Definition
The engine/propeller speed at which the propeller governor becomes active and begins automatically adjusting blade pitch to hold a selected rpm. Below this speed the blades remain on their low-pitch (fine-pitch) stops and rpm changes directly with power; at or above this speed the governor takes over and maintains the pilot-selected rpm by changing blade angle.
Plain English
It is the rpm at which the propeller's automatic speed control kicks in. Below it, the blades are flat and rpm rises and falls with power. At or above it, the system holds the rpm steady by twisting the blades.
Context Anchor
Seen in turboprop engine operation, especially when learning how a split-shaft or free turbine engine controls propeller speed separately from engine gas-generator speed.
Derivation
Governing' comes from the Latin gubernare, meaning to steer or control. A governor is a device that controls speed automatically. 'Propeller governing rpm' is therefore the rpm at which that control device starts steering propeller speed.
Why Pilots Care
Allows the pilot to select an efficient propeller speed that remains constant during power changes, preventing overspeed damage and optimizing thrust.
Grounding Statement
In a free turbine airplane, the propeller can be kept at a selected rpm even while the engine section that produces the hot gas is running at a different speed.
Intuition Check
Governing does not mean a rule written in a manual here. It means actively controlling the propeller speed to hold a selected rpm.
Example Sentence 1
As the power lever is advanced for takeoff, engine speed rises until it reaches propeller governing rpm, after which the governor holds rpm steady and further power increases show up as more torque.
Example Sentence 2
Even with a power reduction the propeller governing RPM stayed at 1,900 because the governor adjusted blade angle automatically.