Flights across Greece were grounded for several hours on Sunday after a radio-frequency outage crippled air traffic control, stranding thousands of travelers and bringing airport operations to a virtual standstill.
There was little clarity on what caused the disruption, which began early Sunday and quickly escalated, affecting the travel plans of thousands on the busy last weekend of the holiday period.
Some overflights across Greek and regional airspace were still being serviced, but restrictions were imposed on airport operations, Greece's civil aviation authority said.
By Sunday afternoon, limited services had been restored after pilots switched to backup frequencies to maintain contact with ground controllers, authorities said.
Greek authorities were still unsure of the cause of the blackout, which disrupted dozens of flights.
"For some reason, all frequencies were suddenly lost .. We could not communicate with aircraft in the sky," Panagiotis Psarros, chair of the Association of Greek Air Traffic Controllers, told state broadcaster ERT.
'Unprecedented' outage
The association later said the breakdown affected all ground frequencies and some frequencies used by Athens Approach, an air traffic control unit responsible for managing aircraft arriving at and departing from Athens's Eleftherios Venizelos Airport.
Among its responsibilities is radar monitoring to ensure safe separation of aircraft in the air, as well as issuing instructions on speed and altitude.
The Air Traffic Controllers Association said controllers were using all available resources to ensure flight safety, calling the scale of Sunday's incident "unprecedented and unacceptable" for an air traffic control system.
Psarros said the problem appeared to be a collapse of central radio-frequency systems at the Athens and Macedonia area control systems, the country's largest air control facility. It monitors the Athens Flight Information Region, a vast expanse of airspace under the control of Greek authorities.
"We haven't been informed about the cause of this problem... certainly, the equipment we have is virtually ancient. We have raised this many times in the past," Psarros said.
Authorities had raised the number of flights leaving Greek airports to 45 per hour by late afternoon, a Greek official said.
Italian airport sees delays following technical issues
The Italian airport of Bergamo Orio-al-Serio, a Ryanair hub for flights to Milan, halted flights on Saturday evening due to technical issues with the landing guidance system and poor visibility, leaving thousands stranded overnight.
SACBO, the company that operates the airport, said in a statement that the technical issue was resolved around midnight, but a warning on the airport's website stated, "Flights may be delayed or canceled."
The incident led to the cancellation of 26 departing flights, with six diverted to other airports and seven rescheduled for Sunday, local media said.
Thousands spent the night at the airport, with images showing people sleeping on the floor and baggage check-in belts.
The Bergamo online edition of Corriere della Sera daily said the first flights were halted at around 5:00 p.m. on Saturday.
Real-time information about flights on the airport's website showed a long list of mostly Ryanair flights delayed, though several had departed on Sunday morning.
A 6:05 a.m. Ryanair flight to Cagliari, delayed to 08:50 a.m., was showing as 'departed' as was a 6:20 a.m. Neos flight to Sharm el-Sheikh, which had been delayed to 08:55 a.m.