Heathrow published its summer figures last week and led with a line about demand.

More than 40 airlines, both ones already flying there and ones that want to start, are trying to get slots at the airport and can't. Around three quarters of the routes they want to launch would be long-haul. Chief executive Thomas Woldbye said the longest queue at Heathrow is now the one made up of those 40 airlines, and that none of them can get a slot because both runways are already full.

It's a good line, and it's aimed squarely at getting a third runway built.

Further down the same release, passenger numbers for the peak holiday month were down.

Both things are true, and that's the interesting part

Heathrow handled 23 million passengers over the peak summer period and 84.26 million across the twelve months to August, up 0.3% on the year before. Flat, in other words, with a dip in the busiest month.

So the airport is simultaneously saying that demand far exceeds what it can handle and reporting that the number of people going through it isn't growing.

Both are accurate. They're just measuring different things, and the gap between them is the whole story.

What's actually full isn't the concrete

This is the bit that gets lost, and it's the reason the two facts sit together.

Heathrow is limited to 480,000 flights a year by government. That's a cap written into planning permission, not a physical limit of the tarmac. The airport has been bumping against that ceiling for years.

A slot is permission to use the runway at a specific time on a specific day. Because there's a hard ceiling on total flights, those slots are finite and fully allocated. The only way to get one is to buy it from an airline that already holds it. Heathrow slot pairs have changed hands for tens of millions of dollars.

So when Woldbye says the runways are full, he means the permitted movements are used up. A third runway would raise the cap to around 720,000 flights.

That's why passenger numbers can stay flat while 40 airlines queue outside. Airlines already inside hold the slots and aren't giving them up, so the only growth available to them is putting bigger aircraft on routes they already fly. Which they've done, which is why traffic has crept up rather than jumped.

Where the third runway actually stands

Longer than most coverage suggests.

An Airports Commission recommended expansion back in 2015. Britain's Department for Transport later selected Heathrow's plan for a 3,500-meter runway with a realignment of the M25 motorway as its preferred option. Heathrow's board approved work on the planning application in January, and the airport is targeting planning permission by 2029, which would put first flights roughly a decade out.

Britain's Civil Aviation Authority has been examining Heathrow's funding model, which matters because the airlines paying for expansion through charges have their own views about the bill. Neighboring councils including Richmond and Hillingdon oppose it on noise and environmental grounds.

By the airport's own numbers, expansion would support 108,000 jobs and raise the UK's trading capacity by 50%.

What I'd take from it

Nothing about the 40-airline number is untrue. It's just being used as a lobbying instrument rather than reported as a neutral fact, and it arrived in a release showing flat traffic.

What I'd want from Heathrow, and haven't seen, is which airlines and which routes. Forty carriers and a list of the long-haul destinations Britain is missing would be considerably more persuasive than a round number. It would also be checkable.

Practically, if you fly through Heathrow: nothing changes for years. What the cap means day to day is that the airport runs with almost no slack, which is why one problem there spreads across the whole schedule so quickly. Keep your connections long.