More than a fifth of all European flights were delayed in May despite the introduction of measures aimed at preventing disruptions that wreaked havoc on travel during last year’s busy summer season.

Monthly data from regional air traffic authority Eurocontrol show 22.1% of flights were delayed compared with 24.5% the same month last year, according to a statement Friday. Flights over central and eastern Germany were among the most delayed.

Strikes by traffic controllers and staff at airlines including Ryanair Holdings Plc led to delays and cancellations during May and the summer months of 2018, costing carriers hundreds of millions of euros. Since then, airlines, airport operators and traffic control authorities have introduced a raft of measures to ease congestion aimed at preventing a repeat. These include allowing pilots to fly at lower altitudes to avoid particularly crowded areas.

Yet the latest data indicates little improvement. “The report for May shows that punctuality improved compared with May 2018; but we are still seeing over 10% of flights more than 30 minutes late,” Eurocontrol Director General Eamonn Brennan wrote on Twitter.

More than 15% of flights passing through the so-called Karlsruhe UAC zone in Germany were delayed due to capacity constraints or a shortage of controllers. The country’s DFS air traffic control authority has said it plans to hire more staff to handle rising air traffic, but has warned of a dearth of prospective candidates and long training times needed for new hires.

Deutsche Lufthansa AG is the worst hit by congestion in Germany. The airline is bracing for another summer of crowded airspace and jammed security lines at its hub airports, .

The company’s Eurowings subsidiary also faces potential cabin crew strikes this summer, with unions saying they would hold a staff ballot in the coming weeks. Cabin crew at the main Lufthansa airline may hold a strike ballot later in the season, the UFO union said.

“Another summer of chaos looms if the company doesn’t change its approach,” union leader Sylvia De La Cruz said in Frankfurt last week.

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Photo Credit: A passenger stands in front of the display board for departures Munich's airport. More than a fifth of European flights were late in May. Peter Kneffel / dpa via AP