Boeing is conducting the final taxi tests of its first 737 MAX 10 aircraft. Eagle-eyed planespotters have spied the MAX 10 out and about at Renton in preparation for its first flight, rumored to be occurring any day. The MAX 10 is the fourth version of the MAX family. To date, 20 customers have ordered 550 of the MAX 10s.

High-speed MAX 10 taxi tests conducted this week

Footage posted on Twitter by Katie Bailey shows N27751 moving about on taxiways and conducting high-speed taxiing tests. The MAX 10's maiden flight has been expected for a couple of weeks. Now, Renton's informed planespotters say the first flight is imminent. The taxi tests are the last set of ground evaluations before the plane takes to the skies.

The MAX 10 was first launched in 2017 at the Paris Air Show. Boeing took 240 orders from 10 customers at the launch. The largest of the four MAX variants Boeing offers, the MAX 10 can fly 3,800 miles (6,110 kilometers) and carry up to 230 passengers (depending on the cabin configuration).

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First deliveries of the 737 MAX 10 now delayed by three years

The MAX 10 uses the same LEAP-1B engines from CFM International as the other MAX variants and has an identical wingspan. But the MAX 10 has a longer fuselage than the other three variants. The MAX 10 is eight meters longer than the smallest variant, the MAX 7.

“Airlines wanted a larger, better option in the large single-aisle segment with the operating advantages of the 737 MAX family," Boeing said when launching the MAX 10.

"Adding the 737 MAX 10 gives our customers the most flexibility in the market, providing their fleets the range capability, fuel efficiency, and unsurpassed reliability that the 737 MAX family is widely known for.”

Boeing has not had an easy ride with its MAX program. The MAX 10 is no exception to the rule. Deliveries of the MAX 10 were originally slated to start in 2020. It's now 2021, and the MAX 10 is yet to take to the skies. Earlier this year, Boeing pushed back the delivery timeline to 2023, representing a three-year slippage.

"This schedule reflects a number of factors, including an updated assessment of global certification requirements informed by continued discussions with regulators and resulting in a management decision to make modifications to the aircraft’s design,” Boeing said in an SEC filing earlier this year.

MAX 10 launch customer eyeing further order

United Airlines, who already flies MAX 9s, has an order for 100 MAX 10s. The Chicago-based airline is lined up as the launch customer. United is also waiting for a large order of MAX 8s to begin arriving. Despite the delays, United is shaping up as a big fan of the MAX. Rumors are swirling that United Airlines is in advanced talks with Boeing for another big MAX order.

In 2017, United did a deal with Boeing to convert part of an existing MAX 9 order to MAX 10s. At the time, United said deliveries would start in 2020.

"With superior fuel efficiency and increased range, the 737 MAX 9 and MAX 10 aircraft will elevate the performance and capabilities of United's existing 737 fleet," said United Airlines at the time.

"The MAX 10 will provide even more flexibility to United's route schedule and give them the best economics of any airplane in the single-aisle segment."

Despite the delays and problems around the broader MAX program, United's rumored new MAX order signals its confidence in the plane. The imminent first test flight of the MAX 10 will further cement that confidence at United and other customer airlines.