IN CHINESE slang, it used to be said that an official who was rising unusually fast up the bureaucratic ranks was “riding a helicopter”. Nowadays it is more common to say that such a person is enjoying a “rocket-style promotion”. Cai Qi is a striking example of someone whose career has thus defied political convention. He was launched in May to the post of Communist Party chief of Beijing, becoming the capital’s most senior official, and is all but certain to be elevated directly to the ruling Politburo later this year, without first tarrying on the lower rung of the Central Committee. Mr Cai’s ascent is a sign that China’s president, Xi Jinping, is tightening his control over the city. He has turned to an unusual personality to help him.

The politics of the capital are hugely important to the central leadership. Beijing’s party chief is responsible for the city’s security, which the government sees as critical to stability elsewhere (in 1989 the Tiananmen Square pro-democracy protests in Beijing sparked nationwide unrest). Whoever holds the job runs a city that, in the party’s eyes, plays a crucial role in shaping China’s image abroad and projecting the country’s “soft power”. Reducing the city’s notorious smog and traffic jams is therefore regarded by central officials as a task of national importance, not just a local one.

There is no doubt that Mr Cai (pictured), who is 61, is the president’s man. Much of his working life has been spent in Mr Xi’s shadow. He served as a senior official in the coastal provinces of Fujian and Zhejiang when Mr Xi was their party boss. In 2014, two years after Mr Xi took over the leadership of China, Mr Cai was brought to Beijing to work in the National Security Commission (NSC)—a body newly established, and led, by the president.

It was in October 2016 that Mr Cai’s rocket broke the sound barrier. He moved rapidly through posts in the capital, from acting mayor to mayor to party chief in just eight months. His elevation to the Politburo—expected after a party congress late this year—would be just as remarkable.

Mr Xi doubtless feels far more comfortable with Mr Cai in charge of Beijing. The city’s previous party chief, Guo Jinlong, was a holdover from the days of Mr Xi’s predecessor, Hu Jintao. Replacing allies of Mr Hu, and of Jiang Zemin before him, appears to be one of Mr Xi’s main aims at the upcoming congress. Mr Guo is now 70, which, according to convention, means he will have to retire from the Politburo at the gathering. His replacement by Mr Cai as Beijing’s chief suggests that is the plan.

The capital’s kingdom

Having an ally in place will help Mr Xi to avoid a problem that has sometimes plagued the city (and that he may have encountered in his dealings with Mr Guo): a local government led by someone who is out of step with the central leadership’s thinking. Peng Zhen, the first party chief of Beijing after the Communist takeover in 1949, was a close confidant of Mao Zedong but was purged by him in 1966 for becoming too critical of the chairman’s policies. Chen Xitong, who ruled Beijing in the years spanning the Tiananmen unrest, was purged in 1995—ostensibly for corruption but more likely for cultivating an independent power-base in the capital.

But the choice of Mr Cai has raised eyebrows. At that level of leadership, officials normally try their hardest to conform. Mr Cai, however, projects a more independent-minded persona. He refrains from dying his hair shoe-polish black (usually de rigueur for men of his age and rank). Instead, he sports short, salt-and-pepper hair. And, although a handful of people who have attended meetings with him say he tends to listen far more than he speaks, he was an avid microblogger before joining the NSC.

Some of the views Mr Cai used to share in his Zhejiang days on Tencent Weibo, a Twitter-like platform, lie far outside the cautious limits of party-speak. In 2011 he bemoaned the fact that Chinese censors block access to Facebook (and Twitter, too, he might have added). Less controversially, Mr Cai intervened in 2012 when a small boy was attacked and badly hurt by a dog in Zhejiang. He had learned of the matter during the annual session of China’s parliament—the boy’s family had posted bloody pictures of him. On his microblog, Mr Cai confirmed the family’s charge that the dog belonged to a government department and arranged compensation.

That Mr Cai tweeted at all was unusual enough for a senior official. That he did so engagingly was truly exceptional. With millions of followers on Tencent Weibo, he became one of the country’s biggest social-media celebrities (“a Bolshevik” is how he described himself in the blurb of his account). One of his most talked-about posts was addressed to a woman who complained that her son, a tax official, kept being forced to drink excessively at official banquets. “Tell me which tax department your son belongs to,” Mr Cai responded. “He won’t have to drink any more.”

During his four years of microblogging, Mr Cai averaged more than six posts a day. He compiled them into a book, which he called “A Room Made of Glass”. He said he chose the title because social media promote transparency and public supervision. These are hardly attributes that Mr Xi seems bent on encouraging with his ever-tighter censorship. But his promotion of Mr Cai suggests he is keen on someone who might be able to win the support of Beijing’s residents, who complain endlessly about the city’s faults, from its foul air to the high cost of housing.

Mr Cai is being helped, for now, by Chen Jining, who took over in May as the city’s acting mayor. At 53, Mr Chen (who attended university in Britain) is someone to watch as a possible member of China’s “sixth generation” of leaders who will take over when Mr Xi retires. Curbing smog will be one of Mr Chen’s priorities, though in his previous role as minister of the environment he made little progress in tackling air pollution. Some analysts believe he will be promoted to deputy prime minister next year.

Mr Cai’s age—he is less than three years younger than Mr Xi—means he is unlikely to be groomed as a possible successor to Mr Xi when he retires in 2022 (assuming he follows precedent; there is some doubt that he will). But Mr Cai has a lot of work to do in Beijing. One task will be to implement Mr Xi’s plans to improve Beijing’s environment by downsizing the city: he wants to build a new one in the neighbouring province of Hebei that will take over some of Beijing’s “non-capital” roles, for example as a centre for business and higher education.

Another job will be to move his own office: to ease congestion in the centre of Beijing, the city administration is due to relocate later this year to Tongzhou, a satellite town about 20km to the east. The grumbling of thousands of uprooted officials will add to that of bigwigs miffed by Mr Cai’s queue-jumping career.