Only one of Germany’s eight Airbus A400M military transport planes is currently ready for use, the German air force said on Thursday, days after one of the aircraft broke down during the German defense minister’s visit to Lithuania.

A spokesman for the Luftwaffe said three aircraft, including the one stranded in Lithuania, were out for unscheduled repairs, with two more in planned inspections and one receiving scheduled retrofits. The seventh aircraft was still going through the acceptance process after arriving on Jan. 31, he said.

“Of course, we’d like to have more of the aircraft ready for use, but it’s also normal that aircraft need to be inspected and maintained,” the spokesman said.

German officials said on Thursday they were still investigating what caused an oil leak in one of the four engines on the aircraft that carried Defense Minister Ursula von der Leyen to Lithuania, and whether it marked a new technical problem for Europe’s largest defense project.

Airbus declined comment on the air force data. The company, which has written off over 5 billion euros on the program after gearbox problems and fuselage cracks, said it is doing all it can to support the current investigation.

Tobias Lindner, a Green party lawmaker and member of parliament, said the latest incident reflected continuing problems with the A400M, and particularly its engines, and he was concerned it could further stall aircraft deliveries.

Airbus has told the German defense ministry that it plans to deliver 10 aircraft this year, down from 12 aircraft initially planned, and 13 in 2018, down from 14 planned.

The defense ministry told Lindner last month that it had asked for compensation of 39.4 million euros from Airbus for delayed deliveries of the first 5 aircraft, of which 27.2 million euros had been received. Payments for the first and fifth aircraft were still being discussed.

Technical problems have put the A400M program years behind schedule, with Germany’s share of the costs having risen to 9.6 billion euros ($10.2 billion) from an initial estimate of 8.1 billion.

Germany is the largest customer for A400M, which was initially developed for seven European NATO nations at a cost of 20 billion euros.

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Picture Rob Vogelaar