Chaos at mainland airports caused by unidentified flying objects is being blamed on an increase in so-called black flights.
Unauthorised flights by private aircraft have become a common annoyance as the country's growing number of billionaires buy planes and helicopters to flaunt their wealth.
But the problem is becoming a potential danger to airlines and passengers, with airports being forced to shut down and commercial flights having to be diverted.
Baotou airport in Inner Mongolia was shut down for more than an hour on September 11 after air traffic controllers reported a UFO buzzing round the airport.
It was the ninth report of a UFO in the skies over Hunan , Sichuan , Shandong , Shanxi , Yunnan and Zhejiang provinces and the Xinjiang Uygur Autonomous Region since June.
In July, another UFO report forced Xiaoshan airport in Hangzhou, Zhejiang's capital, to suspend operations for an hour.
The authorities claimed the aircraft was part of a military test at a nearby air force base.
But people in the neighbourhood said it could have been a private plane whose owners took to the sky without appropriate licences.
In late April, flights to Shanghai's Pudong and Hongqiao airports were delayed or forced to be diverted after an illegal flight by a helicopter.
Private access to the sky is strictly limited by the government. But the fine for those guilty of flying illegally is between 10,000 yuan (HK$11,660) and 100,000 yuan (HK$116,600), a trivial amount for the rich.
Tycoon Liu Boquan, of Dongguan , Guangdong province, took off in his own helicopter in an attempt to trace thieves in July, apparently without the approval of air controllers.
But he received an award for bravery from the city government.
Chen Yan , 40, a cafe owner in Zhuhai and the mainland's first female to obtain a private licence, told the South China Morning Post that black flights were an open secret among pilots.
She said: 'In the United States, flyboys can take to the sky after simply filing a flight plan, but the scenario is totally different in China.
'The mainland's air space is still highly restricted to military and commercial aircraft, and potential fliers need to apply to air traffic controllers seven working days ahead for a single journey.
'It is too difficult to fly on your own in China. I have the opportunity to fly and get a private licence largely because my father-in-law served in China's air force and my husband received his own private licence in 2000.'
Chen said she knew many private jet owners in her hometown in Zhejiang who fly illegally.
'Lots of people do that ... a friend of mine crashed a helicopter to the ground, and his spine was badly hurt during a black flight. Thanks to the surgeons, he finally recovered after sitting in a wheelchair for more than three months,' she said.
Guan Xiwen, general manager of the Guangzhou Suilian Helicopter General Aviation Co, attributed the rising number of black flights to the mainland's administration, which had not recognised the new demands of the rich.
'China neither has laws and regulations to manage air traffic of private planes, nor issues licences to assemble helicopters for leisure and sport purposes, which are very popular in the United States and have grabbed a considerable market share in China, too. It means owners have to fly illegally,' said Guan.
'Black flights at a very low attitude to avoid being detected by authorities' radars are common. [I guess] the number of private helicopters or jets registered with the General Administration of Civil Aviation of China is fewer than 200, far below the real number.'
Scott Jiang Li, a Beijing-based regional director of US private jet manufacturer Cirrus Aircraft, said the mainland's tedious procedures for importing private aircraft and a lack of necessary infrastructure were other factors that make some rich people fly illegally.
He said: 'The regulations on buying private jets abroad are too strict.
'Only certain types of jets are allowed to be imported to China.
'In terms of infrastructure, nobody will build his or her own apron and hire a crew just to fly privately, but that's what Chinese aviation authorities require.'
He added: 'In China, individuals aren't allowed to buy their own planes before they appoint professional operators that can provide an apron, pilots, a maintenance crew and a hangar, and help them apply for tedious flight permits ahead of each journey.'
Jiang said very few operators provided those services on the mainland and there are few aprons available, even in Beijing and Shanghai.
To boost aircraft sales, Jiang's company built its first apron in Zhuhai and provides a maintenance and storage service for private jets. He said the company was discussing with more regional governments the possibility of building more aprons in an attempt to encourage billionaires to buy their planes and fly legally.
Zhu Songbin , 52, a tycoon in Wenzhou, Zhejiang, who was fined for a black flight in March, complained at least two of his sport planes couldn't receive licences from aviation authorities.
'The two helicopters, priced under 2 million yuan (HK$2.3 million) each, aren't recognised by mainland authorities,' he was quoted by Shanghai's Xinmin Weekly as saying.
'Also, I couldn't find an apron nearby or build my own, which is a prerequisite for each authorised journey. I'm doomed to fly illegally.'
Is it a plane?
The eight provinces where UFOs (like this one, right) have been reported since June. Their identity may not be so mysterious after all