Definition
The position of a piston when it has reached the highest point of its travel in the cylinder, with the connecting rod and crankshaft aligned so the piston cannot move any higher. It is a fixed reference point used to time ignition events, valve openings, and other engine functions in a reciprocating engine.
Plain English
The exact moment when a piston is at the very top of its stroke and can't go any higher before it starts moving back down.
Context Anchor
Seen in engine maintenance when setting or checking ignition timing, valve timing, and piston position.
Derivation
"Dead" here is an old mechanical term meaning "motionless" — at the top of the stroke the piston momentarily stops before reversing direction. "Top" distinguishes it from Bottom Dead Center (BDC), the lowest point of piston travel.
Why Pilots Care
Accurate location of top dead center is required for correct ignition and valve timing, which directly affects engine performance and prevents damage.
Intuition Check
“Dead” does not mean the engine has stopped, and “center” does not mean the middle of the cylinder. TDC is an end position of the piston, at the cylinder-head end of its travel.
Example Sentence 1
The mechanic rotated the propeller by hand until the number one cylinder reached top dead center on the compression stroke before timing the magnetos.
Example Sentence 2
Valve clearance is measured when the piston is at top dead center on the compression stroke for that cylinder.