Nanyuan Airport is closer to the capital's downtown area only 25 to 30 kilometres, while the distance from Langfang is more than 40 kilometres.

"Usually an airport is less than 30 kilometres from the city it serves," Guo Feng, assistant manager of the planning department of the Capital Airport Group Corporation, was quoted by Beijing Business Today as saying.

Another advantage Nanyuan Airport has is that it is located in Beijing's Daxing District, within the capital's jurisdiction, while Langfang is part of Hebei Province.

However, currently used by both civilian and military planes, Nanyuan Airport may not have as much available airspace as Langfang.

Both Daxing district government and Beijing Municipal Commission of Development and Reform have refused to comment on the possible site for an airport.

But airport construction specialist Wang Wei, said distance would not be a problem as long as the airport was between 30 and 60 kilometres from downtown Beijing.

"Many cities' experiences show that an airport too close to the downtown area would actually hinder the city's expansion, so 40 kilometres the distance between Langfang and Beijing will not be a problem," said Wang.

According to Li Haijun, the Langfang airport would be an hour's drive from Tian'anmen Square.

In addition, building a second airport in Langfang could stimulate the economic development of the whole Beijing-Tianjin-Hebei area, say experts.

The NDRC recently proposed that the site should be selected taking the whole area's economic development into account.

Initial work was done on finding a site for a second airport for the capital in 2002 and 2003, but the scheme was suspended when it was decided to expand the Capital Airport instead.

Scheduled to be completed by the end of 2007 to serve the 2008 Olympic Games, the expansion project includes the construction of a third terminal and runway.

With these facilities, the capital airport will be able to handle 60 million passengers and 1.8 million tons of cargo per year by 2015.

The current airport facilities can handle 35 million passengers and 780,000 tons of cargo every year.