The Chinese military’s use of ageing J-7 jets in a Taiwan fly-past last month has raised questions about why the second-generation fighters were deployed alongside more modern warplanes. Military sources raised the possibility that the planes had been converted into drones, which offer a cost-efficient way of honing the People’s Liberation Army’s combat drills and testing Taiwan’s response, and added they may also have been a way of testing whether all the island’s warplanes have resumed operations.

The “island encirclement exercise” on June 17 included four J-7s – fighters originally modelled on 1960s Soviet MiG-21s and known as “grandpa fighter jets” in Taiwan – the first time they had been used on such an operation since they started in 2016.

More modern aircraft, including two J-16 multi-role fighter jets and a Y-8 electronic warfare plane, also entered Taiwan’s air defence identification zone during the exercise, prompting questions about why the older models had joined the operation.

According to mainland media reports, China has turned thousands decommissioned second-generation fighters, including J-7s, into unmanned aerial vehicles (UAVs).

A source close to the PLA told the South China Morning Post that some J-7s have been transferred into target drones because its radar cross-section images are similar to Taiwan’s Indigenous Defence Fighter (IDF) jets and US-made F-16s – potentially confusing air defences.

Neither the PLA nor the Taiwanese defence ministry have said whether the four J-7s involved in last month’s fly-past were UAVs or not, but the military source said they had been piloted and added that the operation had been designed to test the reaction from the Taiwanese air force and public.

“The four J-7s made short flights after taking off from an airbase in Shantou, Guangdong province,” the source, who requested anonymity due to the sensitivity of the topic, said. “It was aimed at testing the Taiwan air force’s response, seeing whether all their aircraft have resumed flying.”

In March Taiwan grounded its military aircraft for safety checks – except for those on guard duty or combat-readiness missions – after two pilots were killed in a collision off the south coast of the island. It was the third fatal accident involving active Taiwanese fighters in the space of six months.

Macau-based military observer Antony Wong Tong said the PLA had been using converted J-7 drones for target practice since 1997.

“There are many variants of the J-7s … which were all dubbed ‘mini F-16s,” Wong said. “Mainland China has also exported the variant J-7s to Pakistan, which used them for mock dogfights.”

The state-owned Shenyang Aircraft Corporation started building J-7s back in 1965 and only stopped making them in 2013.

The PLA hopes to withdraw its last second-generation fighters from service by the end of next year.

Lu Li-shih, a former instructor at Taiwan’s Naval Academy in Kaohsiung, said the PLA had started making UAVs because it helped mitigate the impact of a rapidly ageing society following the introduction of the one-child policy in 1980.

A display board at the 2018 Zhuhai Airshow listed the advantages of turning retired fighters into UAVs, saying that they retained many of their original capabilities, but helped save money and reduced the risk of casualties.

“There are hundreds of reasons that the mainland is coming up with some new combat tactics against Taiwan,” Lu added.

Ben Ho, an air power analyst at the S Rajaratnam School of International Studies in Singapore, said he believed China had been studying tactics used in last September’s Nagorno-Karabakh conflict involving Azerbaijan and Armenia, in which the Armenians had been fooled into firing at unmanned versions of the Soviet-era Antonov An-2 biplane.

“The PLA would do well to take a page from the Azeri playbook and similarly use the J-7 as an unmanned decoy for SEAD [suppression of enemy air defence] during a regional contingency,” Ho said.

“This is especially so as China’s potential adversaries like the US possess top-notch air defences such as the Aegis Combat System.”

Ho said: “It makes sense to deploy the J-7 as a target drone to the extent that its physical dimensions are roughly the same as that of the F-16 and IDF, the two mainstays of Taiwan’s air force.”

China regards Taiwan as a renegade province that must be reunified with the mainland – by force if necessary – and has been conducting island encirclement exercises since President Tsai Ing-wen, from the independence-leaning Democratic Progressive Party, was elected in 2016.