Definition
A solid metal fastening pin with a flat, disc-shaped head on one end and a small hole drilled through the opposite end to receive a cotter pin. It is used to join components that need to pivot, hinge, or be quickly disassembled, with the cotter pin acting as the removable retainer that keeps the flathead pin in place.
Plain English
A straight metal pin with a flat top and a small hole near the bottom. You slide it through two parts that need to stay loosely connected, then push a small split pin through the hole at the bottom to stop it falling out.
Context Anchor
Seen in aircraft maintenance when checking or replacing pins in control connections, hinges, or other moving assemblies.
Derivation
Flathead' simply describes the shape of the head -- flat rather than rounded or tapered. The flat head sits flush against the surface and gives the pin something to stop against when pushed through a hole.
Why Pilots Care
Maintains secure, low-profile connections in flight controls and landing gear so components move freely without adding drag or risk of loosening in flight.
Intuition Check
Do not confuse a flathead pin with a flathead screw. A flathead pin is not turned with a screwdriver; it is held in place by a retainer such as a cotter pin.
Example Sentence 1
During preflight, the pilot checked that the flathead pin connecting the elevator pushrod was fully seated and secured with a cotter pin.
Example Sentence 2
During the annual inspection the technician replaced a worn flathead pin in the rudder linkage to restore smooth movement.