JT, #4634,
Re fitting “
a variable downspring in the circuit”, this appears to be the underlying theory of MCAS - electronic ‘down spring’. However the implementation differs between aircraft with ‘conventional’ stick-trim relationships, and those with trimmable tailplanes (737).
Adding a downspring in a stick-elevator-trimtab system results in the force being applied directly to the stick - pilot feel, aircraft ‘stability’.
With the 737 trim configuration the ‘spring’ force is applied to the tail (HStab), which moves the aircraft, then requiring pilot input and change of feel. Although this can be described as backward action, aircraft with these trim systems appear normal to the pilot, they are controlled and fly like any other aircraft.
All is well until the trim system misbehaves, a powerful tail force pitches the aircraft which can overcome the elevator power generating high stick forces.
The trim differences/effects are considered in
MAX’s Return Delayed by FAA Reevaluation of 737 Safety Procedures see the link to the accident report
and
https://www.skybrary.aero/bookshelf/books/2627.pdf
There are few examples of spring fixes in aircraft with a trimmable tail; those which have involve high integrity installations.
There may be options to fix the aerodynamics - with difficulty. I recall seeing a down tab on the trailing edge of the MD90 / B717 engine pylon; are these in service - probably related to stall recovery.