FAA part 25 is readily available in PDF format on the FAA.gov website as is the Advisor Circular AC25-7D (the current issue). I could not find anywhere where the pilots' elevator force is required to overcome full nose down or full nose up trimmable stabilizer force.

The Advisory Circulator did say that with the aircraft stabilizer set at the maximum nose down or nose up position that would not trigger a configuration warning the aircraft should be flyable at the forward and aft CG limits.

The question is then what is the difference between full stabilizer travel and travel required to illuminate the configuration light.

That's just for takeoff. I'm thinking 25.671(c)
.
"to be capable of continued safe flight and landing after any of the following failures or jamming in the flight control system and surfaces (including trim, lift, drag, and feel systems)"

(c)(1)

"(1) Any single failure, excluding jamming (for example, disconnection or failure of mechanical elements, or structural failure of hydraulic components, such as actuators, control spool housing, and valves)."

Also 25.671(c)(3)
"A runaway of a flight control to an adverse position and jam must be accounted for if such runaway and subsequent jamming is not extremely improbable."

Extremely improbable is defined in 25.1309. "they are not anticipated to occur during the entire operational life of all airplanes of one type".

So - airplane must be controllable if a control surface can runaway to a mechanical stop.

Boeing is in a world of hurt if the MCAS is shown to be Part 25 uncompliant.

Boeing has been in that world of hurt for a while. It is in the manual that to recover from a stall you need to apply AND trim and reduce power as the elevator doesn't have enough authority.

That's a violation of pretty basic Part 25 stuff. The lawsuits will be interesting!

"A runaway of a flight control to an adverse position and jam must be accounted for if such runaway and subsequent jamming is not extremely improbable."

So basically we do not know if the 737 was designed to be controllable in the instance of full runaway stabilizer travel or not. The instance of the crash might suggest not.

Extremely improbable means that it is never expected to happen to the whole fleet, ever.
If a simple AOA sensor failure makes it happen and there's no compensation method for it, this will be nasty, NASTY, for Boeing.
I would say this failure mechanism is "probable" according to the guidance in 25.1309.

I don't think you're reading that correctly. The maximum out-of-trim condition that must be correctable with the elevator is that resulting from 3 seconds of trim operation. This is *way* beyond 3 seconds.

The stab wins, it always does. I don't know of any modern jet where the elevator can overcome the stab in worst case scenarios. This should be pretty well covered in groundschool...

If the elevator always won, we wouldn't need the cutout switches.

25.671

""A runaway of a flight control to an adverse position and jam must be accounted for if such runaway and subsequent jamming is not extremely improbable"

I’m going to take a stab at it and say this crew didn’t fully understand what was going on systems wise.
Previous crews did...or got lucky.

Either way, boeing should not have put them in that position.