Originally Posted by
MX Trainer
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.
Thanks, info similar to above on the fuel topping governor would be nice as well.
Originally Posted by
MX Trainer
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!!!
The King Air and Beech 99 I flew did not have the Beta back up system like the Twin Otter, I wonder why?
Also in your info on various PT-6 engines....Confirm that the early models did not have reverse capability. Never heard of that before.