Boeing Faces 737 MAX Deliveries Hurdle Over FMS Software Bug

Boeing is allowing airline customers to roll back 737 flight management system software to version U13 to keep MAX deliveries flowing. The aviation giant faces a software bug introduced in U14.0 and U14.1 that causes vertical navigation to disengage during specific missed approaches, prompting airlines like Southwest and United to refuse delivery of affected aircraft.

The FMS Update 14 Glitch and Vertical Navigation Failures

Flight management system software updates carry immense consequence in modern commercial aviation. For the Boeing 737 MAX fleet, the introduction of FMS Update 14 (U14) and its subsequent iteration, U14.1, brought an unintended operational hazard. When pilots alter a published procedure inside the flight management computer during a missed approach, the aircraft’s automatic vertical navigation (VNAV) unexpectedly disengages. Pilots must manually intervene to secure the correct altitude profile.

Boeing began embedding the U14.0 software load into delivered 737s as early as 2020. They followed up with U14.1 in early 2022. Yet, operator feedback detailing VNAV disconnect snags did not reach Boeing until November 2024. By February 2025, an internal assessment concluded the issue did not meet the necessary criteria for a safety evaluation at that time. It took until August 2026, fueled by broader operator feedback, for Boeing to formally publish information regarding the glitch in its fleet digest.

Software Architecture Evolution Across the 737 Fleet

Software deployment variances across the single-aisle lineup have fractured the delivery pipeline. Baseline 737-8 and -9 models were originally certificated with the U13.0 software load, delivered starting in 2017. These in-service fleets are now slated for updates to U14.0 standards, though carriers can request rollbacks.

Conversely, newer variants face stringent roadblocks. The 737-10, nearing the finish line of its prolonged regulatory approval program, requires U14.1. Meanwhile, the first 737-7 destined for Southwest Airlines features U14.0 as its standard software configuration. Hardware configuration changes mean both the 737-7 and 737-10 must rely on a version of U14 unless engineers modify the software or the flight management computers themselves. Aviation Week Fleet Discovery shows Boeing holds 27 units of the 737-7 and 25 units of the 737-10 in undelivered inventory.

Airline Pushback and Delivery Maneuvers

Major carriers are drawing a hard line against taking possession of aircraft carrying the troubled U14 iteration. Southwest Airlines, which planned to initiate 737-7 deliveries in November, explicitly stated it will not accept aircraft running the problematic software. United Airlines confirmed that none of its operational fleet is affected and that it is declining deliveries featuring U14-version software, despite taking at least 10 737 MAX deliveries since August 1. Alaska Airlines verified its 737 MAXs run version U13, with older 737NGs operating U12.

To prevent gridlock on the tarmac, Boeing is actively rolling back FMS software to version U13 for customers requesting the adjustment. A subset of the active fleet that previously upgraded to U14 or U14.1 has also downgraded to U13 in recent weeks. This software is compatible across both the 737 MAX and 737 Next Generation families.

Regulatory Scrutiny and Certification Delays

The software issue traces its lineage back to previous computing glitches. FMS U14.1 was originally designed to fix a known bug that triggered flight management computer resets when loaded with flight plans containing vectors, route discontinuities, and required navigational performance waypoints—a phenomenon previously flagged by an NTSB investigation into a 2022 737 MAX wrong-runway landing.

FAA Administrator Bryan Bedford addressed the cascading software hurdles during a September 28 press briefing on air traffic control modernization. “We did certify U14.1 as part of the 737-7 approval,” Bedford stated. “It fixed a known bug on the vertical navigation system [and] unexpectedly introduced a new bug.” Bedford noted that while the VNAV issue is smaller than the prior FMC reset issue, the primary concern remains pilot workload under obscure operational scenarios.

Consequently, the FAA is delaying the certification of the 737-10. Regulators are weighing whether to mandate a fix before affected aircraft enter service or to approve an interim operational procedure for flight crews. Software supplier GE Aerospace is currently collaborating with Boeing to fast-track a permanent code correction, though an exact deployment timeline remains unconfirmed.