The Irish Aviation Authority barred aircraft from landing at Ireland's largest airport for about two hours after the radar system failed to display the call signs that normally identify each incoming aircraft. Landings resumed at a reduced rate Wednesday afternoon.
Irish airline Aer Lingus announced that all of its Wednesday night flights involving Dublin would be canceled so that its traffic jam of morning and afternoon flights could get airborne.
The airport's other major user, Ryanair, canceled 10 flights and warned other passengers to expect delays of up to five hours.
In a statement, Ryanair said engineers "shut down the Dublin radar computer but the restart was unsuccessful" and harshly criticized the Irish Aviation Authority, which is responsible for running air traffic control.
The Irish Aviation Authority acknowledged that aircraft call signs have disappeared from controllers' radar screens on several occasions in the past month and engineers from the system's French designer, Thales Group, arrived in Dublin this week to fix the problem.
But Irish Aviation Authority spokeswoman Lilian Cassin said the problem reappeared Wednesday. It meant controllers had to order inbound aircraft to hold their positions as they identified each incoming "blip" by other means, and a traffic jam quickly developed.
She said controllers took the decision to shut down the runway to all inbound traffic shortly before 2 p.m. when the problem worsened.
The Dublin Airport Authority, which runs other parts of the airport, held an emergency meeting to salvage as much of the day's schedule as possible.
Dublin Airport Authority spokeswoman Siobhan Moore emphasized that the airport quickly reopened the runway for departing aircraft, although at a reduced rate of approximately one takeoff every five minutes.
But Aer Lingus spokeswoman Gillian Culhane said all of its flights involving Dublin that were scheduled to depart after 5 p.m. would be canceled.
Moore said more than 10 aircraft were diverted to Belfast, about 100 miles to the north, or to Shannon Airport about 140 miles to the west.
Cassin said the radar fault meant that Dublin controllers could not see the call signs that normally specify aircraft by airline and route. She said controllers lacked confidence in the system following the second breakdown.
"Safety had to be our overriding concern, and air traffic controllers did not believe their systems were sufficiently stable to continue," she said.
Cassin and Moore later confirmed that the radar was relaunched, and landings resumed, by 4 p.m. But Cassin said aircraft were being permitted to land at only half their usual rate.
Ryanair spokesman Stephen McNamara said the Irish Aviation Authority moved too slowly.
"The issues with radar equipment at Dublin have been known for some weeks and it is unacceptable that there is no contingency or back-up plan," he said.