“If you think a third runway is unpopular, try mentioning ‘mixed mode’ in polite Home Counties company.”

That was the private response of one of the 16 transport secretaries this century when I asked whether they had considered allowing both runways at London Heathrow airport to be used for arrivals and departures at the same time.

The shorthand for this technique is “mixed mode”. For a government obsessively focused on growth, it could unlock extra capacity at the UK’s biggest hub for very little additional financial cost – but, as the erstwhile minister indicated, huge political cost.

At present, Heathrow dedicates one strip of asphalt to landings and the other to take-offs.

The only regular use of mixed mode is between 6am and 7am daily, the busiest hour for arrivals into the airport. Planes are allowed to land on both runways.

Intuitively, you might imagine that the most efficient way to operate a two-runway airport like Heathrow is to separate arrivals and departures. In fact, the opposite is true: you can extract more capacity if there is a plane coming into land a few seconds after an aircraft ahead has taken off.

Heathrow at its peak has a landing every 80 seconds and a take-off every 80 seconds. But across at Gatwick, air-traffic controllers can manage an arrival and a departure in as little as 65 seconds. Mixed mode adds capacity without the need for another runway.

When Sir Howard Davies’s Airport Commission looked into mixed mode, they concluded: “The increased operational flexibility could be used to enhance the resilience of the airport’s operations.”

Monday was messy this week at Heathrow: 36 flights were cancelled, affecting 5,000 passengers and one Qatar Airways A380 “SuperJumbo” diverted to Amsterdam after a missed approach because there was not room in the system to accommodate another go.

Such disruption could become much more rare if Heathrow was open to receiving more flights. Of more interest to the airlines – and, by extension, passengers keen on more choice and lower fares – is that the technique could allow up to 60,000 more flights each year.

One senior travel industry figure strongly advocates using mixed mode to increase capacity immediately. Paul Charles, chief executive of travel consultancy The PC Agency, flies through Heathrow at least twice a month. He told me: “It’s embarrassing to see Heathrow held back by the lack of expansion. Airports in most other major cities are growing substantially as their governments focus on growth. The demand to fly from consumers is certainly there.

“I suggest the government apply a two-phase expansion to Heathrow in particular. It could start immediately by allowing greater use of mixed-mode, with aircraft taking off and landing on the same runways, so unlocking greater capacity and flight volumes.

“Then it could agree to a third runway, say from 2035, subject to certain environmental criteria being met. The government would have encouraged growth straight away and Heathrow would have won its long-running request for expansion.”

Many interested parties will insist it can’t happen. Purely pragmatically, just because Heathrow could physically land 15 per cent more planes, doesn’t mean there is the terminal and gate space to handle them.

Next, the concept of respite is extremely important to many of the people living on the flight paths. On Tuesday morning, for example, a procession of planes started landing on Heathrow’s northern runway from 4.30am. The first four aircraft, all coming in from Africa, flew diagonally across south London as far as Woolwich, where they turned sharp left to line up for the final approach to Heathrow.

Deptford, Camberwell, Battersea, Fulham … the noise increased as the aircraft descended. Next in line, Brentford and Isleworth – which just happens to be the constituency for Transport Select Committee chair Ruth Cadbury, who is not a fan of Heathrow expansion. The MP and her constituents at least know that at 3pm the noise will cease, as landings are shifted to the southern runway.

The most dramatic reduction in aircraft noise at Heathrow happened overnight in October 2003: Concorde stopped flying. The windows of west London stopped rattling at teatime and shortly after 10pm each night. Since 2006, Heathrow says, the area most impacted by aircraft noise has reduced by 41 per cent.

The Davies Commission stopped well short of recommending mixed mode. But the airport assessors did say: “Should the delivery timescale for new runway capacity be towards the longer end of the anticipated spectrum, then the case for enabling mixed mode operations at Heathrow may be stronger ... It is conceivable that this issue may become material as part of a transition strategy to the preferred longer-term option.”

Residents beneath the flight path don’t want mixed mode. Heathrow does not advocate the practice. But who knows what the pro-growth chancellor, Rachel Reeves, may recommend as a stepping stone to a third runway?

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