Wednesday, April 3, 2019

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 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 stabilizer, as opposed to an elevator 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; and with relatively limited elevator vs stabilizer control authority (read:  area relationships), 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;
·        Changing the relative areas of the elevator and stabilizer in ordered to create a more balanced control authority between them;
·        Adding a completely independent (from the autopilot, MCAS, and other computer controlled systems) and 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 and/or anti-servo tabs on the elevator and/or 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.
Is this same (737 MAX) trim system installed on all versions of the 737? Have these issues been addressed in earlier versions? If so, were they lost in later design iterations, perhaps not requiring a change to the type certificate?
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
The Practical CFI
CFI ASMEL-I