A software bug may have cause the May 9 crash that grounded Airbus' troubled A400M military transport aircraft.

Airbus has sent an alert to customers instructing them to conduct “specific checks of the Electronic Control Units (ECU) on each of the aircraft's engines”.

Spiegel reports that the bug caused three of the transport's engines to shut down during the pre-delivery test flight. The crash killed four crew.

That A400M was to be delivered to Turkey. Following the crash, Britain, Germany and Turkey grounded their A400M fleets, but France did not.

According to Spiegel, three engines shut down after receiving “contradictory instructions” from the flight control system. The pilots tried to return to Seville airport, but the aircraft struck a power pole, crashed and burned.

An unidentified Airbus staffer told Spiegel the crash probably arose from “a quality problem”.

Reuters says a final diagnosis will need analysis of the plane's flight recorder, but the “black box” data is currently “being kept under wraps by a Spanish judge.”

The four-engined propeller-driven transport aircraft is designed as a replacement for ageing Hercules transports. The project ran late and over budget. ®