flathead rivet. An AN442 rivet used on the inside of an aircraft structure where it is not exposed to the airflow. Flathead rivets are often used because they are easy to install with automatic riveting machines.

flat lacquer (finishing material). A type of lacquer that dries with a flat, or nonglossy, surface. Flat lacquer is used for painting instrument panels and any part of an aircraft that must not reflect light.
flat pattern layout. The pattern for a sheet metal part that has the material used for each flat surface, and for all of the bends marked out with bend-tangent lines drawn between the flats and bend allowances.

flat-rated engine. A turboprop engine whose allowable output power is less than the engine is physically capable of producing. Below a specific density altitude, the engine is limited by the amount of torque the airframe can tolerate. Above this altitude, the engine is limited by the exhaust gas temperature.
flat spin (aircraft maneuver). An aircraft maneuver in which the aircraft is autorotating (being pulled around by aerodynamic forces) as it descends, with one wing flying, and the other actually stalled. In a normal spin, the aircraft is pitched steeply nose down, but since it is descending, it has a very high angle of attack. Recovery is made by moving the control wheel forward (the elevators down). This decreases the angle of attack and allows the stalled wing to begin flying again, and the airplane stops spinning.
In a flat spin, the angle of attack cannot be lowered enough, even with full-down elevators to bring the wing out of its stalled condition. Flat spins are caused by the center of gravity of the aircraft being too far aft, allowing centrifugal force to flatten the spin. Very few aircraft can recover from an unintentional flat spin.
flat washer. A disk of metal with a hole in its center. A flat washer, also called a plain washer, is used under the head of a bolt to provide a bearing surface. It can also be used under a nut to make a surface for the nut to turn against so that it will not dig into the material contacted by the nut.