Monday, October 07, 2019

Pilot Overload and the Boeing 737 Max Accidents

In the last couple of months, new information about the factors leading to crashes of two Boeing 737 Max aircraft and the loss of 346 lives has emerged.  All such aircraft were grounded indefinitely last March after investigators found that a software glitch combined with faulty data from airspeed indicators to start a chain of events that led to the crashes.  Airline companies around the world have lost millions as their 737 Max fleets sit idle, and Boeing has been under tremendous pressure from both international regulatory bodies and the market to come up with a comprehensive fix for the problem.  But as long as both humans and computers have to work together to fly planes, the humans will need training to deal with unusual situations that the computers come up with.  And in the case of the Lion Air and Ethiopian Air crashes, it looks like whatever training the pilots received left them inadequately prepared to deal with at least one situation that led to tragedies.

Modern fly-by-wire aircraft are certainly among the most complex mobile systems in existence today.  It is literally impossible for engineers to think of every conceivable combination of failures that pilots would have to handle in an emergency, simply because there are so many subsystems that can interact in almost countless ways.  But so far, airliner manufacturers have done a pretty good job of identifying the major failure conditions that would be life-threatening, and instructing pilots about how to deal with those.  The fact that Capt. Chesley Sullenberger was able to land a fly-by-wire Airbus A320 plane in the Hudson in 2009 after experiencing failure of all engines shows that humans and computers can work together cooperatively to deal with unusual failures.

But the ending was not so happy with the 737 Max flights, and recent news from regulators indicates that a wild combination of alarms, stick-shakings, and other distractions may well have paralyzed the pilots of the two planes that crashed after faulty readings from angle-of-attack sensors set off the alarms. 

Flying a modern jetliner is a little bit like what I am told it was like being in the army during World War II.  For many soldiers, the experience was a combination of long stretches of incredible tedium interrupted by short but terrifying bursts of combat.  It's psychologically hard for a person to remain alert and ready for any eventuality when the norm is that pretty much nothing out of the routine ever happens the vast majority of the time.  So when the unusual failure of both angle-of-attack sensors led to a burst of alarms and the flight computer's attempt to push the nose down, the pilots on the ill-fated flights apparently failed to cope with the confusion and could not sort through the distractions in order to do the correct thing.

A month after the Lion Air crash in 2018, the FAA issued an emergency order telling pilots what to do in this particular situation.  Read in retrospect, it resembles instructions on how to thread a needle in the middle of a tornado: 

            ". . . An analysis by Boeing found that the flight control computer, should it receive faulty readings from one of the angle-of-attack sensors, can cause 'repeated nose-down trim commands of the horizontal stabiliser'.  The aircraft might pitch down 'in increments lasting up to 10sec', says the order.  When that happens, the cockpit might erupt with warnings.  Those could include continuous control column shaking and low airspeed warnings – but only on one side of the aircraft, says the order.  The pilots might also receive alerts warning that the computer has detected conflicting airspeed, altitude and angle-of-attack readings. Also, the autopilot might disengage, the FAA says.  Meanwhile, pilots facing such circumstances might need to apply increasing force on the control column to overcome the nose-down trim. . . . They should disengage the autopilot and start controlling the aircraft's pitch using the control column and the 'main electric trim', the FAA say. Pilots should also flip the aircraft's stabiliser trim switches to 'cutout'. Failing that, pilots should attempt to arrest downward pitch by physically holding the stabilizer trim wheel, the FAA adds."

If I counted correctly, there are six separate actions a pilot is being told to take in the midst of a chaos of bells and whistles going off and his plane repeatedly trying to fly itself into the ground.  The very fact that the FAA issued such a warning with a straight face, so to speak, should have set off alarms of its own.  And after the second crash under similar circumstances, reason prevailed, but first with regulatory agencies outside the U. S.  Finally, the FAA complied with the growing global consensus and grounded the 737 Max planes until the problem could be cleared up.

When software is rigidly dependent on data from sensors that convey only a narrowly defined piece of information, and those sensors go bad, the computer behaves like the broomstick in the Disney version of Goethe's 1797 poem, "The Sorcerer's Apprentice."  It goes into an out-of-control panic, and apparently the pilots found it was humanly impossible to ignore the panicking computer's equivalent of "YAAAAH!" and do the six or however many right things that were required to remedy the situation. 

It is here that an important difference between even the most advanced artificial-intelligence (AI) system and human beings comes to the fore.  It is the ability of a human being to maintain a global awareness of a situation, flexibly enlarging or narrowing the scope of attention as required.  Clearly, the software designers felt that once they had delivered an emergency message to the pilot, the situation was no longer their responsibility.  But insufficient attention was paid to the fact that in the bedlam of alarms that the unusual simultaneous sensor failure caused, some pilots—even though they were well trained by the prevailing standards—simply could not remember the complicated sequence of fixes required to keep their planes in the air.

Early indications are that the 737 Max "fix," whatever software changes it involves, will also involve extensive pilot retraining.  We can only hope that the lessons learned from the fatal crashes have been applied, and that whenever such unusual sensor failures happen in the future, pilots will not have to perform superhuman feats of concentration to keep the plane from crashing itself.

Sources:  A news item about how Canadian regulators are looking at the pilot-overload problem appeared on the Global News Canada website on Oct. 5, 2019 at  The November 2018 FAA directive to 737 Max pilots is summarized at  I also referred to Wikipedia's articles on the Boeing 737 Max groundings, Chesley Sullenberger, and The Sorcerer's Apprentice. 

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