More than 1,000 passengers were affected when flights were delayed or cancelled and incoming flights were diverted to Shenzhen or Zhuhai on Tuesday (December 18), China Daily reports. The cause was a breakdown in the instrument landing system.

An instrument landing system is a ground-based approach system providing precision guidance to an aircraft approaching and landing on a runway, using a combination of radio signals and, in many cases, high-intensity lighting to enable a safe landing. It is illegal to allow a plane to land at an airport where the system isn’t functioning, a pilot told the newspaper.

Airport authorities said 57 flights were affected by the breakdown. Twenty-five flights — 15 arrivals and 10 departures — were delayed for more than an hour.

Ten flights were diverted to Shenzhen and Zhuhai airports while another 10 arrivals were canceled by about 10 pm. Operations at the airport, one of the three busiest in China, returned to normal after midnight.

There has been much dissatisfaction with flight delays and cancellations in China this year. It is common for a flight to be delayed without warning or explanation.

The most notorious case this year was when 20 people blocked the runway at Shanghai Pudong Airport to protest the delay of a Shenzhen Airlines flight.