The German air-traffic control agency (DFS) had shut air space above airports at major hubs Munich and Stuttgart, as well as regional airports Augsburg, Memmingen and Friedrichshafen, due to a high concentration of ash in the atmosphere above the region.
Most of the airports were closed at 3 pm on Sunday, after the cloud from the Icelandic volcano Eyjafjallajökull reached northern Italy in the morning. The airports of Milan, Florence and Pisa were also temporarily shut down.
The DFS said at the time that airspace above southern Germany was described as "potentially contaminated."
As many as 500 Sunday flights were cancelled in Europe, the Brussels-based flight safety organisation Eurocontrol announced, including dozens from Germany's biggest airport in Frankfurt. This primarily affected flights to Milan, Lisbon and the US.
News magazine Der Spiegel also reported that pilots had complained about Lufthansa's decision to allow certain commercial passenger flights to go ahead according to Visual Flight Rules (VFR) during the closure of airspace in April.
VFR means that pilots fly around dangerous clouds until they reach a cruising altitude above them, guided mainly by their own judgement. According to the magazine, Lufthansa safety pilot Jürgen Steinberg now regrets his approval of the decision.
"That must not happen again. If the situation happened again, my recommendation would be 'Don't do it!'," Steinberg said.
In mid-April, the DFS closed the lower airspace in Germany to flights using instruments because of the ash cloud. But after coordinating with other authorities, the DFS permitted VFR flights.
Lufthansa defended the decision. "That was a completely clean process," company spokesman Klaus Walther told the news agency DPA. "It was totally safe." Walther said that there had not been a single incident, and no pilot had refused to fly. Referring to Steinberg's statement, Walther said, "That is his private opinion."
The cancellation of tens of thousands of flights during six days in mid-April led to billion-euro losses to the flight industry.