Definition
A propeller whose blade angle (pitch) can be changed to suit different phases of flight. The term covers two distinct designs: ground-adjustable propellers, where the blade angle is set by a mechanic on the ground and remains fixed in flight, and controllable-pitch propellers, where the pilot can change the blade angle in flight using a cockpit control. In current usage, 'adjustable-pitch propeller' most often refers to a controllable-pitch system, including the constant-speed propeller found on most complex aircraft.
Plain English
A propeller whose blades can be rotated to a different angle, either on the ground by a mechanic or in flight by the pilot, so the propeller works efficiently at different speeds and power settings.
Context Anchor
Seen in propeller and aircraft performance discussions, especially when comparing fixed-pitch, adjustable-pitch, and constant-speed propellers.
Derivation
Pitch' here borrows from the idea of how far a screw advances in one turn. A propeller blade acts like a rotating screw cutting through the air, and its 'pitch' is the angle that determines how big a 'bite' of air it takes per revolution. 'Adjustable' simply means that bite can be changed, rather than being fixed at one setting for the life of the propeller.
Why Pilots Care
Allows selection of the most efficient blade angle for each phase of flight, improving performance, engine life, and fuel economy compared with fixed-pitch designs.
Analogy
Like the gears on a bicycle. A low gear gives lots of pulling power for starting off and climbing hills; a high gear lets you cover ground efficiently on the flat. A fixed-pitch propeller is a single-speed bike — fine for one situation, compromised in others.
Intuition Check
Adjustable does not mean the pilot can always adjust it in flight. Pitch here does not mean sound; it means the blade angle and the forward-driving effect of each propeller turn.
Example Sentence 1
Because the trainer was fitted with an adjustable-pitch propeller, the student had to learn to set the propeller control as well as the throttle during the climb.
Example Sentence 2
Once level in cruise, the pilot increased the pitch on the adjustable-pitch propeller to reduce engine speed.