MAX’s Return Delayed by FAA Reevaluation of 737 Safety Procedures

[MemberBerry]

This is the section of the JATR report discussing the relation between stall behavior and MCAS, I highlighted the parts I found interesting:

• Recommendation R3.4: The FAA should review the natural (bare airframe) stalling characteristics of the B737 MAX to determine if unsafe characteristics exist. If unsafe characteristics exist, the design of the speed trim system (STS)/MCAS/elevator feel shift (EFS) should be reviewed for acceptability.
  • Observation O3.4-A: The original implementation of MCAS was driven primarily by its ability to provide the B737 MAX with FAA-compliant flight characteristics at high speed. An unaugmented design would have been at risk of not meeting 14 CFR part 25 maneuvering characteristics requirements due to aerodynamics.
  • Observation O3.4-B: Extension of MCAS to the low-speed and 1g environment during the flight program was due to unacceptable stall characteristics with STS only. The possibility of a pitch-up tendency during approach to stall was identified for the flaps-up configuration prior to the implementation of MCAS.
  • Finding F3.4-A: The acceptability of the natural stalling characteristics of the aircraft should form the basis for the design and certification of augmentation functions such as EFS and STS (including MCAS) that are used in support of meeting 14 CFR part 25, subpart B requirements.
• Recommendation R3.5: The FAA should review 14 CFR 25.201 (Stall Demonstration) compliance for the B737 MAX and determine if the flight control augmentation functions provided by STS/MCAS/EFS constitute a stall identification system.
  • Finding F3.5-A: The nose-down pitch identified during Boeing flight tests for stall appears to the JATR team to be the product of system augmentation with flaps and gear up, and is likely due to stabilizer motion from the MCAS function.
  • Finding F3.5-B: The FAA-accepted Boeing flight test technique of freezing column deflection at the onset of EFS was perceived by the JATR team as possibly not meeting the requirements of § 25.201 for natural stall identification from nose-down pitch, not readily arrested. Column/elevator deflection data indicates that there may be an insufficient column input to attempt to arrest the nose-down pitch created by system augmentation.
  • Finding F3.5-C: The JATR team considers that the STS/MCAS and EFS functions could be considered as stall identification systems or stall protection systems, depending on the natural (unaugmented) stall characteristics of the aircraft. From its data review, the JATR team was unable to completely rule out the possibility that these augmentation systems function as a stall protection system.
• Recommendation R3.6: The FAA should review the use of non-standard flight test techniques, such as freezing column position at EFS actuation, when showing compliance with 14 CFR 25.201 (Stall Demonstration). The use of non-standard flight test techniques may not meet the associated regulatory requirements.
  • This recommendation is based on Findings F3.5-A, F3.5-B, and F3.5-C, above.
• Recommendation R3.7: The FAA should review how compliance was shown for the stall identification system on the B737 MAX with respect to inadvertent operation due to single failures.
  • Finding F3.7-A: The JATR team considers that system features on the B737 MAX might constitute a stall identification system. This system is vulnerable to inadvertent actuation due to a single failure, which would not meet the accepted guidance contained within AC 25-7C, Chapter 8, Section 228.

I found especially interesting that last finding, F3.7-A, because it would explain why Boeing insists MCAS is not related to stall prevention. This is the AC the report refers to, section 228 is at page 334 of the PDF:

https://www.faa.gov/documentLibrary/...2025-7C%20.pdf

Some relevant parts of the AC:

A means to quickly deactivate the stall identification system should be provided and be available to both pilots. It should be effective at all times, and should be capable of preventing the system from making any input to the longitudinal control system. It should also be capable of canceling any input that has already been applied, from either normal operation or from a failure condition.

And, while the cutout switches can be used to stop MCAS, they don't cancel "any input that has already been applied". Both the original and the "fixed" MCAS would fail this requirement if MCAS is a stall identification system.

Also from the same AC:

To ensure that inadvertent operation of the stall identification system does not jeopardize safe flight, and to maintain crew confidence in the system, it should be shown that:

1. No single failure will result in inadvertent operation of the stall identification system;

The original version of MCAS didn't meet that requirement.