Once the SRB ignition is complete and combustion pressure has reached its full operating value, there is a greater effect on the walls than on the joint because the joint is more rigid to hoop stress. Thus, the joint rotates and the gap expands. The last thing you want.
https://www.ntsb.gov/investigations/AccidentReports/Reports/AAR0201.pdf
Let me reiterate, aircraft must be maintained with care and at all cost.
Alaska Airlines expanded rapidly in the years before this accident. With the goal of becoming more profitable, as they became bigger and busier the pressures to keep their planes on schedule put increasing stress on their maintenance facilities. And this took its toll. The Federal Aviation Administration seemed to know this, but was nowhere effective in preventing the tragic chain of events.
So N963AS began its fateful path in a C-check years before falling to the ocean. Its maintainers found a jackscrew that needed to be pulled, but no spare was found and, as the part was arguably acceptable, they pushed the plane back into service, with no watch list, no trailers, or orders to keep track of its condition. There were no specific procedures to do so, and no one thought enough to ask for one.
The aircraft was arguably, that is technically, legal, and it was probably safe, if it was carefully greased. It was not, we know that without question. And the lengthened inspection intervals were such that it was not to be looked at again, until it was in our laboratory.
When it finally failed, ground support from Alaska Airlines seemed to encourage the crew to proceed with a broken plane on to their scheduled destination, for reasons perhaps of convenience both to passengers and maintenance -- we won't really ever know.
But the impression is inescapable. An aircraft that had been hustled out the door three years earlier for the convenience of scheduling was now encouraged to keep to its appointed routing. It is less coincidence than culture.
