Definition
The system inside a constant-speed propeller hub that rotates each blade about its long axis to change blade angle, thereby controlling propeller load and engine RPM. In most light aircraft it uses governor-supplied engine oil pressure acting on a piston in the propeller hub, opposed by springs and the aerodynamic and counterweight forces on the blades, to twist the blades to a coarser or finer pitch as commanded by the propeller governor.
Plain English
The mechanical system that physically turns the propeller blades to a steeper or shallower angle while the engine is running, so the propeller can hold the RPM the pilot has set.
Context Anchor
Seen in discussions of constant-speed propellers, propeller overspeed, and failures that prevent the propeller blades from moving to the correct angle.
Derivation
“Pitch” in this use comes from the idea of slope or angle. For a propeller blade, pitch refers to the blade’s angle as it cuts through the air, not the airplane’s nose position.
Why Pilots Care
It prevents engine damage from overspeed and lets the pilot select and hold an efficient RPM across different phases of flight.
Intuition Check
Do not read “pitch” here as the airplane’s nose-up or nose-down attitude. In this term, pitch means the angle of the propeller blades.
Example Sentence 1
When the governor senses the engine slowing below the selected RPM, it directs oil to the blade pitch change mechanism to twist the blades to a finer angle.
Example Sentence 2
A malfunction in the blade pitch change mechanism can allow the blades to flatten and drive the engine into an overspeed condition.