China’s fifth-generation Chengdu J-20 stealth fighter, operated by the People’s Liberation Army Air Force’s elite First Air Brigade, reportedly flew undetected through the Eastern Channel of the Korea Strait on 27 July, according to the South China Morning Post. Neither South Korea nor Japan is said to have tracked the aircraft during its transit — a claim that raises uncomfortable questions about regional air defence readiness.

Beijing has so far maintained a neutral NCND (neither confirm nor deny) posture on the report, subtly fuelling speculation that its stealth platforms may have slipped through the surveillance networks of Seoul, Tokyo, and even Washington.

Following the alleged incursion, China prominently displayed its J-20 and carrier-capable J-35 at the Victory Day military parade on 3 September and again at the Changchun Air show on 20 September. The move showcased Beijing’s growing stealth aviation confidence and signalled its technological ambitions in contested airspace near the Korean Peninsula.

The reports resurfaced during a parliamentary audit at the Air Force Headquarters in Gyeryong, where lawmakers questioned the Air Force’s ability to detect low-observable aircraft flying near its borders. Representative Lim Jong-deuk raised concerns that an unidentified fighter could have approached within minutes of South Korean airspace without detection.

Air Force Chief of Staff General Son Seok-rak responded that the flight date was unverifiable and that there was “no concrete evidence” of a J-20 operating in the region. He reaffirmed that the Eastern Channel lies outside South Korea’s Air Defence Identification Zone (KADIZ), limiting their capacity to confirm or deny detection.

Radar logs from the Master Control and Reporting Center also showed no record of a J-20 flight near 27 July. Military analysts suggested that if such a sortie occurred, it would have required aerial refuelling given the distance from Chinese bases — raising doubts about the validity of the claim.

Defence observers believe China’s ambiguous stance may serve strategic information goals — projecting the illusion of undetectable stealth capabilities to test regional responses. However, without radar confirmation, it remains unclear whether a flight occurred or whether it was a calculated psychological manoeuvre aimed at demonstrating air dominance.

The incident underscores escalating air power dynamics amongst China, Russia, and North Korea. The U.S. Defence Intelligence Agency has identified China’s J-35 and FH-97 stealth UAV as key next-generation assets, mirroring similar advancements in Russia’s Su-57 programme. North Korea, seeking to upgrade its outdated air fleet, may attempt to access allied stealth technologies through this growing trilateral cooperation.

Comparatively, the radar cross-section (RCS) of Beijing’s J-20 ranges between 0.01 and 0.05 square metres — notably larger than that of the U.S. F-22 or B-2 Spirit, estimated at just 0.0001 square metres. Russia’s Su-57 sits around 0.5 square metres, though both Beijing and Moscow continue investing heavily to reduce their aircrafts’ radar signatures.

South Korea’s long-range radar systems, which constitute the backbone of its air surveillance network, are facing severe reliability issues. Of the 12 FPS-117 radar stations in service, eight date back to between 1987 and 1992, with the remaining four introduced in 2004. Many have exceeded their designed operational lifespan.

The Air Force logged ten breakdowns of the FPS-117K this year alone, totalling approximately 100 hours of downtime, while the newer FPS-117E1 units suffered another 90 hours of outages. Maintenance requirements now demand between 100 and 200 hours of annual shutdowns — a dangerous vulnerability amid intensifying regional stealth activity.

Officials admit that existing systems can barely detect stealth aircraft even under ideal conditions. While adjusting sensitivity can occasionally track small objects such as migratory birds, specialised counter-stealth radar remains technologically and financially out of reach for now.

Seoul plans to invest 260.3 billion won (£142 million) by 2029 to comprehensively upgrade its long-range radars. However, analysts warn that without accelerated implementation, South Korea risks prolonged “farsighted blindness” in the face of evolving stealth technologies. For now, the country must rely on ageing sensors — an uneasy situation as China’s shadow in the East Asian skies grows longer.

This article was originally written in Korean and translated by a bilingual reporter with the help of generative AI tools. It was then edited by a native English-speaking editor to refine the article