Actual demand for services could fall short of expectations, they said.

Nguyen Thien Tong, former chairman of the aviation engineering department at the Ho Chi Minh City University of Technology, told VnExpress International a second airport in Hanoi was not needed given that there’s no certainty air travel demand will continue to surge in the next decades.

He was referring to a recent proposal of the Hanoi Department of Planning and Architecture, which said that the number of air passengers in the capital city could reach up to 150 million in the future and therefore a second airport is needed to be built in the southern part of the city starting 2035.

Tong said that there was no solid ground for this forecast. The number of passengers passing through Hanoi's Noi Bai International Airport has been growing by 10 percent a year and reached 29 million in 2019, and no one can guarantee that the same growth rate could be maintained in the next 50 years.

Architect and airport designer Ngo Viet Nam Son shared similar concerns. He said an already envisioned upgrade for the Noi Bai International Airport would have it accommodate 100 million passengers a year, so an additional airport in the capital city might be redundant.

A 100-million passenger airport is a very big one and if Noi Bai has that capacity it would be able to comfortably handle the demand in Hanoi and several surrounding cities. The staff needed for such big airport, estimated at 100,000, will be enough to cause traffic congestion in their daily commute, he added.

This means that authorities should only think of building a new airport if there are specific plans to develop several big cities around Hanoi to form a large urban complex, otherwise the airport will likely suffer losses, he said.

"Investment should be prioritized toward building more roads and metro routes connecting other areas of Hanoi with the existing Noi Bai International Airport," Son said.

There is valid reason for experts being concerned about losses when commenting on new airport proposals.

Only six of 22 airports in Vietnam are making profits, according to the Airports Corporation of Vietnam (ACV). All six are located in major cities or tourism hotspots: four in Hanoi, Ho Chi Minh City, Da Nang City and Thua Thien Hue Province; and two in Nha Trang and Da Lat.

Such concerns have not stopped several localities from proposing new airports in recent years, all based on expectations of high air travel demand in the future.

The central province of Ha Tinh Province recently proposed the building of an international airport to operate flights to Taiwan and South Korea, expecting a capacity of 500,000 by 2030 and two million by 2050.

Another central province, Ninh Thuan, wants to use its military airport for commercial purposes to receive an expected influx of tourists and investors in the coming years.

In 2018, the southern province of Ba Ria – Vung Tau also proposed its own civilian airport after discussing the idea with a private resort developer.

Tong said localities were "racing" to build new airports without carefully studying actual demand and this could lead to too much investment in air travel.

"Money should be poured into other much-needed infrastructure such as expressways and high-speed railways," he said.

However, some experts support the idea of building a second airport in Hanoi.

Nguyen Bach Tung, a former airport design researcher, said many cities in the world with populations of 10-15 million have built two international airports, so Hanoi will need another one in upcoming years.

Other experts have said that an airport in southern part of the capital city will benefit surrounding localities like Ninh Binh, a travel hotspot.

In Vietnam, demand for air travel was rising consistently until Covid-19 broke out, with its airports handling 116 million passengers last year, up 12 percent from 2018, according to the Airports Corporation of Vietnam.

The country has 22 civilian airports, including 10 international air hubs.

Tan Son Nhat in HCMC and Noi Bai in Hanoi are the largest.