Definition
A condition in which all forces and moments acting on the airplane are balanced, producing no acceleration. The airplane maintains a steady state — constant airspeed, constant altitude, and constant attitude — because thrust equals drag and lift equals weight.
Plain English
Everything pushing and pulling on the airplane cancels out, so it keeps doing what it's already doing without speeding up, slowing down, climbing, or descending.
Context Anchor
Seen in constant-speed propeller discussions when the handbook explains how the propeller governor holds a selected RPM.
Derivation
From the Latin aequilibrium — aequus meaning 'equal' and libra meaning 'balance' or 'scales.' It literally means 'equal balance,' which is exactly what is happening to the forces on the airplane.
Why Pilots Care
Loss of equilibrium allows blade angle to change, producing unintended RPM shifts that can affect performance or cause engine overspeed.
Analogy
Like a balanced seesaw that stays level until weight is added to one side.
Intuition Check
Equilibrium does not mean everything is stopped. Here it means the forces are balanced while the engine and propeller are still running, so no RPM correction is needed at that moment.
Example Sentence 1
Once the airplane was trimmed for cruise, it settled into equilibrium and held altitude and airspeed without further input.
Example Sentence 2
A sudden change in airspeed can upset equilibrium, so the governor immediately adjusts blade angle to restore it.