Definition
The flat, imaginary surface in which an applied force acts on a spinning gyroscope. When a force is applied to a gyro, it does not produce a reaction at the point of application; instead, the reaction occurs 90 degrees ahead in the direction of rotation, in a predictable plane relative to the spin axis. This is the principle of precession, and the plane of force is one of the three reference planes (along with the plane of rotation and the plane of precession) used to describe how a gyro responds.
Plain English
The flat surface, like a sheet of glass passing through the gyro, that shows the direction in which a push or pull is being applied to it.
Context Anchor
Seen in explanations of turn indicators, where the handbook describes how a gyro reacts when the aircraft turns.
Derivation
Plane here means a flat two-dimensional surface, from the Latin planum meaning a level surface. Force keeps its everyday meaning of a push or pull. Together, the phrase describes the flat surface along which a push or pull acts on the gyro.
Why Pilots Care
Correct understanding prevents misreading the turn needle during coordinated or uncoordinated flight.
Analogy
Picture pushing on the side of a spinning wheel. The plane of force is the flat direction your push is acting in, not the wheel itself.
Grounding Statement
If you press down on the rim of a spinning gyro at one spot, the plane of force is the flat surface running through that push -- the surface that shows which way the force is being applied.
Intuition Check
Plane does not mean airplane here. It means an imagined flat surface showing the direction in which the force is acting.
Example Sentence 1
When the pilot applies rudder, a force is introduced in one plane, but because of precession the gyro tilts in a different plane.
Example Sentence 2
The instrument is designed so the plane of force aligns with aircraft yaw to produce a visible indication.