Airbus said on Thursday it had identified and contained a "quality issue" in the fuselage structure of its A321neo aircraft, but that planes could continue operating because it did not pose a safety risk.
The defect affects about 500 aircraft, including roughly 250 already in service and 250 in production, according to a source familiar with the matter.
The A321neo, Airbus' largest and most popular single-aisle jet, seats up to 244 passengers and is designed to fly routes of up to 4,000 nautical miles (7,400 km). The defect does not affect the planemaker's longer-range A321XLR, the source said, who could not speak publicly about the private matter.
It is the second industrial problem to affect the fuselage of the industry's best-selling model in nine months after Airbus reported defects with fuselage panels last December.
Airbus stock hit by new discovery
The company's shares fell more than 1% on Friday after the European planemaker announced the quality flaw, describing the issue as a quality problem with no safety implications, but saying the situation has caused several months of delivery delays.
According to the latest reports, shares in the planemaker fell about 1.3% in early trading, lagging other CAC 40 blue-chip stocks.
Airbus said it had discovered what it described as a "deviation" in the surface protection, or coating, applied to skeletal components known as stringers that run lengthwise in a fuselage. It said it made the discovery while parts were already undergoing rework on the production line.
Airbus has identified and contained the source of the issue, and a remedy is available, it added.
Snag comes as Airbus aims to increase production
The latest snag comes as Airbus tries to increase production of narrow-body jets to 75 a month from about 60.
The source said Airbus first spotted the problem over the summer and the first results from closer analysis came back recently.
This part of the plane is made by Airbus' in-house aerostructures unit Stelia, which was not immediately available for comment outside of business hours.
Customers are continuing to take aircraft deliveries without insisting on extra work being carried out in advance, and Airbus has told customers there is no immediate requirement to repair, the source said.
European regulators may, however, issue a directive requiring the defects to be repaired within a certain period. US carrier Delta Air Lines, which operates A321neo planes, said it was continually in contact with manufacturers and would "always take action to meet or exceed operator recommendations and regulator directives."
Industry publication the Air Current first reported the issue on Thursday.