German carriers Lufthansa and Air Berlin have expressed scepticism over the volcanic ash risk, and the need to keep airspace closed, after neither detected any technical problems during a series of positioning flights.
Lufthansa has repositioned 10 Boeing 747-400 and Airbus A340 aircraft from Munich to Frankfurt, in preparation for eventual clearance to operate. The aircraft mainly performed under visual flight rules, limiting their altitude to 3,000m (9,800ft), although one of the jets was taken to 8,000m to assess the atmospheric conditions.
A spokesman for the airline says that none of the aircraft showed any sign of volcanic ash damage.
These results are generating a steadily-growing backlash against regulatory authorities and increasing doubts about the scientific basis for keeping European airspace closed.
The Lufthansa spokesman argues that decisions appear to be founded on volcanic monitoring forecasts rather than actual atmospheric testing.
"Everyone's basing decisions on estimates from computer simulations," he says. "We need additional tests and analysis, test flights need to be done. But this is not being done quickly enough."
Air Berlin yesterday carried out positioning flights of three Airbus aircraft on the Munich-Dusseldorf and Nuremberg-Hamburg sectors.
The airline says that technical inspections "did not reveal any adverse effects" on the jets, and is similarly questioning the rationale behind the airspace restrictions.
"We are amazed that the results obtained from test flights carried out by Lufthansa and Air Berlin...did not have any influence whatsoever on the decisions taken by the aviation safety authorities," says Air Berlin chief Joachim Hunold.
The carrier is offering to conduct formal test flights to examine the risk and is pressing the German transport ministry to establish a crisis-management operation.
Pilots from the Netherlands are calling on governments to allow partial restoration of services. Dutch pilots' union VNV believes the concentration of volcanic particles is "so small that it presents no danger".
Dutch carrier KLM is planning to operate another series of flights today following an initial airborne test on 17 April.
KLM has secured permission for another nine flights. The first has been conducted out of Dusseldorf, at 06:30 today, with 20 crew members on board but no passengers.
Seven aircraft are stranded in Dusseldorf and KLM aims to reposition these back to its Amsterdam Schiphol base.
Its initial test flight, with a Boeing 737-800, aircraft indicated no operational problems and KLM says the "quality of the atmosphere is in order". KLM insists, however, that safety "remains the chief priority".
Chief executive Peter Hartman stresses that the decision on resuming normal air operations lies with the Dutch and European authorities, but that it hopes to restore services as soon as possible if restrictions are lifted.
Air France is carrying out its own test flight today, using an Airbus A320, and similar flights are reportedly to be undertaken in the UK.