737 MAX: FAA Prelim Summary Released, NPRM Imminent


The latest chapter in the Boeing 737 MAX saga debuted today, with the FAA releasing a “preliminary summary” of its formal review of the tasks required to return the airliner to service. The Notice of Proposed Rulemaking (NPRM) outlining compliance measures is due to be published this week; it contains a 45-day comment period that opens upon publication. Boeing and the FAA recently completed compliance flight checks in Washington state.

“During its evaluation of Boeing’s proposed modifications, the FAA has retained all compliance findings and approvals associated with the design changes related to MCAS,” says the agency, reinforcing that it has not allowed Boeing to self-certify at this stage. “This thorough review has taken more than 18 months and included the full-time work of more than 40 engineers, inspectors, pilots, and technical support staff. The effort represents more than 60,000 FAA hours of review, certification testing, and evaluation of pertinent documents. This has so far included approximately 50 hours of FAA flight or simulator tests and FAA analysis of more than 4,000 hours of company flight and simulator testing.”

The changes have been telegraphed almost from the beginning and boil down to fundamental changes in the way the Maneuvering Characteristics Augmentation System (MCAS) functions. The FAA will require several changes, including reliance on both angle-of-attack sensors (not the one, as previously configured), elimination of MCAS’s ability to provide multiple nose-down commands and a limit on the total amount of nose-down trim the system can apply, and making the “AOA Disagree” annunciation, which had been an option on some aircraft, standard across the board. The FAA is also requiring changes to the flight-control computers to detect other stabilizer-trim failure modes. There are also changes in training and maintenance of the AOA sensors.

During the MAX crisis, the FAA and Boeing have faced increased scrutiny from civil aviation authorities (CAA) around the world, with many threatening to demand fixes beyond what the FAA might propose in order to clear the MAX in other countries. “This Preliminary Summary is part of FAA’s extensive outreach to technical experts from CAAs around the globe to keep them apprised of the agency’s progress and address their concerns about the aircraft,” the agency said. “The FAA is also actively supporting concurrent validation activities of the aircraft by the CAAs of other States of Design of large transport airplanes. Each authority will make its own assessment of the FAA’s process and findings, as well as Boeing’s actions to address the findings.”

The 737 MAX is expected to be cleared for revenue service in October, with aircraft returning to a decimated travel market some weeks after that.

Marc Cook
KITPLANES Editor in Chief Marc Cook has been in aviation journalism for more than 30 years. He is a 4000-hour instrument-rated, multi-engine pilot with experience in nearly 150 types. He’s completed two kit aircraft, an Aero Designs Pulsar XP and a Glasair Sportsman 2+2, and currently flies a 2002 GlaStar.

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  1. I hope Boeing gives more thought to the possible impacts of “lot rot” than they gave to the potential single point failure of an AOA sensor. I would be much more worried about flying a 737 Max now than if they would have pushed out a partial fix in a few months.
    I mean if there was a GA plane that had been sitting untouched at your local airport with a for sale sign on it for a year would you want to be the first person to take it for a test flight? Multiply that concern by a factor of about 100 when you think about all of the complicated hydraulics and such it takes to make a heavy fly. Just the thought of it makes me want to sell the bit of Boeing stock that I bought on the cheap this spring…

    • Complex aircraft like this will have a pre-defined set of maintenance tasks to be performed to put an aircraft into short-, medium-, and long-term storage, potentially a series of tasks to be performed to check it during its period of storage, and a set of maintenance tasks and inspections to be performed when they’re returned to active service. This has all been thought out during design.

      • The Boeing owned aircraft, at least, have a crew exercising controls and running systems on a weekly basis and testing fuel and hydraulics. My guess is that most other operators do the same. They’re not just sitting in the back 40.

        • Yes; there are procedures for “mothballing” as it’s a common practice. Many airliners have had a hiatus in their lifetime, especially some jumbos.

  2. All 737’s since the 300 (Likely more and likely other types as well) have a characteristic which could easily prevent recovery if a runaway trim condition is mishandled or not corrected early on

    Once a certain aircraft nose down trim setting is reached, neither the manual trim nor electric can overcome the aerodynamic forced required to restore nose up trim.

    Altitude permitting, the only solution is to reduce the forces by applying significant nose down pressure to the column. It’s unlikely this was possible, or known by the crews of the two accident airplanes, one of which allowed the trim to run nose-down unchallenged for more than 9 seconds.

    • Yeah, it would require two coordinated pilots and yarbles of steel to shove the yoke forward when the ground is already in the windshield. It’s been done on a sim, but you know you aren’t going to die.

  3. It says “The effort represents more than 60,000 FAA hours of review, certification testing, and evaluation.
    I am sure all of these FAA employees who have been milking the system all this time will be sorry to see completion!