in existence, and investment in the technology is increasing. If planes are powered by electricity generated from renewable sources such as wind and solar, flying could become zero-carbon. But the weight of batteries is currently considered a major barrier to scaling up electric flights, especially for long-haul journeys. A technological breakthrough in battery weight is possible, but most analysts believe widespread electric air travel is not a realistic prospect in the near future.

Heathrow reports that in 2023, an average of 232 planes a day – over a third of all arriving aircraft – were held in one of four "stacking areas" over London, circling at 7,000ft or above until a slot became available at the airport. They spent an average of 6.85 minutes in the stacks. Loitering at low altitude is less fuel-efficient than cruising at high altitude because there is more air resistance. As a result, since 2014, Heathrow and air traffic control company NATS have been working with their European counterparts to slow down inbound flights from as far as 350 miles away, in an attempt to avoid delays over London.

Supporters of Heathrow's expansion argue that it is crucial for boosting national economic growth, especially as demand outstrips supply. Heathrow reported that in 2024, a record 83.9 million passengers passed through its terminals. This was 4.7 million more than in 2023, and 3 million more than the pre-pandemic peak in 2019. The airport has no available landing slots, meaning that airlines wishing to expand their services at the airport have to buy slots from other carriers. For example, in 2016, Oman Air reportedly paid $75m (£60m) for a pair of early-morning landing slots at Heathrow. According to research commissioned by Heathrow, these costs are ultimately passed on to passengers through fares.

An independent report commissioned by the previous government, led by Sir Howard Davies, concluded in 2015 that a new runway was needed in the south-east of England, and that the "best answer" was to expand Heathrow. However, by approving a third runway at Heathrow, Reeves appears to be at odds with the advice of the government's own independent emissions advisors, the Climate Change Committee (CCC). The committee has repeatedly warned that airport expansion should not take place without a framework to manage the overall capacity of the nation. In a report to parliament last July, the committee stated: "No airport expansion should take place until a UK-wide capacity management framework is in place that assesses and, where necessary, controls the sector’s greenhouse gas emissions and non-CO2 impacts annually."

In response, the government said last December that it "recognises the role that airport expansion can play in delivering economic growth while meeting our legally binding net-zero targets and strict environmental standards. We are currently considering wider approaches to aviation decarbonisation". If the government increases the pace of emissions reductions in other parts of the economy while allowing aviation emissions to increase, it could theoretically still remain within the government's statutory carbon budgets. However, achieving deeper reductions in other areas will be difficult and cannot be guaranteed. It is unlikely that Reeves and Prime Minister Keir Starmer will still be in power by the time a third runway at Heathrow is completed, which would hinder mitigation plans and accountability for its impacts.