At least 100 flights to and from Tokyo's Haneda airport were delayed or forced to turn back Thursday after its radar system developed a hazardous tracking glitch, the transport ministry said.
The airport said the glitch made it difficult to track the planes by preventing their speeds and altitudes from appearing on their radar screens.
This forced Haneda to scramble to reduce landings by ordering Narita International Airport in Chiba Prefecture to delay its flights.
The ministry is looking at the possibility that a "concentration of data" is to blame. The two airports integrated their radar systems the same day.
The problem, which began at around 10:40 a.m., forced Narita airport to lengthen its takeoff intervals to five minutes instead of two, causing delays to several flights until shortly after 11 a.m.
Haneda airport fixed the problem at around noon but has been unable to pinpoint the cause.
As of 1 p.m., Haneda had forced seven flights to go back to their departure points and two others to change destinations, while canceling 19 incoming and outgoing flights, the Land, Infrastructure, Transport and Tourism Ministry said.
Another 73 flights were delayed by up to 2 1/2 hours, it said.