United Airlines (UA, Chicago O'Hare) has deferred the reactivation of its Pratt & Whitney PW4000-powered B777s to at least mid-May 2022 as it continues to work with the regulators on safety concerns related to these powerplants, CNBC has reported.

The carrier said in an internal memo that the aircraft would not return to commercial operations before May 12, with an additional two weeks (until May 25) before they could start flying to Hawaii and on international routes.

"Due to the delay in the return of our PW B777 aircraft to active service, the May flight schedule is being reconfigured to account for the lack of these aircraft," the carrier informed its pilots.

United Airlines operates 52 PW4000-powered B777s, including nineteen B777-200s and thirty-three B777-200(ER)s.

The Federal Aviation Administration grounded all PW4000-powered B777s immediately after a United aircraft suffered an uncontained engine failure on February 20, 2021. While JAL - Japan Airlines (JL, Tokyo Haneda) subsequently retired all such aircraft in its fleet and ANA - All Nippon Airways (NH, Tokyo Haneda) announced a permanent reduction of its subfleet, United Airlines did not waver and reaffirmed that once the type had been cleared, it would reactivate it.

Japan lifted its ban on PW4000-powered B777s in March 2022. ANA operates two B777-200s, eight -200(ER)s, and one -300 with the PW4000 engines but has yet to reactivate any of them. The powerplants are also used by Korean Air (and its LCC subsidiary Jin Air) and Asiana Airlines in South Korea, where all aircraft of the type are also parked. Ukraine International Airlines has a single B777-200(ER) with PW4000 engines which has been in storage since December 2019 and which was unlikely to return to service due to fleet cuts even before the Russian invasion stopped all flights from and to Ukraine.