Missile drills in the South China Sea and aeronautical limitations were the reasons a Taiwanese aircraft was denied entry into Hong Kong airspace en route to the Pratas Islands last week, according to a military insider.
"The People's Liberation Army (PLA) was conducting an air-to-air missile exercise in the South China Sea in the morning (on Thursday) when the Taiwanese aircraft was heading to the Dongsha Islands," a Beijing-based military source close to the PLA said, using the Chinese name for the group of three atolls.
The UNI Air flight carrying military and coastguard personnel was forced to turn back when Hong Kong's civil aviation authorities told their counterparts in Taipei there were "dangerous activities" taking place below 26,000 feet.
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"Most passenger airliners fly above 26,000 feet," said the source, who asked not to be identified because of the sensitivity of the matter. "But the Taiwanese aircraft was a propeller-powered ATR 72 that can't climb that high."
According to aircraft websites, the ATR 72 has a maximum altitude of 25,000 feet.
On Friday, the day after the incident, Taiwan's Minister of National Defence Yen Te-fa rejected the explanation given by Hong Kong and appealed to the city's authorities not to "disrupt the order of international aviation".
According to Yen, there were no military activities taking place in the area and China's maritime safety agency had not issued any warnings.
"We hope that Chinese authorities will abide by the international aviation regulations and place importance on safety," he said.
Hong Kong's Civil Aviation Department had earlier rejected the suggestion it deliberately impeded the flight and said its staff were following "established practices and procedures".
It said it notified Taiwan's air traffic control centre of "the minimum safe altitude to be observed for flights" and that the centre indicated it would "cancel the request" for the flight to enter Hong Kong's airspace.
The incident came amid an increase in military exercises by the PLA in disputed regional waters as well as in the skies close to Taiwan's airspace. Beijing regards the self-ruled island as part of its territory and has never rejected the use of force to reunify it.
"This would never have happened if the Taiwanese military had chartered a more advanced aircraft, like a C-130 transport plane, that could fly higher," the source said, though he added there appeared to have been a breakdown in communication on both sides.
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