Australia’s shortage of air traffic controllers is showing signs of nearing a breaking point after the country’s busiest airport recorded at least four near misses in the past month. Since late July, incidents at Sydney Airport included a passenger jet aborting its take-off because there was a plane in its path. In a separate close call, an aircraft that was taxiing had to slam on the brakes to avoid colliding with another jet. Short-staffed control towers have long been a problem globally, but...