Europe’s Airbus said on Friday it was ordering immediate repairs to 6,000 of its widely-used A320 family of jets in a sweeping recall affecting more than half the global fleet, threatening upheaval during the busiest travel weekend of the year in the United States.

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 directive, some 3,000 A320-family jets were in the air.

The fix mainly involves reverting to earlier software, but it must be carried out before the planes can fly again, according to a bulletin to airlines seen by Reuters. Numerous airlines on Friday said the repairs could potentially cause flight delays or cancellations.

The world’s largest A320 operator, American Airlines, said some 340 of its 480 A320 aircraft would need the fix. It said it mostly expected these to be completed by Saturday with about two hours required for each plane.

Other airlines said they would take planes briefly out of service to do the repairs, including Germany’s Lufthansa, India’s IndiGo, and U.K.-based easyJet.

Colombian carrier Avianca said the recall affected more than 70 per cent of its fleet, around 100 jets, causing significant disruption over the next 10 days and prompting the airline to close ticket sales for travel dates through Dec. 8.

There are around 11,300 A320-family jets in operation, including 6,440 of the core A320 model, which first flew in 1987. Four of the world’s 10 biggest A320-family operators are major U.S. airlines: American Airlines, Delta Air Lines, JetBlue and United Airlines.

For about two-thirds of the affected jets, the recall will theoretically result in a brief grounding as airlines revert to a previous software version, industry sources said.

Still, that comes at a time when airline repair shops are already overrun by maintenance work, as hundreds of Airbus jets have been grounded due to long waiting times for separate engine repairs or inspections.

Recent incident leads to probe

Airbus said a recent incident involving an A320-family aircraft had revealed that solar flares may corrupt data critical to the functioning of flight controls.

Industry sources said the incident that triggered the unexpected repair action involved a JetBlue flight from Cancun, Mexico, to Newark, New Jersey, on Oct. 30, in which several passengers were hurt following a sharp loss of altitude.

That flight made an emergency landing at Tampa, Florida, after a flight control problem and a sudden uncommanded drop in altitude, prompting a Federal Aviation Administration investigation.

JetBlue and the FAA had no comment.

The European Union Aviation Safety Agency late on Friday issued an emergency directive mandating the fix, and the FAA was expected to follow suit.

‘Two hour’ repair

An Airbus spokesperson estimated the repairs would affect some 6,000 jets in total, mixed between several variants, confirming an earlier Reuters report.

The temporary groundings for repairs for some airlines could be much longer since more than 1,000 of the affected jets may also have to have hardware changed, the sources said.

Launched in 1984, the A320 was the first mainstream jetliner to introduce fly-by-wire computer controls.

It competes with the Boeing 737 MAX, which suffered a lengthy worldwide grounding after fatal crashes in 2018 and 2019, when faulty flight-control software pushed the planes’ noses down.

The Airbus bulletin seen by Reuters traced the problem to a flight system called ELAC (Elevator and Aileron Computer), which sends commands from the pilot’s side-stick to elevators at the rear. These in turn control the aircraft’s pitch or nose angle.

The computer’s manufacturer, France’s Thales, said in response to a Reuters query that the computer complies with Airbus specifications and the functionality in question is supported by software that is not under Thales’ responsibility.