Definition
A specially shaped, rotating cam used in the fuel control unit of a turbine engine that simultaneously schedules fuel flow against two input variables — typically engine speed and compressor inlet conditions (such as pressure or temperature). Its profile varies along both its rotational axis and its length, so the position of a follower riding on the cam represents the correct fuel schedule for any combination of those two inputs.
Plain English
A rotating part inside the fuel control that is shaped in three dimensions instead of just one. As the engine's operating conditions change, the cam turns and slides so that the part touching it always sits at the correct height, which sets the right amount of fuel for those conditions.
Context Anchor
Seen in turbine engine fuel control and fuel metering system descriptions during powerplant maintenance study.
Derivation
Cam comes from the Dutch 'kam,' meaning 'comb' or 'tooth,' referring to a shaped piece that pushes a follower as it rotates. A normal cam has a profile that varies in only one dimension as it spins. A three-dimensional cam adds a second varying dimension along its length, letting one part schedule two inputs at once.
Why Pilots Care
The three-dimensional cam is what allows a turbine fuel control to deliver the correct fuel for any combination of engine speed and inlet conditions, keeping the engine within safe operating limits during acceleration, deceleration, and steady-state running.
Analogy
Think of a small follower riding over a shaped hillside. Different spots on the hillside are different heights, and those height changes tell the mechanism what position to take.
Intuition Check
Do not read three-dimensional cam as just “a cam that has physical thickness.” Here it means a cam whose shaped surface carries more than one control input and helps produce the correct fuel setting.
Example Sentence 1
The three-dimensional cam in the fuel control rotates with engine speed and shifts axially with compressor inlet pressure to set the correct fuel flow.
Example Sentence 2
The three-dimensional cam translated throttle movement and altitude changes into the correct fuel schedule without electronic sensors.