The setback appears to be among the largest recalls affecting Airbus in its 55-year history and comes weeks after the A320 overtook the Boeing 737 as the most-delivered model. At the time Airbus issued its bulletin to the plane's more than 350 operators, some 3,000 A320-family jets were in the air. The fix mainly involves reverting to earlier software and is relatively simple, but must be carried out before the planes can fly again, other than repositioning to repair centers, according to the bulletin to airlines seen by Reuters.