Heathrow airport eyes a series of “viaducts” over the M25 as a simpler solution to its £17bn third runway

Heathrow Airport is considering plans to construct its third runway across a series of “viaducts” that would cross above the M25.

These would consist of three parallel bridges, the widest of which would be the runway itself and two thinner ones which would be taxiways for planes.

The plans were presented to an engineering conference by Heathrow’s expansion director Phil Wilbraham, the Sunday Times reported.

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Building viaducts over the M25 is, as well as being a simpler solution, also believed to be a cheaper option than other alternatives that would require more elaborate infrastructure.

Heathrow has been scratching its head as to how to deliver the £17bn, two-mile third runway over the major motorway since the government gave its endorsement to the project in October.

It began with a proposal to construct a 650 metre tunnel for the motorway, later vying with the option of putting the runway on a slope above the road.

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Transport secretary Chris Grayling last year said the latter would be “cheaper and quicker” than any movement of the M25.

Last month, British Airways owner IAG hit out at prospective plans to build over the M25, estimating that bridging the motorway will cost £2bn to £3bn on top of the airport’s £17bn existing bill for the third runway.

IAG wants a shorter runway that does not breach the M25.

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A shorter runway had been floated amid Heathrow’s earlier proposals for expansion, but was considered more troublesome in terms of noise concerns.

A public consultation on plans for the third runway ended on 25 May.

At some point later this year or in early 2018, MPs will vote on a national policy statement on the third runway.