Singapore's first four next-generation fighter jets, the F-35 Joint Strike Fighter, will have their initial training and evaluation conducted in the United States, said Major-General Kelvin Khong yesterday.

The Chief of Air Force said the evaluation would not just be on the aircraft's performance, but also on how it can be integrated into the Republic of Singapore Air Force's (RSAF's) warfighting systems.

"We believe it is not sufficient for the platform itself to be capable. It needs to be integrated into the RSAF system so that the effects of its combat power can be multiplied," he added.

In a wide-ranging e-mail interview with local and foreign media that was tied to the Singapore Airshow, Maj-Gen Khong acknowledged concerns over the F-35s, which included design deficiencies and cost overruns.

"We are convinced that the F-35 programme has matured to a stage where confidence in the eventual delivery of a cost-effective fifth-generation fighter is high," he said, adding that most of the issues had been resolved in recent years.

The US government last month gave the green light for the sale of up to a dozen of the stealth fighters and related equipment to Singapore for US$2.75 billion (S$3.8 billion), although Singapore's purchase must still be approved by Congress.

He noted that the price of the aircraft has been falling steadily because of "healthy orders" from the US and other countries like Britain and Australia, and the cost of one F-35 today has dropped by more than 40 per cent since 2010.

He estimated that the total cost of acquiring and operating an F-35B, a pricier variant of the F-35 with short take-off and vertical landing capabilities which Singapore has requested, over the lifespan of the aircraft is comparable to that of an F-15SG.

Giving an update on the RSAF's other hardware upgrades, he said that the F-16 upgrade programme which started in 2016 is on track, and the first upgraded F-16 aircraft should be rolled out around 2021.

The H225M and CH-47F helicopters will replace the Super Puma and older CH-47D Chinook that have been in service since 1983 and 1994, respectively. Delivery is expected towards the later part of the year, he said.

On the recent agreement with the US to set up a new fighter training detachment in Guam signed last December, Maj-Gen Khong said its "sole purpose" is to meet the air force's training needs.

$3.8b Cost of up to a dozen of the stealth fighters and related equipment that the US government has agreed to sell to Singapore, although the purchase must still be approved by Congress.

"Like our other overseas detachments, Guam gives us access to larger training areas that will help us mitigate airspace constraints in Singapore. We will be able to conduct bigger-scale and more complex training under realistic conditions to hone our operational capabilities."

He added that discussions with the US are ongoing, with the detachment scheduled to be established by the end of the decade.

In his first interview since he was appointed air force chief last March, Maj-Gen Khong also spoke about how the security landscape had become more complex in recent years, along with the challenge of protecting Singapore's skies.

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In the past 10 years, air traffic around Singapore has grown by about 59 per cent, with an average of more than 1,000 flights in and out of the Republic every day.

"Our air defence operators need to be vigilant to monitor the air traffic around Singapore and spot any abnormal behaviour. When there are threats, they need to respond to make sure these are swiftly dealt with," said Maj-Gen Khong, a trained fighter pilot who has flown the F-16 and F-15SG.

Drones are another threat that has emerged over the past few years, he said.

This is not just from small, commercial drones like those that caused disruptions at Changi Airport last June.

"Larger, military-grade weaponised drones are perhaps less easy to acquire and operate, but can wreak far greater damage, and so continue to present a serious threat that we must be prepared to guard against," he said. "We have built up capabilities to prevent such drones from attacking Singapore, but we will need to continue to enhance these capabilities as the drone threat continues to evolve."

On the RSAF's manpower challenges, Maj-Gen Khong emphasised that members of the air force are its greatest asset.

"We have a highly skilled and educated people, and they are dedicated to the mission of defending our nation's skies. We will continue to invest in developing our people and equipping them with the skills to exploit technology so they can do their jobs better."