The carrier was forced to ground its Boeing 737 Max 9 fleet for three weeks after a mid-air cabin blowout on an Alaska Airlines flight in January.
United said that pushed it to a pre-tax loss of $164mn for the first quarter.
The airline said it "would have reported a quarterly profit" otherwise. However, the loss was smaller than Wall Street expected and United's shares rose by more than 5% after the announcement.
United has 79 Boeing 737 Max 9s in its fleet, more than any of its rivals, and second only to Alaska Airlines.
United and Alaska were forced to cancel thousands of flights as inspections were carried out in January before the US aviation regulator cleared the planes to resume flying. Earlier this month, Boeing paid $160mn to Alaska to make up for losses the airline has suffered. And speaking at a conference in March, the firm's chief financial officer Brian West said that its customers like United Airlines had been "supportive of everything we're trying to do to enhance safety and quality for the industry". Referring specifically to the pay-out due to the events on January 5, he added: "We continue to stand behind our customers with that responsibility.”
United told investors that its operations were also impacted by delays to deliveries of Boeing planes. "We've adjusted our fleet plan to better reflect the reality of what the manufacturers are able to deliver," United's chief executive Scott Kirby said in a statement.
Boeing recently faced new pressure after a whistleblower reported safety concerns over the manufacturing of some of its planes to US regulators. Engineer Sam Salehpour accused Boeing of taking shortcuts in the construction of its 787 and 777 jets. He said he was "threatened with termination" after raising concerns with bosses. But Boeing said the allegations were "inaccurate" and added it was confident its planes were safe.
Salehpour is set to be the key witness at a US Senate hearing. On January 5, a door plug on an Alaska Airlines 737 Max 9 blew off shortly after take-off, terrifying passengers, and forcing an emergency return to the Portland, Oregon airport.
The latest incident raised fresh questions about the safety of Boeing's new aircraft.
The company faced intense scrutiny after two fatal crashes of 737 Max 8 passenger jets in 2018 and 2019, which killed 346 people.
Boeing is expected to face criticism at a US Senate hearing from a whistleblower who says he feared its planes could fall apart mid-air. Engineer Sam Salehpour told NBC News he had raised concerns about the 787 Dreamliner, which Boeing dismissed.
Salehpour discussed the alleged safety issues with the 787 Dreamliner in an interview with NBC ahead of Wednesday's hearing. He claimed he feared the plane could simply "drop to the ground" unless manufacturing issues were addressed. He referred to gaps between components that emerged during the plane's assembly.
The Federal Aviation Administration (FAA) is investigating Salehpour's claims, which were first reported last week by the New York Times.
Boeing, one of the world's two major producers of commercial planes, has again insisted its jets are safe, and disputed the engineer's assertions.
Boeing told NBC: "These claims about the structural integrity of the 787 are inaccurate. The issues raised have been subject to rigorous engineering examination under FAA oversight."
At an earlier event on Monday, the company said 99% of gaps conformed to the standard, and that testing showed "zero fatigue".
Elsewhere, airline regulators in the US have begun an investigation after an engine cowling on a Boeing 737-800 fell off during take-off and struck a wing flap.
The Southwest Airlines flight returned safely to Denver International airport at about 08:15 local time after originally departing to Houston.The aircraft had 135 passengers and six crew members on board and climbed to around 10,300 feet before initiating a descent for an approach and landing.
Southwest Airlines said its maintenance teams would review the Boeing 737-800 after its cowling, which covers the plane's engine, fell off. The airline confirmed it was responsible for maintenance of such parts.
"We apologise for the inconvenience of their delay, but place our highest priority on ultimate safety for our customers and employees," a statement said.
The aircraft was manufactured in 2015 according to regulator Federal Aviation Administration (FAA) records, and the 737-800 is an earlier generation of the 737 from the latest Max model.
The aircraft in the latest incident was powered by CFM56 engines while the 737 Max models use the CFM-Leap engine. Both are made by a joint venture between General Electric Aerospace and Safran Aircraft Engines. The FAA said the Boeing aircraft was towed to the gate after landing.
