Airbus' troubled A400M military plane has hit turbulence once again.

The cargo plane whose rollout went billions over budget and years over deadline is being grounded by numerous governments after a crash in Spain this weekend killed two pilots and two flight test engineers.

Though the crash is still unexplained, many of the governments who first ordered the A400M are keeping them in the hangars out of precaution, with leading buyer Germany openly criticizing its design. Shares in Airbus Group fell on the prospect of more trouble with the model.

Here's a look at the case and the plane's history of problems.

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SPAIN CRASH

Airbus and government officials haven't publicly announced the cause of the weekend's crash near Seville, but the voice and flight data black boxes have been recovered and delivered to a Spanish judge investigating the case.

Airbus said the A400M has clocked roughly 2,000 hours in various military operations.

But concerns that the crash could be tied to the plane's history of technical problems hit Airbus on Monday.

Four of the five countries that have received A400Ms — Britain, Germany, Malaysia and Turkey — grounded the plane. France has the most in operation of any country — six — and said it was would only use the aircraft in urgent operations.

Airbus shares slumped over 4.5 percent before recovering slightly to close 2.1 percent lower.

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STORMY TAKEOFF

From its conception, the A400M — billed as an alternative to airlifters like Lockheed Martin's C-130 Hercules — was ambitious in operational and organizational terms.

With seven launch countries, and seen as a bellwether of European cooperation on military matters, the A400M was forced to reconcile many competing constituencies. Countries had differing conceptions of how they planned to use the plane.

The result was that the 20 billion-euro ($22 billion) project ran about 5 billion euros over budget and four years behind schedule.

In 2010, the program nearly collapsed and saved only after an infusion of new government funds.

European nations have long been hampered by the shortfall in strategic military airlift capabilities. In the 1990s, they struggled to deploy forces to nearby trouble spots in Bosnia and Kosovo without using U.S. Air Force transports such as the Boeing C-17 Globemaster III.

The A400M uses the largest turboprop engines ever fitted onto a Western aircraft. Because the turboprops are mounted high on the wing, Airbus says, the plane can fly into unprepared airstrips where jet-powered transports with engines that are slung low under the wing face the danger of ingesting runway debris.

Airbus has projected to sell hundreds of A400Ms. So far, it has received order for 174 and delivered twelve.

Germany in particular has been eager for this plane, ordering 53.

But it has not been completely happy with the result. German Defense Ministry spokesman Uwe Roth said the military's only A400M was in a test phase, and had shown a "large number of shortcomings" so far. Germany was set to receive five more this year, but Roth said Airbus had notified the German government that there would be delays and that they would only know in the second half of the year how many could be expected and when.

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OUTLOOK

Airbus says it is continuing the A400M flight-test program as planned in Toulouse, France, where the company has its headquarters.

In a symbolic show of confidence in the plane, Fernando Alonso, the head of military aircraft at Airbus Defense and Space, is expected to take part in Tuesday's test as a flight engineer, company officials said. Alonso took up his post in January, after Airbus dismissed his predecessor following complaints by governments about continued delays.

Meanwhile, analysts say the financial hit to Airbus is likely to be limited as the countries backing the program are in too deep to pull out now.

"It's a ten-year program that unfortunately has faced a lot of delays, but the plane flying today ... doesn't have a competitor," said Ludovic Eyt-Dessus of Montsegur Finance in Paris. "I don't think the incident this weekend will have an impact on the program and its development, but customers may just have to wait before using the airplane."

The plane has also had some successes it can point to. France has used it in hard-to-reach areas of Africa and to deploy troops in Mali — where French forces have played a crucial role in keeping Islamic militant fighters at bay. Turkey, which has two, has used the plane to shuttle troops and gear to Afghanistan.

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Alan Clendenning and Ciaran Giles in Madrid, David Rising in Berlin and Nicolas Garriga in Reims, France, contributed to this report.