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 (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.
Phil Hertel


Wednesday, April 13, 2016

How spatial disorientation can trap pilots

A pilot must learn, under actual threat of death, that it is impossible to maintain an airplane in controlled flight without visual reference to the horizon, even if that horizon is an artificial one.
Loss of control accidents caused by spatial disorientation continue to regularly claim the lives of pilots and passengers in spite of required training and the best efforts of instructors to instill the importance of non-instrument rated pilots recognizing and avoiding instrument flying conditions.

There are many ordinary circumstances in flying that can result in a pilot inadvertently encountering instrument flying conditions where there is no effective natural horizon:
  • Flying over water on a clear moonless night can easily result in disorientation, intense vertigo, and a loss of control.
  • Flying under the same conditions over land in sparsely populated areas has the same effect.
  • Flying on top of a horizon-to-horizon seemingly flat deck of solid cloud that is not aligned with respect to the real (invisible) horizon below, can be disconcerting to the point of serious nausea and worse.
  • Clouds are easy to inadvertently enter when flying at night, even over a large city.
  • In addition to acting alone, haze, smoke, and fog exacerbate the above conditions.
It is almost impossible to imagine the difficulty or impossibility of controlling an airplane under these seemingly ordinary circumstances without actually experiencing the conditions. It’s not stubbornness or lack of training that causes the continuing occurrence of this type of accident; it’s our anatomy that confronts a pilot with often insurmountable problems.
We maintain the position of our head in space using a combination of sensory inputs, obviously including vision, but less obvious and equally important are the effects of gravity and inertia on our muscles and on the fluid in the semi-circular canals in our inner ears. Even so, most of us can walk, swim or run with our eyes closed.
Speaking of the inner ear, we have three semi-circular canals (tubes, really) partially filled with fluid, in each ear. There is one tube in each of our ears for the three dimensions of angular rotation that we must learn to contend with when learning to walk:
  • The pitch axis – head tilt forward and back (nose up or down in an aircraft, controlled by the elevator).
  • The roll axis – head tilt left and right (wings banked, controlled by the ailerons, if you are in flight).
  • The yaw or vertical axis – think figure skater in a spin (controlled by the rudder pedals in an aircraft).
That last one is not always as useful as the other two axes in controlling an aircraft, but it is still critically important, as it is good at adding to the confusion that often results in disorientation and vertigo.
Our brain processes these hundreds, if not thousands, of simultaneous sensory inputs and directs the rest of the body to position the head accordingly.
Inner ear diagram
The system works really well… until it doesn’t.
By the time we decide to learn to fly, the portion of our brain which is responsible for balance and spatial orientation – our position in space – is thoroughly convinced that it knows best. It is more than a little difficult to convince it that it still has more to learn, despite the fact that the logical portion of the brain has been studying, and is insisting otherwise.
If we allow it, its survival instincts are strong enough to kill us.
Centrifugal, centripetal and inertial forces in flight mimic the effects of gravity and, in combination with the actual effects of gravity, just confuse the hell out of the brain. Without sufficient study, re-training, practice and experience, it reacts, often inappropriately, when it becomes disoriented.
I observed all this in action with my buddy Max.
My student Max, like many before and after him, could just not bring himself to believe that he could not fly the airplane by the seat of his pants without visual references outside the cockpit in spite of instruction and all the materials he had read about spatial disorientation and vertigo.
One evening we flew to a grass strip about fifty miles south of the city in the middle of sparsely populated farm country, where an old Piper Cub was being restored in the proverbial barn. We had a great time hangar flying with the owner and when it was finally time to leave and actually go flying, it was pitch black outside. It was a beautiful night, perfectly clear, no moon – you could see a hundred miles if you were high enough.
Max was shocked. “There are no runway lights, what do we do now?” He had never been allowed to use the “landing light” during night flights at our lighted home field – that’s another story – but I explained that it was actually not a bad takeoff light. “We’ll just taxi the length of the runway to make sure there are no deer eating it, and it’ll be just like driving your car,” I told him.“Cool,” said Max.
Max took off and we began to climb. A couple of hundred feet from the ground, I asked how he liked flying from a grass strip. He was having a ball. Not long thereafter Max, the airplane, and I were in a ten-degree bank to the left. “How are we doing Max?” Max was ecstatic. This was fun! At 400 feet, and now in a 30-degree bank, I asked how the attitude indicator looked. It must have looked a little funny, because Max allowed that it must be “a little off.” At around 600 feet, we had pretty much stopped climbing, and the attitude indicator was telling us that we were in a sixty-degree bank. Max was perplexed, but insisted that the instrument must be failing. He was finding it difficult to believe that we were anything but wings level.
When we were well past ninety degrees of bank, I said to Max “I have the airplane” and, as he had been trained to do, he immediately removed his left hand from the control wheel, his right hand from the throttle, and pulled his feet well back from the rudder pedals, positively relinquishing control of the airplane to me. No big deal, just routine until I rolled the airplane back to wings level. His reaction was involuntary and violent as he crashed into the pilot’s door as the recalcitrant portion of his brain fought to remain “upright.”
Given the subdued light in the cockpit, and our less than perfect visual night adaptation, there were just enough bright stars in the sky to match the scattered barn lights on the ground. There was no visible horizon on that beautiful, clear night.
Max became a true believer at that point and at last check, he was still alive and flying.