The Core Causes of the Boeing 737 Max Flights 302 and 610 Tragedies
Updated 01:27 JST 21 March 2019.
The following describes my sense of the core causes of the Boeing 737 Max Flights 302 and 610 tragedies and why their underlying design and management failures might be viewed as egregious.
In my estimation Boeing, the FAA, and many others will suffer substantially from these tragedies because further investigation will reveal truly awful design, corporate management, and institutional oversight failures. And even aviation enthusiasts will bear some shame - as the Flight 610 investigation revealed the core vulnerability which ultimately caused that tragedy most of us failed to raise red flags as vigorously as we should have before the Flight 320 tragedy ensued.
Here's a brief review of why the tragedies occurred as I perceive the evidence:
First it's important to understand some terminology and mechanics. Please refer to stabilators and elevators. I include a quote from the stabilator page below but the graphics and greater detail on the Wikipedia pages are very helpful. (I contributed to the Wikipedia talk page for Ethiopian Airlines Flight 302 but my content below is equivalent. The article itself is useful but provides less technical detail.)
From stabilators: "Most modern airliners adjust the tailplane angle of incidence to trim during flight as fuel is burned and the center of gravity moves. These adjustments are handled by adjustable incidence horizontal stabilizers. However, such adjustable stabilizers are not the same as stabilators; a stabilator is controlled by the pilot's control yoke (or stick), whereas an adjustable stabilizer is controlled by the trim system."
Airline pilots are well aware of the difference between a stabilator and an adjustable stabilizer. And they know adjustable stabilizers, not stabilators, are used almost universally in modern jetliners.
In both cases the entire structure rotates. However a stabilator has no elevators. Rather the pilot's yokes or sticks control the angle of attack of the entire structure. (Small trailing edge trim tabs are likely incorporated too.)
An adjustable stabilizer incorporates modest sized trailing edge elevators (which likely incorporate small trailing edge trim tabs too). The pilot's yokes or sticks control only those elevators. The angle of attack of the entire adjustable stabilizer is controlled by completely separate aircraft systems, and is evidently commonly referred to as 'trim control'.
Normally adjustment of the angle of attack of the entire adjustable stabilizer is a trimming function. And in my personal estimation most pilots believed it only operated as a trimming function - I suspect most believed an aircraft's systems would only rotate the angle of attack of the adjustable stabilizer modestly and slowly - that it would move only in a trimming mode.
But if an aircraft system commands substantial angle of attack changes over short periods of time it's not performing a trim function, but rather a flight maneuvering function - in that case it operates the adjustable stabilizer as if it were a stabilator. And rotation of the adjustable stabilizer exerts a more powerful pitch control than the elevators.
My sense is that most jetliners can't do that - they're not able to swiftly rotate the entire adjustable stabilizer as a flight maneuvering control surface like a stabilator, but rather can only rotate it in a slow trimming manner. (This needs confirmation however.)
But a 737 Max system can do so - it's able to change the angle of attack of the adjustable stabilizer rather swiftly and to its limits, utilizing it as a flight maneuvering control surface - as a stabilator. And since rotation of the adjustable stabilizer exerts a more powerful pitch force than the elevators a 737 Max system can overwhelm yoke control, forcing an aircraft pitch change even when a pilot tries to counter the action with full opposite yoke command.
I suspect this was not understood by many pilots. Which invited tragedy because pilots who weren't well studied with the unique ability of the 737 Max's autonomous systems to overpower the comparatively modest control available from the elevators, and thus their yokes, were unprepared to manage a system failure event, leaving them far too little time to reason through it and then execute the multiple steps necessary to regain control. (And it's not yet clear whether even superbly trained pilots could regain control swiftly enough in response to system failures which occur at low altitudes.)
The jackscrew which controls the angle of attack of the adjustable stabilizer was found in the full nose down command position in both tragedies.
And the jackscrew position is controlled by autonomous systems (or manual trim if the autonomous systems are disabled), not by the yokes or sticks. And in the 737 Max those systems aren't limited to trimming duties - they can exert swift full control over the aircraft's pitch, and were tasked to do so to prevent or limit stall conditions.
Two key elements seem likely to be debated at great length:
1. The authority levels and vulnerabilities of the 737's Maneuvering Characteristics Augmentation System (MCAS), the system which can autonomously operate the aircraft's adjustable stabilizer in a flight maneuvering mode as if it were a stabilator.
2. The magnitude of training required to insure pilots are competent to retain control of aircraft so equipped even when system failures occur.
In my estimation only limited defense will be available - in both elements the design and institutional failures appear to have been quite bad - bad enough that attorneys might successfully wield the word egregious in hearings.
And the days ahead might be even darker because human beings chronically fail to comprehend the immense magnitude of difference between clarity of hindsight and clarity of foresight. And perhaps no species in the Cosmos is more fond of a witch hunt than homo sapiens. And a lot of wonderful people died in a quite horrible manner. Which adds up to a rather perfect storm of hardship ahead for Boeing, the FAA, and many others.
So sadly the grief from these tragedies is far from over...
These are just my personal perceptions and judgments. They are inaccurate in at least some measure, and possibly in large measure. All who seek to thoroughly understand these tragedies must of course assimilate much more information from wiser and more learned sources than me.
I abandoned further work for my Deadly Autopilot site due to time constraints. However I remain very concerned about that issue too and feel another Flight 370 type tragedy awaits if the autopilot shortcomings I describe there haven't been well corrected. I haven't researched the matter in the last couple of years but offhand have heard of no significant progress. Which concerns me of course...
Copyright 21 March 2019 JST, Howard Bruce Campbell, AirplaneHome.com.
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