Both cause the stabilizer to tilt. Both cause the stabilizer wheels next to the pilot/co-pilot to turn. If the plane is pitching down, and you pull back the yoke and that still doesn't recover, don't you at some point check the stabilizer angle? If it's wrong, you use the stabilizer trim switch on the yoke under your thumb to correct it. Won't you see the stabilizer wheel turn when you do that? If you let go and the wheel immediately starts turning again the wrong way (pitching the plane down), doesn't that look like runaway stabilizer?
Also, how did the off-duty pilot figure it out if they are nothing alike?
BTW, when you write "runaway stabilizer feels nothing like MCAS" are you speaking from personal experience?
Pilots are trained well for runaway stabilizer, MCAS was essentially the same thing, but it didn't FEEL like the same thing, so a stressed pilot's mind may not immediately go to that solution/memory item.
Now things are starting to make sense.
Boeing claimed (and F.A.A agreed) that pilots didn't need training because they already had training for a memory procedure which would have solved the issue.
They both appear to have overlooked is the MCAS symptoms were sufficiently different to the runaway stabiliser scenarios pilots trained on, that pilots are having problems knowing which procedure to apply.
Besides obviously making the sensors used by MCAS truely redundant and limiting its extreme behavior of moving the trim up to the stops, a large part of fixing the MAX might be just having a big warning light for MCAS operating.
First he’s gonna be behind the throttle quadrant. The stabilizer wheels are roughly in the center of his vision, so he will notice them moving and when they start and stop. The pilots have them down near their thighs, so they’re not even necessarily in their peripheral vision. They will be less likely to notice every time they start and stop.
Second the jumpseat guy has no flight responsibilities. He can focus on thinking about what’s wrong without dealing with all of the other stuff you’re doing on departure. Radio calls, navigation, performance monitoring, configuration changes or being agitated by the stick shaker.
My assumption is that the jupmseat guy took a very generous interpretation of the checklist. But there may have been more panic about the trim on the earlier flight. If you compare trim inputs on flight 610 to the previous flight, the pilots on the earlier flight were making much shorter, more rapid inputs than the pilots on flight 610. The plane also pitched down far more dramatically than on flight 610.
The graphs for flight 610 seem to indicate that the pilots thought they had the trim situation under control (and they did right up until they didn't).
https://reports.aviation-safety.net/2018/20181029-0_B38M_PK-...