@gulliBell
AP2 is powered off the essential bus, as are thestick trims. Interruption of essential bus power the collective stick trimsystem no longer holds the collective in the set position and it will probablylower slightly. This is consistent with observation of the FP reporting to theCaptain
This is not consistent with your conclusion of loss of the force trim. PF’s statement is “the collective being heady and the power dropping anytime he pulled to maintain power” and this doesn't support your conclusion that the case is the lost of the Force Trim.
The most likely scenario is the jamming or the partial jamming of one rod of flight controls. If one of the rods is jammed then the whole rod line is jammed. The jamming could be located in to the Mixing Unit, or before or after the Mixing Unit. If the jamming is in somewhere in the rod line of the collective then the collective movements are transferred via Mixing unit forward. Because of the collective rod line in not able to move (due to jamming) then the whole movement of PF’s collective will be transferred to be movements of the other freely moving flight control rods (collective rod line stucked or partial stucked).
If the case is that the “the collective rod is jammed” (doesn’t matter where) but still the collective is able to move even its feels very heavy (“The PF called my attention to the collective being heavy and the power dropping anytime he pulled to maintain power”). In this case the all of the collective movements has been transferred only to the Mixing unit’s compensating rods. Then the all movements of collective rod have been transferred in to Mixing Unit’s only the freely moving rods. These only freely moving rods will transfer PF’s control inputs incorrectly. This has the most likely caused the loss of the controllability of the helicopter.
It doesn't matter which "control rod line is affected" the Mixing Unit behaves like in the scenario of jammed collective rod. In every case the jamming on of the control rod line is "really mixing helicopter's steering and it will make the helicopter's flight controls to be illogical. This scenario will be the most likely and it will lead to the loss of the helicopter controllability or partial controllability and to the situation where the helicopter is unflyable.
@gulliBell
...simultaneous AP failure is about as likely as both engines failing at the same time.
I agree that! It is possible to lose both of the AP's at same time. It will be very likely in a case where e.g. the flight controls are jammed or partial jammed. In this situation where the helicopter AP computers / electrical line actuators are not able to maintain a helicopter's stability or AP computers commands to the line actuators has lost. If this is the situation then the AP1 and AP2 will be kicked off immediately (ATT and SAS modes).
I assume that the "loss of the autopilots" means in this case that they have lost the both of ATT and SAS modes?
These both scenarios are logical, and it will not be a surprise to for certain S76 accident investigators of accidents which have occuered in tha past.