nomorecatering
There are actually 4 governors on the PT6A
1. Hydraulic Propeller Governor
2. Hydraulic Propeller Overspeed Governor
3. Pneumatic Propeller Fuel Topping Governor
4. Fuel Control Unit Governor.
They are all interconnected and perform various functions.
The reason for the Hydraulic Propeller Overspeed Governor is primarily for a back up for the failure of the Propeller Governor - It is not field adjustable - has no input from the flight crew and basically sits there like a lump until needed. If needed you will be very glad it was installed. In 40 years of working on the PT6 I have only seen a couple of Propeller Governor complete failures - so system is pretty robust. I would assume that part of the reason for having the overspeed governor has to do with back up protection as required by the pesky regulations set by the authorities to cover as many possible failure modes as they deemed a requirement.
Here is a link to some very good stuff on the PT6 small engine.
File Catalog - FreeBee
I don't know of any small turboprop engines that have electronic Fuel Topping functions but it is a possibility - all you would need would be electronic speed sense for propeller speed - and the computer to tell the fuel control to reduce fuel flow and it should work just fine - unless of course the computer control has been compromised and you are now running on "Manual" . I would think that those pesky authorities would take that into consideration and then require a manual backup for the computer failure. Probably just as easy to just make the one unit a manual one.
Also don't forget that many PT6 engines have/had a "Beta Back-up" system - This was used to prevent the propeller blades from going below the Flight Idle blade angle with the Power Lever at Flight Idle or above. Electric blade position sense plus microswitches adjusted on the Power Lever - activates a "Lock Pitch" solenoid on the propeller governor to move blades towards coarse. This system prevents un-commanded blade movement towards reverse while in flight - which would not be a good thing on the Beech!!!
Hope this helps.