SINGAPORE — The world’s biggest aircraft manufacturers expect airlines in the Asian-Pacific region to spend close to $2 trillion on shiny new jets over the next two decades, but some industry-watchers are questioning if carriers really need all those planes.

Much of the growth in Asian commercial aviation has been fueled by aggressive new budget carriers like Malaysia’s AirAsia Bhd. and Lion Air of Indonesia. Low-cost airlines, nearly unheard of in Asia just 10 years ago, now account for a quarter of the seats in the region.

Most airlines in Asia have ordered more planes than their current fleets in the hope of capturing a bigger slice of the market, and that is raising concerns among analysts that the rise in capacity could outpace that of passenger demand.

Airbus Group EADSY -0.55% NV on Tuesday said it expects airlines in the Asian-Pacific region to buy planes worth $1.8 trillion over the next 20 years. That view is close to the $1.9 trillion estimate Boeing BA +0.91% Co. gave a day earlier.

Asia’s “macro economic context is looking good in the long term but every airline believes its market share will grow. That cannot happen,” said Bertrand Grabowski , the head of aviation finance division of Germany’s DVB Bank SE.

He said some airlines in Asia will fail and their aircraft will as a result need to be redeployed.

“We aren’t yet talking about over-ordering, but certainly there’s very aggressive ordering,” he said.

However, Boeing and Airbus say the orders are in line with their growth projections as they rake in new deals.

“There is a good balance between supply and demand,” said John Wojick , who heads global sales of commercial aircraft at Boeing, noting that he is comfortable with the company’s current global backlog of 5,080 planes.

Smaller regional airlines in Asia are making a big push for new capacity as they foresee market growth. At the Singapore Airshow on Tuesday, Vietnamese budget carrier VietJetAir signed firm orders for 63 Airbus A320 jets, as well as purchase rights for 30 more, in a deal valued at $9.1 billion at list prices.

The order is at least several times the size of the current fleet of 11 leased A320s at VietJetAir, which flew 4 million passengers last year.

Also on Tuesday, Myanmar’s state-owned Myanma Airways entered a leasing agreement for 10 Boeing 737 jets that will nearly double the company’s current fleet of 12 planes.

The deals add to the hundreds of jets already on order among Asia’s premium carriers and low-cost airlines.

AirAsia and Lion Air have more than 1,000 new jets on order between them that will be delivered over the next decade. The order books at Boeing and Airbus are nearly full. An airline looking to buy single-aisle planes, the workhorse of the industry, is now unlikely to get delivery slots over the next four to five years.He said Indonesian budget carrier Lion Air is likely to look to place some of its aircraft with other airlines through leasing deals, taking advantage of its existing delivery slots with both Boeing and Airbus.

Lion Air is likely to receive about 45 new planes this year from Boeing, Airbus and European turboprop aircraft maker ATR. Still, the airline is confident it can fill them.

“The Indonesian market is still growing by 15% per year. We feel there’s still opportunity to grow domestic services in Indonesia. Plus Lion has enormous opportunity to expand internationally,” said Leithen Francis, a Lion Air spokesman.

Concerns of rapid expansion in the industry come as even as many of the region’s traditional airlines like Singapore Airlines Ltd. and Malaysian Airline System Bhd. are struggling from intense competition.

A spokeswoman for AirAsia said some of the new orders will replace older aircraft “to ensure we keep a young and efficient fleet.”

Asia’s full-service carriers have, in recent years, seen budget airlines eat into their market share on shorter routes. On longer routes to the U.S. and Europe, they have to compete against international airlines, particularly Gulf carriers, such as Emirates Airline and Qatar Airways.

Still, Boeing and Airbus are eager to sell more of their flagship jets to the region’s airlines, as reflected by their expectations that the region will take delivery of over a third of total planes being produced through 2034.

Airbus brought its entire leadership team to Singapore for the city-state’s biennial air show, where its latest jet, the widebody A350, made its first flying display at an international air show on Tuesday.

“We are on track to certify the A350 and deliver it to Qatar Airways by the end of this year,” Fabrice Brégier , said Airbus president and chief executive.

Airlines in the Asia-Pacific will need 10,940 new planes in the next 20 years, Airbus said Tuesday, with 4,100 of those being widebody jets that typically carry 300-500 passengers. Airbus’s projections are slightly below the 12,820 jets that Boeing expects region to take.

– WALLSTREET JOURNAL