The decision by Germany's DFS air traffic control agency to restrict flights over Hamburg, Bremen and Berlin on Wednesday was based on much thinner information than Transport Minister Peter Ramsauer previously indicated, the Financial Times Deutschland reported.
Ramsauer had said that flights were grounded because the concentration of ash exceeded the permissible limits calculated partly on information from aircraft manufacturers.
But according to the German pilots' association Cockpit, the aviation industry has avoided carrying out tests to determine reliable data on what the limits should be.
Hundreds of flights were cancelled on Wednesday and tens of thousands of passengers affected after a cloud from the Icelandic volcano Grimsvötn spewed out ash. By Thursday, the volcano was mostly blowing steam, news agency Reuters reported, indicating that further flight disruptions are unlikely.
Ramsauer also said a reliable system of measurements provided the necessary data about the ash concentration in the air over Germany. But the FTD reported that the 52 stations operated by the German Weather Service (DWD) could only measure whether there were particles in the air – and not, crucially, the actual concentration of these particles.
Indeed German air safety authorities were relying foremost on simulations from the London-based Volcanic Ash Advisory Centre (VAAC), whose findings have previously been controversial. A reading taken of German air space in April concluded that the VAAC's findings were incorrect.
A spokesman for Ramsauer confirmed the essential facts of the FTD report but argued that the VAAC data was more precise than it had been last year during the lengthy and widespread closure of airspace during the eruption of another Icelandic volcano, Eyjafjallajökull.
A DWD spokesman told the paper that there was no alternative to these simulation predictions.
“Otherwise we have disrupt flights ad hoc, if we detect ash particles,” he said.
According to the FTD, the VAAC data still showed a high ash concentration over Hamburg when the DWD issued its warning, which was why Lufthansa kept its planes on the ground for several extra hours.
Speaking Wednesday night in Saalfeld in the state of Thuringia, Ramsauer defended the airspace closure.
“If a volcano were to erupt again, I would do proceed in exactly the same way as what took place in recent days,” he told reporters.
The information provided by engine manufacturers had played a vital role in his decision-making, he said. The companies could not rule out a risk to safety in the event of an ash concentration of two milligrams per cubic metre or more. Therefore flights had to be grounded as soon as the concentration exceeded that level.
Pilots' association Cockpit said the aeroplane manufacturers had a responsibility to carry out reliable tests to determine the level at which ash clouds became dangerous.
“Then we would have solid data upon which the foundation for an upper limit would be determined,” spokesman Jörg Handwerg told the Ruhr Nachrichten.