US airlines flying the 737 Max 8 and 9 have refused for years to install Boeing's latest flight management software, operating instead on the U13 standard certified with the first Max in 2017 — and in United's and Southwest's case, demanding Boeing deliver new jets with older hardware so they can roll back the code.

The Federal Aviation Administration on Friday convened a corrective action review board (CARB) and determined that a vertical navigation (VNAV) glitch, first encountered in service by WestJet in November 2024, poses no safety risk "because pilots retain full control of the aircraft and the indications to the flight crew are clear and unambiguous." The finding should clear the 737 Max 7 for delivery at year-end and the Max 10 for certification in the coming days.

The determination also puts Boeing on course to unlock as much as $2 billion in cash next year from Max 10s already built, according to BNP Paribas analyst Matthew Akers.

Why did airlines reject U14?

The fourth major update of the 737's flight management system — software developed for Boeing by GE Aerospace and fielded as U14 in 2020, then U14.1 — introduced a catalogue of anomalies the carriers judged worse than the bug they already had.

One U14 fault contributed to a June 2022 incident in which a United Airlines 737 Max 9 landed on the wrong parallel runway at Pittsburgh International Airport. The NTSB's probable cause statement cited a dual reset of the flight management computer displays as a factor in distracting the crew on final approach; the fault, traced to "refinements made to the earth model," was reproduced in the lab and absent from U13 and U14.1.

FAA documents posted in July 2026 state that U14 "does not properly process" certain controller-pilot datalink messages, causing an abrupt termination of the connection and barring aircraft from en route domestic CPDLC. The same documents show the issue has affected Next Generation 737s as well.

American Airlines, Alaska Airlines and United confirmed none of their single-aisle jets run U14 or U14.1. "We are not taking deliveries of new [Max 8 and 9] aircraft with the new FMC software," United said in a statement.

A Boeing spokesman said: "Operators of 737 airplanes can continue safe operations with all approved versions of the 737 Flight Management Computer software. We will continue to follow the lead of the FAA as our engineers work on the next software update." GE Aerospace said it was "working closely with Boeing and will support the FAA's evaluation."

What does the rollback involve?

Rolling back is not simply a software choice. GE's newer C2 flight management computer, the baseline for the Max 7 and 10 and now in early production on the Max 8 and 9, is incompatible with U13. Aircraft with C2 cannot be rolled back at all.

United and Southwest have therefore asked Boeing to deliver Max aircraft with the older C1 hardware configuration, preserving the certified path back to U13. C1 has since gained hardened memory protecting against solar radiation interference — a previously unreported hardware change GE was required to make.

That legacy pairing carries its own limitation: aircraft with C1 hardware and U14.1 software are prohibited from RNP AR approaches requiring lateral accuracy below 0.3 nautical miles — a constraint felt keenly in Alaska, where RNP AR pioneered precision approaches in poor weather.

Is the old software itself a problem?

U13 carries a known defect the FAA rates as a low safety risk. When the air data inertial reference unit feeds bad data to the FMS, the system can provide "misleading vertical guidance," commanding the aircraft low while the cockpit display shows a safe trajectory. The fault emerged in simulator sessions and has never been encountered in service, to the FAA's knowledge.

The agency is preparing a notice of proposed rulemaking ahead of an airworthiness directive, and may give airlines as long as 54 months to comply. The expected compliance path is adopting U14 — the version that contains the VNAV glitch — or waiting for U14.2, expected in 2028.

For now, US carriers "see U13 as the lesser of the two evils," one person familiar with the situation said.

What happens next?

The VNAV bug surfaces during altered missed approaches, when the FMC can drop to a simpler pitch mode. FAA test pilots flew missed approaches in a simulator to experience the anomaly directly. "The issue that we're looking at is the workload component," FAA Administrator Bryan Bedford told reporters on Monday.

Boeing plans a bulletin to help crews recognize the bug; the FAA will follow with a Special Airworthiness Information Bulletin for US airlines and foreign regulators. Training is unlikely to require simulator time — possibly just e-learning and an interactive FMS tutorial — though one person familiar with the multi-step recovery procedure, which involves clearing and re-entering the route, called it "a little bit dicey" and said it "has to be trained."

Southwest, which needs all 12,000 of its pilots qualified across 737 variants, expects its Max 7s — certified with U14 — to deliver at year-end and enter service around spring 2027 to accelerate 737-700 retirements. Alaska anticipates its first Max 10 deliveries in 2027.

The Friday determination resolves Boeing's most immediate certification obstacle, but the accumulated defects in U14 and U14.1 leave Boeing and GE substantial software work before the 2028 U14.2 release can reconcile the fleet onto a single standard.