The TL;DR: The tail of 737 MAXs going up to 4 years back is not attached correctly. What could go wrong?
As an example; this could be as minor as using a riveting tool that hadn't been inspected on the correct schedule but was still operating as expected. It could be far more serious; perhaps that riveting tool was operating at a dangerously low pressure resulting in a poorly attached vertical fin.
We don't know yet. If this had happened before the 737 Max debacle I would have complete trust in Boeing's and the FAA's assessment that "...that there is no immediate safety issue.”. As it is I can't help being a little skeptical, that loss of trust is a real shame.
As an example; this could be as minor as using a riveting tool that hadn't
been inspected on the correct schedule but was still operating as expected.
It could be far more serious; perhaps that riveting tool was operating at a
dangerously low pressure resulting in a poorly attached vertical fin.
No immediate safety issue pretty much means what it says. Planes aren't likely to be falling out of the sky today, but given enough time things will likely fail.
This tragically happened to an airbus flown as American 587 shortly after 9/11. It crashed into a neighborhood in New York City after it encountered wake turbulence from another jet and one of the pilots responded too aggressively with the rudder. Truly a horrifying incident.
Parenthetically I don’t recall whether this was a training issue, a software issue, or a pilot error. Accordingly, I don’t want to sound like I’m slamming the pilot.
On the other hand I’ve seen a B-52 lose just about the entirety of its vertical stabilizer and it didn’t seem to care at all; landed safely.
It was a design issue, which many people like to blame on the pilot because they don't know any better.
The problem was that the rate-of-change of control-force per rudder angle is/was negative for that particular Airbus model... which is generally known to lead to PIO (pilot induced oscillations). When I say generally known, I mean it has been documented since at least as early as pre-WWII NACA (the precursor to NASA) studies.
In fact, that's what led to the installation of MCAS on 737 Max's: The FAA requires that, for the elevator (but not rudder), the control force should increase as deflection increases. So Boeing tried to simulate that behavior with elevator trim + software.
In other words, MCAS was a bad, hacky "solution" to a problem very similar to what happened on AA587 (but for pitch instead of yaw).
> On the other hand I’ve seen a B-52 lose just about the entirety of its vertical stabilizer and it didn’t seem to care at all; landed safely.
Yea, but a B-52 probably has more elevator authority than most commercial airliners (both because it is meant to go supersonic and need to deal with Mach tuck and because B-52 is expected to shed some "payload" while airborne). The problem is that the vertical stabilizer weighs a ton and is waaaay out at the end of the fuselage, so it really throws off your weight and balance if it falls off.
My limited knowledge about the BUFF is that if you achieve supersonic flight in one, you are _definitely_ doing it wrong.
My impression of what the purpose of MCAS is for has, accordingly, not been influenced much by your comment.