SR71,
Non-RSEP a/c you are correct; full rudder pressure will return with a 45% N1 difference at any altitude.
Quote from Vol 2:
A rudder pressure reducer is connected to the A system hydraulic line upstream of the main rudder PCU. Hydraulic pressure to the rudder is reduced when the airplane climbs above 1000 feet AGL. Hydraulic pressure returns to normal when the airplane descends through 700 feet AGL, or if B hydraulic system depressurizes, or whenever the N1 difference between the left and right engines exceeds 45%.
However for RSEP a/c the only factor is airspeed:
At speeds above approximately 135 kts, both hydraulic system A and B pressure are each reduced within the main PCU by approximately 25% each. This function limits full rudder authority in flight after takeoff and before landing.
Pulling this back to the thread again does Vmca change depending upon your a/c mod status???
It is all very confusing.
