BUt... I am missing something in BBN's rationale; if the power levers are retarded, then the blade pitch will fine until achieving the FFPS, at which point it will result in an under speed condition that is countered by fuel scheduling, ie it becomes a minimum thrust condition. ( it is not uncommon for various TP's to have cases where a rapid power lever closure will result in a momentary increase in thrust at part of the operation). The blade at all times is free to increase pitch without any limitation, up towards feather. The FCU/Prop Gov must indeed be unusual.
Some info on D8 prop O/S governing:
D8 O/S governing is by one unit on each engine hydraulically and pneumatically. The O/S gov will provide O/S governing in constant speed
and in flight beta range. If NP exceeds 1256 rpm, the O/S gov will hydraulically increase blade angle. When NP decreases below the overspeed point, the O/S gov goes back to normal prop governing. If the prop continues to overspeed, the cycle will repeat until the cause is removed. In ground beta range, only the pneumatic section of the O/S gov is available to reduce fuel flow. Pneumatic is also backup O/S governing in flight beta and constant speed mode.
Beta backup and beta lock out protection:
In case of prop control unit malfuntion, and bladeangle goes below 16.5 degrees with PL's above the gate, the beta backup system will operate the feather solenoid on the PCU to increase blade angle. The added Beta Lockout System is not found on all D8's, but will prevent low blade angle if the PL's are inadvertently brought below the gate in flight.. Keep those PL's above the gate in flight!
O/S governing with PL's below the FI gate:
As PL's go below the FI gate, the PCU will isolate the speed governor control and terminate O/S control by the hydraulic section of the O/S gov. This leaves O/S governing to the pneumatic section of the O/S gov, there will also be no beta backup protection.