The city of Amami received 133 reports of low-altitude flights by the US military in the 12 months through March. That's nearly six times more than the previous year.
Many are believed to be US Marines' Ospreys. Residents have recorded video on their smartphones of the tilt-rotor aircraft -- which have had a number of serious accidents in the past -- flying over residential areas, sometimes after sundown.
"I see Ospreys several times a week. They fly so low that I can clearly see the shape of the aircraft," says one worker at a gas station in the city.
Residents like Jomura Norifumi are concerned. He has been watching the situation closely, as the head of a local civic group. Jomura collects sighting reports of US aircraft and sends them to the Defense Ministry's Kyushu bureau via the Kagoshima prefectural government.
"US military aircrafts are flying low over schools, hospitals and preschools. It would be a total disaster if one of them crashes. If the US side actually cared about us I don't think they would operate this way," Jomura says.
The US Marines released a report back in 2012 showing the designated routes along which Osprey drills take place in Japan. One originates in Okinawa, which hosts most of the American military facilities in the country, and passes over the western edge of Amami-Ohshima. But not over any inhabited areas.
NHK asked officials at US Forces Japan about the uptick in military flights. In an email, they said the USFJ is "committed to the training, readiness, and deterrent posture of our forces in today's evolving security environment in order to consistently meet high readiness standards."
"These flights and the readiness maintained by this type of training is a necessity for US forces in order for the USG to meet its defense of Japan responsibilities in our Treaty of Mutual Cooperation," the email continued.
Maeda Tetsuo, an expert on defense issues, says training flights are rising in response to China's growing military presence in the East China Sea. He believes US forces are holding exercises to show they're ready to defend Taiwan, which Beijing considers a Chinese province.
"Relations between China and the United States have become increasingly tense since the end of the Trump administration. There are signs that this has been carried over to the Biden administration, although Biden hasn't disclosed his comprehensive China policy," says Maeda, who is a former professor at Tokyo International University.
"It's natural that tensions with China are reflected in the operations and training plans of US forces in the region. And it's quite possible that the sense of urgency in Washington is having an impact on the US forces in Japan."
Abiding by the rules?
The Japan-US Status of Forces Agreement allows for the transfer of personnel and equipment between US bases in the country. Maeda says the US military may have broadened its interpretation of the rules to conduct low-altitude flights during such transfers, which may help explain recent complaints in other parts of the country.
"Recently, it was revealed that US forces are carrying out low-altitude helicopter flights around the Tokyo Sky Tree area, in the capital of the country. It also came to light that they had landed at Yokohama airport's North Dock without permission. Low altitude flights by US forces are spreading across the Japanese archipelago," Maeda says.
Sightings of low-flying US military aircraft have been reported in other parts of the country, such as the Shikoku region and Hokkaido.
Under Japanese law, in densely populated areas airplanes and helicopters must fly a minimum of 300 meters above the tallest building, within a horizontal distance of 600 meters. In less populated areas, the minimum safe altitude is 150 meters.
But these standards have not been applied to U.S. military aircraft in the country. A Japan-US committee of foreign and defense officials has agreed that the US military will abide by the standards of Japan's civil aviation law when conducting low-altitude flight training. But the system doesn't allow for legal liability in the event of violations.
For its part, US Forces Japan told NHK that the US "operates and trains within limits of applicable bilateral agreements."
Prefectural governors want more information. They are asking the central government to give them advance notice about the timing of these drills and the flight routes.
Japan's Defense Ministry says it isn't informed about the details of US training exercises. But it's urging American military authorities to put safety first and limit the impact on residents.
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