Without saying how many were in operation, the People’s Liberation Army Air Force confirmed on Friday that the J-20, the country’s fifth-generation fighter, had entered combat service, meaning it was combat-ready.

However, the aircraft was equipped with inferior engines designed for earlier warplanes when it first joined the air force in March last year because “critical problems” with its tailor-made WS-15 engine, exposed by an accident in 2015, had not been fixed, two independent military sources told the Post.

“The WS-15 engine designed for the J-20 exploded during a ground running test in 2015,” one source said, adding that no one was injured in the accident.

“The explosion indicated the WS-15 is not reliable, and so far there is no fundamental solution to overcome such a problem … that’s why the J-20 is using WS-10B engines now.”

The WS-10B is a modified version of the WS-10 Taihang engine, which was designed for the country’s fourth-generation J-10 and J-11 fighters.

The explosion was confirmed by another source close to the military, who said the reasons it happened were complicated, with one being the quality control of its single-crystal turbine blades, the key component for such a powerful turbofan engine.

The new-generation, single-crystal turbine blades designed for the WS-15 were supposed to be able to cope with the increased mechanical loads caused by significantly higher rotational speeds and extremely high temperatures.

Former Aviation Industry Corporation of China (AVIC) chairman Lin Zuoming told state media last year that China’s third-generation single-crystal turbine blades could withstand temperatures of up to 3,632 degrees Fahrenheit (2,000 degrees Celsius) in WS-10 engines, extending the engine’s lifespan from 800 hours to 1,500 hours when compared with those using the previous generation of turbine blade.

However, the thrust-to-weight ratio of the original WS-10 engine was only 7.5, while that of the WS-10B tops out at about nine. The thrust-to-weight ratio of the all-direction, vector turbofan WS-15 Emei engine is more than 10 – one of the basic requirements for giving the J-20 “supercruise” ability.

Supercruise allows stealth fighters like the US’ F-22 Raptor to fly at supersonic speeds without using afterburners, making them harder to detect. The F-22 is powered by the world’s most advanced jet engine, the Pratt & Whitney F119.

But achieving supercruise would require the single-crystal turbine blades of the WS-15 engine to cope with temperatures even more extreme than those handled by the WS-10.

“Chinese technicians are able to produce cutting-edge-quality single-crystal turbine blades when concentrating on a specific single item,” the second military source said. “But they’ve still failed to turn the advanced technology into a standard product for mass production. It’s a bottleneck problem that needs more time to overcome after countless experiments and tests, based on Western experiences.”

“Using the WS-10B engines is just a temporary, expedient stage of the J-20’s engine development … in the future, the aircraft may use another new engine, the improved performance engine (IPE) version of the WS-10, the WS-10 IPE, until the development of the WS-15 is successful,” the first source said.

“It’s so embarrassing to change engines for such an important aircraft project several times … just because of the unreliability of the current WS-15 engines. It is the long-standing core problem among home-grown aircraft.”

It is at least the third time the J-20 has changed engines. The aircraft’s maiden flight was powered by two Russian AL-31 engines, which are less capable than China’s WS-10B.

Russia has refused to export its most advanced engines to China because they are the core technology of its aviation industry. That prompted China to develop an indigenous alternative, the WS-15, as part of its drive towards producing one of the world’s most advanced combat aircraft.

Beijing has stepped up the development of sophisticated jet engines since the turn of the century, with at least 150 billion yuan (US$23.7 billion) invested between 2010 and 2015, military insiders told the Post in 2016.

State-owned Aero Engine, a company formed in August 2016 through the merger of 24 AVIC subsidiaries with about 10,000 employees, has led China’s development and production of aircraft engines and gas turbines in recent years. One of the former AVIC subsidiaries, Shenyang Aeroengine Research Institute, developed the WS-10 and WS-15.

China Central Television boasted last year that the performance of the WS-15 had matched that of the F119, with a documentary aired in May claiming the WS-15 engines would be widely used in the J-20 by 2020.

That would make China the third country to have a fifth-generation fighter – after the US and Russia. Justin Bronk, a research fellow specialising in combat air power at the Royal United Services Institute, told Business Insider the design of the J-20 gave it “longer range, more internal fuel capacity, and larger internal weapons capability” than its US counterparts, the F-22 and F-35, and that meant it presented a real threat to US forces in the Pacific.

However, military analysts said it was too early for such prognostications because the underdeveloped WS-15 engine was not yet as sophisticated as the F119, which had a lifespan of more than 4,000 hours.

The prototype of Russia’s first fifth-generation fighter jet, the Su-57, took to the air for the first time in January 2010 but its maiden flight powered by the new Izdelie 30 engine, which was specially designed for it, only took place last month. The new engine will enable the Su-57 to achieve supercruise speed and fly consistently at 1,242mph (2,000km/h), but it still faces many tests, with the Russia Beyond website saying it would not be mounted on the Su-57 before 2019 or 2020.

China’s WS-15 project started in the 1990s, with the first prototype delivered in 2004 and the first successful ground-running test staged in 2015, Antony Wong Dong, a Macau-based military observer, said.

“China took 11 years to test the WS-15 after its prototype was created,” Wong said. “It’s very normal to take three to eight years for further development.

“And it’s still a significant achievement for China to develop the WS-10B as a stopgap for the J-20. China’s advanced aircraft engine technology foundation is very weak because its research and development started several decades later than its Western counterparts.”

America’s Pratt & Whitney and General Electric spent more than 12 years developing prototypes of the F119 engine in the 1980s, followed by another 14 years of testing after the engine’s maiden flight, fitted to an F-22, in September 1997.

The US also began developing the F-35, a single-seat, single-engine, all-weather stealth multirole fighter in the 1990s, with the first squadron – using Pratt & Whitney F135 engines, a derivative of the F119, joining its air force in 2016.

The US has started deploying F-35s at its military bases in the Asia-Pacific region, with 12 arriving at the Kadena Air Base in Japan in November. South Korea has said it plans to take delivery of 40 F-35s this year.

Earlier reports said Japan’s Maritime Self-Defence Force was also considering buying the F-35B “jump jet” version for its two new 25,000 tonne, flat-topped Izumo-class helicopter carriers. That pushed Beijing to put the J-20 into service as early as possible, military sources said.

Andrei Chang, editor-in-chief of Canadian-based Kanwa Asian Defence, said the J-20 might need at least eight years to improve its engine performance, but it was still the PLA Air Force’s most capable combat aircraft when it came to countering America’s F-22.

“The J-20 is more advanced than China’s fourth-generation fighters because of its stealthiness,” he wrote in the latest edition of the magazine, adding that it could also carry a greater quantity of more advanced air-to-air missiles.

Chang said he did not expect China to put the J-20 into mass production until the WS-15 project was wrapped up, but that training of J-20 pilots had been going on for a while. He said the PLA Air Force had deployed J-20s to the Dingxin Test and Training Base in Gansu province, where they and other warplanes had been used to study counter measures against the F-22 and F-35.

Hong Kong-based military commentator Liang Guoliang said there was a “very urgent” need for China to develop a carrier-based stealth fighter based on the J-20 because of the regional security environment was becoming more challenging.

“The US F-35B is much more powerful than China’s current carrier-based fighter, the J-15,” Liang said. “In case there is a war, the fighting capability of a Japanese Izumo helicopter carrier could come up to that of a Chinese aircraft carrier if each ship is equipped with five to six F-35Bs, with each one the equal of 10 J-15s.”

But, he cautioned, it would take at least a decade to turn a land-based fighter jet like the J-20 into a carrier-based aircraft because of the extensive modifications required, adding that the US had been developing carrier-based aircraft since the 1930s, while China had just started doing so this century.

China’s carrier-based J-15 is based on the Russian Su-33, a variant of the fourth-generation Su-27 fighter.

“China should hurry up; the central leadership should make a decision as soon as possible,” Liang said.