THE hijacking of the China Southern Airlines plane which landed at Taipei's Chiang Kai Shek International Airport has exposed a loophole in the country's air defence network, a Taiwanese newspaper reported yesterday. According to the United Evening News, the foundation of a radar system, which normally monitored one route used by incoming aircraft to Taoyuan, was unstable and in danger of coming down in strong winds. The radar system had been closed temporarily for the foundation to be reinforced, the newspaper quoted unnamed sources as saying. The Air Force was said to be extremely concerned and had adopted contingency measures to fill the gap. Air Force commander General Tang Fei also took a personal interest in the problem. But the newspaper quoted sources as saying that it was only after the China Southern Airlines plane had flown past Houlong, 42 nautical miles southwest of Taoyuan, that the Air Force was alerted. ''It appears the air defence loophole has not yet been plugged,'' said the newspaper. However, Taiwan's Defence Ministry said in a statement yesterday that when the plane sent out an emergency signal at 9.18 am, it was immediately discovered by the radar system. When the aircraft approached the middle of the Taiwan Strait at about 9.40 am, six F-5E fighters took off to intercept it. The Chinese aircraft was finally escorted down to Taoyuan. The statement said relevant military and national security authorities, as well as the Mainland Affairs Council, were informed of the hijacking shortly after the Chinese plane crossed the Taiwan Strait. Taiwan defence forces also re-examined surveillance capacities after a hijacking incident in 1988, when a Boeing 737 was commandeered from Xiamen to Taichung.