The percentage of A321 deliveries within Airbus single-aisle output has trebled since the start of the century to 16 percent and Airbus hopes to lift that to 50 percent - part of a battle with Boeing for the lucrative "middle" of the jet market. Responsibility for building the jet lies in Hamburg, which has been struggling to implement a complex new "Airbus Cabin Flex" (ACF) version offering airlines multiple layouts. Pressure on Airbus to get things back on track was highlighted on Friday when the chief executive of British Airways owner IAG called recent delivery delays "unacceptable". IAG defected to Boeing medium-haul jets with a draft order for 200 of the grounded 737 MAX last month.