Olympic athletes and spectators might be counting down to the July 27 opening ceremony of the London Games, but Heathrow Airport managers are focusing much of their attention on the day after the closing ceremony.

This is because though athletes and officials traditionally arrive at an Olympic Games over several days, most leave in one fell swoop.

Heathrow is planning for 137,800 passengers on Aug 13. -- a 45 percent increase in departures -- making it the busiest day in the airport's history.

According to Nick Cole, head of Olympic and Paralympic planning for Heathrow, every single seat on every aircraft will be full, something the airport has never seen before.

There are eight days when our Olympic and Paralympic traffic will overtake our business as usual, Cole told Reuters.

It will feel like a busy summer for the majority of the time at Heathrow; July 16 and 26 will feel slightly busier for arrivals and August 13 to 15 will feel very busy for departures.

In addition, August 22 and September 10 will be peak days either side of the Paralympics.

The forecasts, included in a detailed 'six months to go' document published by Heathrow Airport on Thursday, also drill down into luggage figures, which on August 13 will rise by 35 percent to 203,000 items.

While oversized items like canoes and bicycles are set to test baggage systems -- which famously failed during the 2008 opening of Heathrow Terminal 5, leading to the cancellation of 300 flights -- a new challenge comes in the efficient handling of 2,000 firearms carried by Olympic and Paralympic athletes for use in shooting events.

For international visitors to the London Games, close-to-capacity Heathrow will be the first and last impression of the host country.

Unlike December 2010, when a slow response to snow and ice conditions incurred heavy criticism, the airport must prove it can cope with business as unusual.

Describing the task as a unique operational challenge, Cole said Heathrow is more than just its owner BAA.

There are hundreds of companies who operate here, 94 airlines, countless retailers, bus and train companies, and the UK Border Agency. Uniting us into a common purpose has been the biggest challenge of my job to date, he said.

He added that there will be more staff deployed on immigration lanes and in security.

To fast-track arrivals and departures for the 10,100 competing international athletes and their 37,900 bags, Heathrow announced on Thursday that a special Games Terminal will begin construction on March 1 on a site between terminals four and five.

It will only operate between August 13 and 15.

Cole, who has visited previous Olympic host city airports to see their plans in operation, got the idea from Vancouver, site of the 2010 Winter Games.

The temporary terminal, staffed by up to 50 full-time workers and 1,000 meet-and-greet volunteers, will also offer bag collection from the Olympic and Paralympic Village that are to be checked-in the night before.

BAA, which owns six airports including Heathrow, have said its Olympic operations will together cost 20 million pounds to implement.

Due to lack of airfield capacity during the Olympic period, charter flights and private jets will use other airports such as Stansted.

Heathrow's preparations for the Games can be found at http://www.heathrowairport.com