BOEING 737 MAX (Part 1)
On August 6, 2020, the Federal Aviation Administration (FAA) posted Document ID FAA-2020-0686-0001 (see https://www.regulations.gov/document/FAA-2020-0686-0001) apprising the public of a proposed Airworthiness Directive (AD) to supersede Airworthiness Directive 2018-23-51 (applicable to all The Boeing Company Model 737-8 and 737-9 (737 MAX) airplanes), the agency having determined that final corrective action would be necessary to address the unsafe condition. The agency stated that comments on the proposed AD must be received no later than September 21, 2020.
On August 14, 2020, this writer received an email from Regulations.gov under the title, Your Comment Submitted on Regulations.gov (ID: FAA-2020-0686-0001), stating that the comment (submitted earlier that day by this writer) was successfully submitted, the comment (posted by the FAA at https://www.regulations.gov/comment/FAA-2020-0686-0080) being reproduced in said email. That comment is reproduced (from the email) as follows (with minor administrative and privacy-related deletions by this writer):
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Comment Tracking Number: 1k4-9idq-l8vj
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Agency: Federal Aviation Administration (FAA)
Document Type: Rulemaking
Title: Airworthiness Directives: The Boeing Company Airplanes
Document ID: FAA-2020-0686-0001
Comment:
My technical background is in physics and system safety engineering.
My concerns are twofold:
(a) that a high-consequence hazard is not being eliminated at source (i.e., aerodynamically) instead of being addressed via ancillary automation (i.e., MCAS); and
(b) that the acceptance of the specific hazard in question with its proposed amelioration via ancillary automation is actually precluded by FAA regulations.
With respect to my first concern, Boeing sought to overcome flight performance limitations due to the less than adequate aerodynamic characteristics of the MAX design through ancillary automation, thereby parting with the first principle of safety engineering—that of designing out the hazard. (The principle of physically removing high-consequence hazards at the design stage is the preferred practice across industry worldwide.)
With respect to my second concern, the raison d'être for MCAS is stated, for example, in the last sentence of footnote 8 of the NPRM as follows: "MCAS makes pitch trim commands to the horizontal stabilizer during a high AOA event so that the 737 MAX handling qualities are compliant with FAA regulations (including 14 CFR 25.173)." I contend that the decision to incorporate MCAS (or any similar system) reflects a misinterpretation of FAA regulations, more specifically, 14 CFR 25.203 Stall characteristics, that states:
“(a) It must be possible to produce and to correct roll and yaw by unreversed use of the aileron and rudder controls, up to the time the airplane is stalled. No abnormal nose-up pitching may occur. The longitudinal control force must be positive up to and throughout the stall. In addition, it must be possible to promptly prevent stalling and to recover from a stall by NORMAL [my emphasis] use of the controls.”
While MCAS was developed in response to the aerodynamic shortcomings of the MAX, apparently in respect of one specific requirement of 14 CFR 25.203(a), namely, the “no abnormal nose-up pitching” requirement, its actual implementation nevertheless breaches another specific requirement of 14 CFR 25.203(a), namely, the requirement “... to promptly prevent stalling and to recover from a stall by NORMAL use of controls.” (again, my emphasis)
SUBPART B - FLIGHT (14 CFR 25.21 - 14 CFR 25.255) implies (in a number of sections) inherently stable airplane aerodynamics and other features by precluding the necessity for “excessive control forces” or “exceptional piloting skill, alertness or strength" to otherwise compensate for design inadequacies. That underlying concept applies not only to the pilot but to the autopilot (and hence, implicitly to excessive or exceptional automation in general). A prime example in keeping with that philosophy is 14 CFR 25.161 Trim that states:
“(a) General. Each airplane must meet the trim requirements of this section after being trimmed, and without further pressure upon, or movement of, either the primary controls or their corresponding trim controls by the pilot or the automatic pilot.”
Obviously, 14 CFR 25.161(a) does not differentiate between the pilot or the automatic pilot. Its focus is on maintaining trim control exclusively. The objective of 14 CFR 25.161(a) is to ensure that the airplane is aerodynamically capable of meeting the trim requirements and sustaining the required trim without additional trim control after being trimmed.
With respect to 14 CFR 25.203(a), the objective is “to prevent stalling and to recover from a stall” without implementing abnormal or extraordinary use of the controls. Again, the focus is on NORMAL use of the controls. Clearly, it makes no difference whether the abnormal or extraordinary use of the controls involves either the pilot or an automatic system or a combination thereof. In the case of the Boeing 737 MAX, the abnormal use of controls involves ancillary automation (i.e., MCAS) applied to the horizontal stabilizer to compensate for inadequate aerodynamic performance near or under stall conditions. The Boeing 737 MAX design incorporating any version of MCAS or any manner of ancillary automation is therefore NOT COMPLIANT with 14 CFR 25.203(a).
The implementation of ancillary automatic systems to ameliorate hazards arising out of designs that exceed fundamental mechanical or dynamical integration limits represents a misapplication of control technology. Given the safety-critical nature of the civil aviation industry and the magnitude and seriousness of the aerodynamic deficiency in question, the first tenet of hazard control, that of designing out the hazard, is paramount. In the present case that tenet is upheld by FAA regulations. Unless and until Boeing is able to correct the aerodynamic integration issue to render the 737 MAX inherently safe (and therefore compliant) in respect of the 14 CFR 25.203(a) requirement to “promptly prevent stalling and to recover from a stall by NORMAL [again, my emphasis] use of the controls,” it is imperative that the 737 MAX remains grounded.
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