Boeing 737 MAX 9 (B39M)

The Boeing 737 MAX 9 is the second-largest member of the MAX family, the re-engined 737 that Boeing launched in August 2011 to counter the Airbus A320neo. Where the MAX 8 replaced the 737-800, the MAX 9 replaced the 737-900ER, keeping that aircraft's 42 m fuselage and 220-seat ceiling while adding CFM LEAP-1B engines, split-scimitar winglets and a redesigned tail cone for around 14% lower fuel burn per seat.

The first MAX 9 flew on 13 April 2017, and Lion Air's Thai Lion Air affiliate put it into service in March 2018; United, the largest customer, followed a few months later. Within a year the whole MAX family was grounded. The MAX 8 crashes of Lion Air 610 in October 2018 and Ethiopian 302 in March 2019, both linked to the MCAS flight-control software, led to a worldwide grounding that lasted until late 2020, and MAX 9s sat in storage alongside the 8s while Boeing redesigned the software and regulators revised training.

The MAX 9 then earned a grounding of its own. On 5 January 2024 a mid-cabin door plug blew out of Alaska Airlines flight 1282 shortly after departure from Portland; the aircraft landed safely but the FAA grounded all 171 MAX 9s fitted with plugs for three weeks while operators inspected them. The investigation found the plug had been reinstalled at Boeing's factory without its retaining bolts, and the FAA capped Boeing's 737 production rate while it overhauled quality control.

Despite that history, the MAX 9 has settled into service with United, Alaska, Copa, Turkish, Icelandair, flydubai and others as the long-fuselage MAX, pending the even longer MAX 10 whose certification has run years late.

The MAX 9 shares the 737-900ER's fuselage, with the mid-cabin exit aft of the wing that allows up to 220 seats; airlines that fit fewer seats, such as Alaska and United, have that exit deactivated and replaced with a plug, the component at the centre of the 2024 incident. The LEAP-1B engines are much larger than the CFM56-7B they replace, so Boeing mounted them higher and further forward on new pylons, lengthened the nose gear by 20 cm for ground clearance, and changed the aircraft's pitch behaviour enough to require the MCAS system that was later redesigned.

The split-scimitar 'Advanced Technology' winglets, the serrated engine nacelles and the 787-style tail cone are the visible aerodynamic changes. The cockpit keeps the 737NG's basic layout with four large displays, preserving the common type rating that was central to the aircraft's sales pitch. Range is about 3,550 nautical miles with the optional auxiliary tanks.

United Airlines operates the world's largest MAX 9 fleet, using it on domestic trunks and to Hawaii and Latin America in a 179-seat layout; Alaska Airlines flies it across its West Coast network with 178 seats. Copa in Panama, Turkish Airlines, Icelandair, flydubai, Aeroméxico, Lion Air and Corendon are other operators. Typical duties are two- to six-hour sectors where 170–190 seats fit better than a MAX 8, including Icelandair's transatlantic routes and United's Honolulu flights.

A MAX 9 cabin is the 737-900ER's cabin with new engines: the same narrow 3-3 layout of 17-inch seats, the same long walk to the back, and the Boeing Sky Interior with its sculpted sidewalls, larger pivot bins and LED mood lighting on every aircraft. The LEAP engines are noticeably quieter than the NG's, especially behind the wing, and the ride is typical 737. United and Alaska fit a first-class cabin and extra-legroom rows forward, with 30–31 inches of pitch in economy; Copa and Turkish are similar. If the aircraft has a deactivated mid-cabin exit you will see a plain wall where a door would be. For the best seat, take a window ahead of the wing or an exit-row seat; the rear ten rows are the longest wait to deplane on a 190-seat single-aisle.

All 737 MAX aircraft share the giveaways: large LEAP-1B engines with serrated chevron nacelles, split-scimitar winglets that point both up and down, and a tail cone that ends in a flattened point rather than the NG's blunt tip. The MAX 9 is the long one in current service, 3.6 m longer than a MAX 8, with the mid-cabin exit (or its plug, visible as a slightly different window spacing) aft of the wing. Against a 737-900ER it shares the length but has the chevron engines and the new winglets. The MAX 10, longer still, is recognisable by its extra length and new semi-levered main gear.