737 MAX: A Software Fix Might Not Be the Complete
Answer
A very talented mechanic in the food processing industry
once explained to me that there would always be those who knew how to do a thing (think highly skilled
engineers in the case of the 737 MAX) and those who knew what to do; think generalists with less specialized knowledge, but across
a wider range of disciplines, those whose talents and interests allow them to
“think outside the box”. I think this element may be missing in the search for
solutions in case of the 737 MAX.
A few days ago I watched (on YouTube) Mentour Pilot’s co-pilot
struggle to adjust the 737 Max’s pitch trim manually in the simulator.
The copilot needed his (Mentor Pilot’s) assistance because
of the forces required to accomplish this, even in the calm and low pressure
environment of the simulator. In the real world the effort required by this
mechanically flawed system would have caused both pilots to be limited in their
ability to find other solutions to the immediate problem.
In an earlier video, I had noted (with discomfort) the
apparent difficulty of being able to grip the runaway trim wheel and stop it by
force. He also stated that the manual system is a mechanical system based on
cables, which I think is generally a good thing. Hindsight is easy but…
From a human
engineering and safety standpoint, the design of the horizontal tail
surfaces and the pilot’s mechanical trim system on the 737 MAX seems poorly
thought out and poorly implemented. The speed at which the trim wheel runs in
the simulator video seems to indicate a poor choice of mechanical advantage and
mechanical ratios in the manual trim system, producing large forces that the
pilots must overcome manually in an emergency and also require too many
revolutions of the trim wheel to accomplish the needed trim control adjustments.
If this is in fact true, large forces would be required to control a runaway
trim by force; or to simply use the manual trim (which may not exist) in normal flight. In order to get a good grip on the runaway
trim wheel one would need to be able to grasp the outer perimeter of the wheel
using the full capabilities of one’s grip. This appears impossible in the video
because of the design and placement of the trim wheel.
If such a trim system
was disabled after the stabilizer had been run to an excessive nose down trim
position before the MCAS system was disabled and under a busy and pressured
emergency environment; with an excessive number of turns of the (difficult to
operate) trim wheel required to correct the stabilizer position it is understandable
that the pilots of the Ethiopia flight might have elected to re-engage the
(faster) electric trim system, while unfortunately possibly simultaneously
re-engaging the MCAS system.
The Boeing engineers seem to have made an (unsuccessful) attempt
to address some of these issues by providing a fold-out handle attached to the trim
wheels, but it appears to be marginally effective. I also doubt that it (the
handle) could be accessed while the trim wheel is running. There are numerous
other ways that these difficulties could be addressed:
·
Increasing the mechanical advantage available to
the pilots by changing the mechanical ratios involved;
·
The trim wheels could be a larger diameter,
clearing the console and thus providing the ability to better grasp the wheel.
·
Adding a completely mechanical independent (from the
autopilot, electrical, MCAS, and other computer controlled systems) or adding a redundant second
trim motor and control system for the pilot’s emergency use;
·
Utilizing a (recirculating) ball screw mechanism
in place of the conventional jackscrew
in order to reduce forces and the number of turns of the trim wheel required
to be effective. Since ball screw mechanisms require significantly less
force to operate and can be “self-driven” their use opens up additional
possibilities for redundancy in the system.
· Another advantage of ball screw actuators is
that they do not require lubrication; think of the Alaska Airlines Flight 261
accident. The probable cause was stated to be "a loss of airplane pitch
control resulting from the in-flight failure of the horizontal stabilizer trim
system jackscrew assembly's acme nut threads. The thread failure was caused by
excessive wear resulting from Alaska
Airlines' insufficient lubrication
of the jackscrew assembly", similar to the system we are discussing on
the 737 MAX.
·
Self-actuating aerodynamic servo tabs on the horizontal
stabilizer (while un-conventional) might be a part of the solution.
·
Other aerodynamic, possibly self-actuating,
solutions having nothing to do with the trim system may be possible in
addressing the thrust vector caused issues that MCAS was designed to address.
The industry, the FAA, and many others worldwide have created
perhaps the safest transportation system the world has ever seen, but we need
to maintain that system under constant review, surveillance, and improvement by
competent parties to ensure decisions and rules are made, and compromises
decided upon, by those best qualified to do so.
Design always involves compromise and trade-offs. This
requires good judgment, good management, and oversight by qualified people, but
the teams can become too specialized and lose sight of the forest. When you
bring in one or more “outside” team members into a discussion, their seemingly un-informed
insights can be profound. For example, in another YouTube video
“Sully” explained the root cause of the Air France
Flight 447 accident (poor human engineering,
very similar to the 737 MAX issues we are discussing). I would guess that his
wide range of separate areas of knowledge and experience (while seemingly
unrelated to engineering) allowed him to reach this insight, and I expect that
he probably has many others related to possible improvements of the
characteristics of the control system of the A330 and other Airbus aircraft.
Unfortunately I have seen no evidence of response to his accident prevention
insight and lesson by Airbus or the industry in general.
Sincerely,
Phil Hertel
thePracticalCFI®
CFI ASMEL-I
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