United becomes second US carrier to return the 737 MAX to service

Boeing's journey to return the beleaguered 737 MAX to service passed another major milestone on Thursday.

Chicago-based United Airlines became the second U.S. airline to return the MAX to the sky, with the first commercial flight from Denver (DEN) to Houston (IAH) since the plane's 20-month grounding.

Flight 1864 departed the Mile High City at 7:47 a.m. local time, with a full load of passengers in business class and about 150 passengers in coach. The jet, N37523, a one-year-old MAX 9 painted in the new Blue Evolution livery is slated to land on time at 11:15 a.m. local time.

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United plans for 23 other MAX flights on Thursday, according to Cirium timetables, on many routes through Denver and Houston, where the plane will be based as the carrier ramps up service.

In March, United has scheduled 1,997 MAX-operated flights, so it's only a matter of time before the carrier plans to deploy its entire fleet of 28 MAX 9s. In the coming years, United will take delivery of additional MAX 9s, as well as MAX 8s and MAX 10s, according to Cirium fleets analyzer.

Thursday's return to service has been months in the making, starting with the November 2020 recertification order from the Federal Aviation Administration (FAA). After the jet's two fatal crashes in late 2018 and early 2019, the FAA and aviation authorities worldwide grounded the MAX indefinitely.

Related: A tale of two flights: Anxiety, excitement mix onboard return of the 737 MAX

Boeing worked together with the government agencies to address faults in the plane's flight control system related to the MCAS, or maneuvering characteristics augmentation system, before devising a return-to-service plan. On Nov. 18, the FAA ungrounded the jet, paving the way to return the MAX to the sky.

Airlines need to install a four-hour software update to the cockpit computers to fix the MCAS issues, and all pilots must pass a simulated-based training exercise to get recertified to fly the MAX.

American Airlines became the first domestic carrier to get the MAX flying again. Less than a month from the ungrounding order, the carrier flew a media-only demonstration flight with the jet. On Dec. 29, American put passengers back on the MAX with once-daily flights between Miami (MIA) and New York (LGA).

Related: A pilot and mechanic explain what’s needed to bring the 737 MAX back

In February, American is slated to operate 1,267 flights with the MAX, according to Cirium schedules.

Executives at American have cited the plane's fuel efficiency and lower operating costs as a key reason to bring it back so fast. With the pandemic raging, airlines are looking to cut costs wherever they can.

By mid-March, the MAX will be flying for four U.S. carriers. Alaska, which recently took delivery of its first MAX, will start flying passengers on March 1 with daily round-trip flights between Seattle and San Diego, and Seattle and Los Angeles. The airline is then expected to take delivery of its second 737 MAX 9 later in March.

Southwest's return-to-service plans include 32 MAX-operated flights on March 11, from a host of cities nationwide.

Related: How to tell if you're booking on a Boeing 737 MAX

To date, airlines have repeatedly reaffirmed that customers aren't specifically booking away from the MAX. “As we take a look at our loads, there’s really nothing to distinguish our MAX flights from the rest of our operation," American's president Robert Isom told the press before the carrier's first commercial MAX flight on Dec. 29.

Nonetheless, all U.S. carriers have added flexibility for travelers concerned about flying the MAX, and they've all pledged to be transparent about which flights are scheduled on a MAX, too.