The carrier is in final negotiations for the planes that are due to be delivered between 2025 and 2027, the people said, asking not to be identified as the discussions are confidential. Bloomberg News first reported in January that Airbus SE was also looking for A321 production slots to woo United.

Delays in certifying Boeing’s 737 Max 10 aircraft have jeopardized growth plans at United, which was slated to be the first major customer for the plane. Converting some of those orders to Airbus would give the European planemaker a rare opportunity to undercut its chief rival.

United declined to comment Thursday on whether it was close to deals for the Airbus aircraft.

Certification of the Max 10 keeps sliding as regulators more carefully scrutinize new Boeing aircraft entering the market and the federal government is rigorously assessing the company’s quality control.

United Chief Executive Officer Scott Kirby said earlier this week that the carrier was in the market for A321s, but cautioned that he wouldn’t overpay for aircraft just to hit a growth target and would only do a deal “where the economics work.” The airline is likely to end up with a mix of Max 9s and A321s to replace the Max 10s, he said.

Kirby also disclosed that United has told Boeing to stop building Max 10s for its fleet and switch to the smaller Max 9 until the planemaker can secure federal certification for the 10. United has ordered 277 Max 10s, with options to buy 200 more.

“It’s impossible to say when the Max 10 is going to be certified,” he said. The carrier removed the plane from its fleet planning earlier this year. It is a major part of United’s strategy to shift to larger aircraft.

Boeing had originally targeted beginning commercial flights for the Max 10 in 2020, but the plane’s certification has been pushed back indefinitely as Boeing addresses issues in its factory and contends with increased scrutiny from the Federal Aviation Administration and other US agencies.

Read More: United Tells Boeing to Stop Making Its Long Delayed Max 10s

Kirby has been one of Boeing’s most outspoken critics after a fuselage panel blew off of a 737 Max 9 operated by Alaska Air Group Inc. in early January. United, the biggest operator of the variant, temporarily took dozens of planes out of service while federal investigators probed the accident. The National Transportation Safety Board has found that workers at Boeing had failed to affix four bolts holding the door plug in place.