EasyJet and tour group TUI said they had joined the action led by Manchester Airports Group (MAG) aimed at challenging the government for an alleged lack of transparency over travel rules.
Under the agreement, the airline will purchase 15 of Boom's "Overture" aircraft once they meet United's safety, operating and sustainability requirements, with an option for an additional 35 aircraft.
United said the aircraft would be optimized to use 100% sustainable aviation fuel (SAF) from its entry to service, projected to be in 2029.
The company said it now expects first-quarter revenue to fall 66% to $3.2 billion from the same period in 2019. It had previously forecast a drop of between 65% and 70%.
The airline said its average daily cash burn for the quarter is expected to be about $9 million per day, an improvement of about $10 million compared to the fourth quarter of 2020.
Emirates said last week passenger flights to and from Nigeria had been suspended until further notice in line with government directives, but did not give details.
"Here we would actually recommend increasing the number of tasks to be performed if you have a low utilisation," she said. "This is always an engineering driven and safety driven analysis."
In its first judgments on those rules, the Luxembourg-based General Court will assess a French scheme allowing airlines to defer certain aeronautical taxes.
Ryanair in its filings to the court faulted EU competition enforcers by allowing EU countries to grant aid only to airlines with EU operating licences issued by their countries.
Airlines have been among the hardest hit by the coronavirus crisis that has led people to avoid air travel.
Air Canada, which was forced to cut more than 20,000 jobs in 2020, has been negotiating with the Liberal government since November for aid to support jobs.
Privately-held WestJet, the country's second-largest carrier, announced further cuts to its schedule, introduced a hiring freeze.
WestJet's announcement follows new Canadian rules that passengers would need to test negative for the coronavirus before boarding a plane bound for Canada, starting on Jan. 7.
Airlines have developed training programmes for pilots re-entering service ranging from theory refreshers to multiple simulator sessions and supervised in-flight checks, depending on the length of absence.