It’s no secret that several of Boeing’s latest models of commercial aircraft are going to enter service many years behind schedule, with delays plaguing the certification of both the narrowbody 737 MAX 7 and 10 models and the widebody 777X family. What makes this situation especially complex is the fact that many of these aircraft have already been built, with, for example, 26 widebodies from the 777X family having been produced to date.
However, until these aircraft are certified for entry into commercial service, they cannot be delivered to their customers, but they do still need to take to the skies on a test basis every now and then to keep them fresh. Away from the 777X family, this is also something that has been observed on the narrowbody side of things, with data from Aviation Flights showing that United’s undelivered Boeing 737 MAX 10s have amassed over 300 hours.
Already Racking Up The Hours
According to present fleet data made available by ch-aviation, United Airlines has a grand total of 167 Boeing 737 MAX 10 jets in its current order book, with those that have already been produced clocking in at an average age of 2.6 years old. The oldest of these is N27751, which, having first flown in June 2021, is now 4.5 years old. Its most recent test flight, a circular journey from Boeing Field (BFI), took place on December 16.
This month has also seen two other United Airlines-bound Boeing 737 MAX 10 aircraft take to the skies, with N27752 in the air as recently as December 17 when it flew from Boeing Field to Eugene, Oregon, and back. Meanwhile, N27753 made a circular test flight from Boeing Field on December 4. Needless to say, plenty more flight hours will be racked up before these jets can join United, which said back in 2017 that:
“The 737 MAX 10 will enable us to continue using larger and more efficient aircraft within our domestic network and better meet the needs of our customers today and into the future.”
One Of The Aircraft Has Been Used As A Boeing ecoDemonstrator
Arguably the most notable use of a 
As part of this initiative, the special aircraft, which, as seen in the picture embedded above, wore a colorful striped livery in this role, operated test flights powered by Sustainable Aviation Fuel (SAF). When in the air for this purpose, it was trailed by a Douglas DC-8 from NASA to measure its emissions.
This allowed researchers to gather data concerning the differing emissions levels produced by SAF compared to conventional jet fuel, as well as the differences in contrail ice particles. Boeing added that “the researchers’ aim [was] to understand how advanced fuels, engine combustor designs, and other technologies may reduce atmospheric warming,” given that “some research has suggested certain contrails can trap heat in the atmosphere.”
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United Airlines & The Boeing 737 MAX 10
United Airlines hasn’t had the easiest of times with regard to its order for the Boeing 737 MAX 10, with the carrier having to deal with years of delays as various factors have hindered the certification process of the largest variant of the next-generation narrowbody. Indeed, in 2017, when the carrier first announced its order for 100 examples of the MAX 10 at the Paris Air Show, United was expecting to begin taking delivery of the type in late 2020.
Since then, the Boeing 737 MAX grounding, the coronavirus pandemic, and various other holdups at Boeing on the quality control side of things have repeatedly caused the certification timelines for the MAX 10 (along with the MAX 7 and 777X) to slip. According to reporting by Aviation Week in July, United is now ‘hopeful’ of beginning to receive these aircraft in 2027, having previously removed the type from its fleet plan for the near future.
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