Air traffic at China’s fourth busiest international airport, in Chengdu, Sichuan province, was interrupted by illegal drone flying in the air space in mid-April. Dozens of flights ended up getting delayed, and some were required to execute forced landings.

According to Sichuan's public security department, a total of 23 flights leaving Chengdu were delayed, and 22 flights on their way to Chengdu were forced to land elsewhere due to a drone flying just 3.7 kilometers away from an airport runway on April 18. Within 10 minutes, another drone was detected flying 14.8 kilometers away from the airport.

Twelve flights made forced landings on April 17, and three were forced to make detours on April 14 before ultimately landing, all due to similar circumstances.

Local authorities have already started investigating these cases, which are seen to have endangered public security. To encourage tip-offs regarding these cases, Sichuan police are offers rewards of no less than 10,000 RMB to anyone who provides useful information.

This is not the first time Chinese air traffic has been disrupted by illegal drones, but the number of incidents has lately been on the rise. In 2015, there were just four such cases, but the number rose to 23 in 2016. As of April, there had already been 13 cases in southwestern China alone, five of which took place in Chengdu. Nevertheless, not a single suspect has been identified, reported Thepaper.cn.

According to the Southwest China Metropolitan Daily, the airport runway protection area for Chengdu Shuangliu International Airport takes the shape of a circle with a radius of at least 20 kilometers from the end of its runway.

“That means the whole city of Chengdu is within the protected area,” an anonymous drone flyer told the newspaper, adding that he flies his drone away from airport and city center, and keeps its altitude below 100 meters.

However, all drones flying without approval from local military and civil aviation authorities are considered illegal, even though drone enthusiasts have complained about the complicated paperwork required to obtain official approval. As a result, more than 90 percent of the drones flown in Chengdu are flown illegally. In 2016, there were just over 100 applications submitted for the whole city, the newspaper noted.

Meanwhile, Thepaper.cn found that a favorite area for drone operators to deploy their gadgets is an open space only 3 kilometers away from the airport, where they defiantly ignore a sign barring any flying activities.

Several witnesses claim that they have seen drones flying in the area, criticizing the devices for the loud noise they produce. Some passersby have allegedly been injured by drones in the process of landing or taking off. One witness specifically claimed that he saw two men flying a 1-meter-tall “taxiing plane” on April 18, whose sound could be heard from several hundred meters away.

In response, police have vowed to strengthen patrols in this area.