Unproven ‘aircraft bridge’ is key to “Heathrow Lite’s” £3bn savings

An ‘aircraft bridge’ could be Heathrow’s bold new idea to shave £3bn off the cost of its Third Runway bid.  But in the rush to circumvent 4 years of critique of its published plans is Theresa May being pushed to make a decision on a device that is more science fiction than proven engineering capability?

An 'aircraft bridge' over the M25 is key to Heathrow shaving £3bn off the cost of a Third Runway ... but few have ever been built for more than taxiways.

An ‘aircraft bridge’ over the M25 is key to Heathrow shaving £3bn off the cost of a Third Runway … but few have ever been built for more than taxiways.

With Theresa May’s Government set to make a decision this week on airport expansion, residents locally will continue to be starved of the detail of just what it is her Cabinet sub-committee will be deciding on.

The Airports Commission’s report recommending Heathrow cost £20 million after several well publicised consultations and public hearings that followed publication of the draft, and final, proposals from Heathrow, Hub, and Gatwick.

But with a rush to shave £3bn off the cost of Heathrow’s £18 billion scheme – with smaller terminal buildings and even less spend on critical transport infrastructure – the public has been denied the opportunity to review the revised plans.  A 4 year process has been compressed into less than 4 weeks and residents have had to pay the price.

While Heathrow has talked of re-routing the M25 around the new runway and the airport’s new chair has referred to the possibility of “some sort of bridge”, The Sunday Times today provides the first real insight of what its “lite” scheme might involve instead of a tunnel … “a shallower underpass”.  An ‘aircraft bridge’ is the technical term, apparently.

An entry on Wikipedia created in 2013 has, coincidentally, seen a rash of updates since the Summer. And, while a number of real aircraft bridges are listed, there remain very, very, few actual runway bridges – which are an altogether far bigger engineering challenge.

heathrow-tunnel.png

The Sunday Times says a “shallower underpass known as an ‘aircraft bridge’ could be Heathrow’s cost-saving option”.

Any runway bridge for a Heathrow Third Runway must be able to accommodate the weight of the Airbus 380, currently the world’s largest airplane in operation, with a maximum take-off weight of 575 tonnes (1,268,000 lb).  14 long haul destinations are currently served from Heathrow by the A380: Los Angeles, Doha, Dubai, Abu Dhabi, Singapore, Dusseldorf, Hong Kong, Perth, Kuala Lumpur, Bangkok, Johannesburg, San Francisco, Washinghton DC, and Miama – so it’s unlikely that Heathrow will want to scale back this technical specification.

Aircraft bridges must be designed for “the substantial forces exerted by aircraft braking, affecting the lateral load in substructure design”

But that is no mean feat:

“Aircraft bridges must be designed for the substantial forces exerted by aircraft braking, affecting the lateral load in substructure design. Braking force of 70 percent of the live load is assumed in two recent taxiway bridge designs. And “deck design is more apt to be controlled by punching shear than flexure due to the heavy wheel loads.”

The structural challenges of such schemes are immense and, as Wikipedia notes,”numerous taxiway bridges have been proposed but not built”.

Heathrow’s revised plans also apparently see an end to the expansion of the 14-lane expansion of the M25.  But the airport says its expansion plans will have no impact on traffic levels to the airport in any case given its push for more rail links and plans to move employees to public transport.

As Nigel Milton, Heathrow’s Director of External Affairs, wrote this week in a letter to The Guardian, the airport’s confidence in its “triple-lock guarantee” on air quality even extends to taking action, if required, “to reduce road traffic”.

With local roads taking the brunt every time there is the slightest hiccup on the M4 or M25 even pro-expansion residents are likely to be unimpressed with the lack of detail on the scaled down proposals.

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