Some Assembly Required

Henry Ford didn't invent the assembly line method of manufacture, that was about 1853, but Henry is the avatar for its use. The Boeing door plug saga is a lesson in doing the right thing for the wrong reason. Or, may haps, doing the wrong thing for the wrong reason.

Years ago, Volvo among a few others, tried building cars in situ with a dedicated cadre of workers who did most of the assembly tasks as a unit. The effort was abandoned rather quickly.

If all goes as planned, the plant will be organized into work teams - each of which will ultimately assemble a complete car by itself.

Now, it can be argued that the assembly line method is the most efficient way to build widgets.

Ford was inspired by the meat-packing houses of Chicago and a grain mill conveyor belt he had seen. If he brought the work to the workers, they spent less time moving about. Then he divided the labor by breaking the assembly of the Model T into 84 distinct steps. Each worker was trained to do just one of these steps. Ford called in Frederick Taylor, the creator of "scientific management," to do time and motion studies to determine the exact speed at which the work should proceed and the exact motions workers should use to accomplish their tasks.
[my emphasis]

For what it's worth, Taylor is infamous for the art of de-humanizing labor.

It can be argued that assembly line manufacture really only works when the widget is actually made in an in situ linear process, the widget moved step by step to the next task. Today's autos are built with an even more capitalized assembly line. The key to success is that each task point is limited and specific. The 737 has an estimated 500,000 individual parts. A car about 30,000. And, of course, a car can be assembled in a traveling assembly line.

One might wonder whether aircraft assembly is amenable to the method. Based on today's reporting (and earlier similar), Boeing's method rely heavily, if not exclusively, on ad hoc manual reporting, "paperwork", to keep track of the status of a plane. That's not really assembly line method.

As it happens, I am addicted to a Canadian series, "Air Disasters", which tells the stories of airplane crashes (you thought it was about mahjong games?). More than a few incidents are driven by maintenance work which didn't keep proper paperwork and let the aircraft crash. Maintenance isn't done in an assembly line fashion, and when the work exceeds a shift and/or team, paperwork must follow the plane. No paperwork, no task completed, no part installed. It appears that aircraft manufacture has the same issue.

What happened with this 737 is that, according to the existing record, the removal and replacement of the plug door was done by multiple groups doing a single task; single from the outsider's point of view. In particular, hard to believe, is that the group which put the plug door back in the fuselage, wasn't tasked to bolt it in. Now, the assembly line method is based on reducing the work done in each task to the minimum possible. But here we find a case, if true, that the following group(s) can only know to replace the bolts if there's paperwork in the flow to tell them to. It's not clear from my reading of the record whether the replaced door plug was such that the missing bolts were not viewable; that is, was the interior panel set in place at the time the door plug was re-fitted?

Deming said that the best policy is to do it right the first time. From the report, we get this

To avoid the problem in the future, Boeing is considering adding a warning light in the cockpit that would alert pilots if the door plug moves even a little bit — well before it could blow out in the kind of accident that occurred on the Alaska Air flight. Since it is a known phenomenon that aircraft fuselages flex during pressurization cycles, won't such an alarm fire every flight?

Given how the door plug eventually failed, after ~150 flights, one might wonder whether such a warning light would do any good? If the plane is at 35,000 feet, and the warning light flashes, how much time would the pilot have to get the plane back on the ground before the door plug flew away? A second? A millisecond? Stupid.

It would appear that calling aircraft manufacture an "assembly line" process has led the industry to complacency. Since it's an assembly line, then we must know that a plane can't get from Step X to Step X+1 without Step X being completed, by definition. Guess not.