Definition
The position of a piston in a reciprocating engine when it has reached the extreme end of its travel in the cylinder and is momentarily stationary before reversing direction. Top dead center (TDC) is the position at the top of the stroke; bottom dead center (BDC) is the position at the bottom of the stroke.
Plain English
The exact point where the piston stops for an instant at the very top or very bottom of its travel before starting back the other way. At that moment, the piston isn't moving up or down -- it's switching direction.
Context Anchor
Seen in aircraft engine maintenance, especially when checking piston position, valve timing, or ignition timing.
Derivation
Dead' here means motionless -- the same sense used in 'dead stop.' At the top and bottom of its travel, the piston is briefly dead (not moving) before reversing. 'Center' refers to the centerline of crank rotation that places the piston at its extreme position. Knowing this helps avoid the assumption that 'dead center' means a midpoint -- it actually means the endpoints of piston travel.
Why Pilots Care
Accurate dead center positioning is required for setting ignition timing and adjusting valves so the engine runs safely and efficiently.
Intuition Check
Dead center does not mean the exact middle of the engine, and it does not mean a failed part. In this context, it means the piston is at one end of its travel and is about to reverse direction.
Example Sentence 1
The mechanic rotated the propeller until the number one piston was at top dead center on the compression stroke before checking magneto timing.
Example Sentence 2
Valve clearances are measured when the piston is at dead center on the compression stroke.