Definition
In a reciprocating engine, the position of a piston at any given moment as it moves between top dead center and bottom dead center within the cylinder. It describes how far the piston has traveled along its stroke at a particular instant.
Plain English
Where the piston is sitting inside the cylinder at a given moment as it moves up and down.
Context Anchor
Seen in chart plotting, instrument or system descriptions, and maintenance procedures when a position is measured from a starting, neutral, or reference point.
Derivation
From the Latin 'dis-' (apart) and 'placere' (to place), meaning 'moved from its original place.' In engine terms, it refers to how far the piston has been moved from a reference point in its stroke.
Why Pilots Care
A displacement position only makes sense if you know the reference point. Using the wrong reference can put a plotted location, control setting, or system indication in the wrong place.
Grounding Statement
Picture starting at a known point on a chart, then drawing a line showing the distance and direction traveled; the end of that line is the displacement position.
Intuition Check
Do not read displacement position as a special parking spot, gate, or displaced runway threshold. Here, it means a position described by movement away from a reference point.
Example Sentence 1
Ignition timing is set so the spark fires when the piston is at a specific displacement position before top dead center.
Example Sentence 2
During preflight, the pilot verified the aileron displacement position was within limits before engine start.