Accidents occur when everything goes wrong at once

Increasingly we know that accidents, especially airline accidents, occur when many independent things all go wrong at the same time. We engineer and plan for the expected errors. We have a very hard time anticipating the sudden intersection of two or three or four simultaneous errors.

William Langewiesche has written a fantastic article for New York Magazine on the two Boeing 737 Max crashes. So many great parts:

An old truth in aviation is that no pilot crashes an airplane who has not previously dinged an airplane somehow. Scratches and scrapes count. They are signs of a mind-set, and Lion Air had plenty of them, generally caused by rushed pushbacks from the gates in the company’s hurry to slap airplanes into the air. Kirana was once asked why Lion Air was experiencing so many accidents, and he answered sincerely that it was because of the large number of flights. Another question might have been why, despite so many crashes, the death toll was not higher. The answer was that all of Lion Air’s accidents happened during takeoffs and landings and therefore at relatively low speed, either on runways or in their immediate obstacle-free vicinities. These were the brief interludes when the airplanes were being flown by hand. The reason crashes never happened during other stages of flight is most likely that the autopilots were engaged.

What Really Brought Down the Boeing 737 Max?


The 737 features two prominent toggle switches on the center pedestal whose sole purpose is to deal with such an event — a pilot simply switches them off to disengage the electric trim. They are known as trim cutout switches. They are big and fat and right behind the throttles. There is not a 737 pilot in the world who is unaware of them. Boeing assumed that if necessary, 737 Max pilots would flip them much as previous generations of 737 pilots had. It would be at most a 30-second event. This turned out to be an obsolete assumption.


This time he was ready when the MCAS engaged, and he managed to avoid a dive by counter-trimming and hanging tight. The surprise was that after the assault ended, the MCAS paused and came at him again and again. In the right seat, Harvino was fumbling through checklists with increasing desperation, trying to figure out which one might apply. Over in the left seat, Suneja was confronting a rabid dog. The MCAS was fast and relentless. Suneja could have disabled it at any time with the flip of the two trim cutout switches, but this apparently never came to mind, and he had no ghost in the jump seat to offer the advice. The fight continued for the next five minutes, during which time the MCAS mounted more than 20 attacks and began to prevail.

The whole article is a study in design, human performance, complexity, and tragic expedience.