United Airlines announced an incremental order for four Boeing 777-300ERs today, adding widebody growth capacity in 2018.
The Chicago-based carrier will take three of the aircraft by the summer of 2018, and the remaining aircraft late that year, says Howard Attarian, senior vice-president of flight operations at United, in a letter to pilots today.
The first three 777-300ERs will be "in operation for our planned summer growth in 2018", he says.
United was not immediately available for further comment on the aircraft.
The airline also converted 100 Boeing 737 Max 9s to the larger Max 10 variant at the Paris air show today. It now has firm orders for 61 Max 9s and 100 Max 10s.
The 777 order complements United's fleet of 10 777-300ERs and four firm orders, bringing its total commitments for the type to 18, the Flight Fleets Analyzer shows.
The first 14 777-300ERs are replacing the US carrier's Boeing 747-400 fleet, which stood at 20 in April the carrier's latest fleet plan shows.
Scott Kirby, president of United, has said that the airline could replace its 747s with fewer 777s and maintain capacity due to the improved operational reliability of the new aircraft.
United's 747s have an average age of 21 years, Fleets Analyzer shows.
The carrier is scheduled to take delivery of 26 aircraft in 2018, including the four 777s announced today, its first 787-10 and two 787-9s, Fleets Analyzer shows.
However, the 2018 delivery numbers include United's first three Airbus A350-1000s - deliveries that have been in question since chief financial officer Andrew Levy said in February that it was considering "different alternatives" for the aircraft.
United has a firm order for 35 A350-1000s.