As you know; engines, flight controls are located on the wings. Wings produce lift force and stores a certain amount fuel. Engines produce thrust force and provide support to wings for lift force. In the same time pilots control and command aircraft with flight control surfaces.
Faults occuring in these structures highly effects safety of flight.
Crews should control these structures in spesific times. There are black and small triangle sign on the panel in the side of certain seats.
These triangle signs indicates where the wings can best be observed. So these black triangle signs are for visual inspection. Crew can make best observation and control of wings, engines, flight controls from here.
Thus any faults or problems are quikly perceived.
When occurs any trouble in wings, systems transmit a signal as electrically to cockpit. But even if fault is transmitted to cockpit with help of systems, visual inspection is important.
Crew can control the accuracy of the warning. Because system can transmit wrong signal because of a different fault. In this case crew use window marked with a black triangle.
Triangles are providing best reference for inspection and observation important structures of aircraft (wing, engines, flight controls and such)
Location of triangle changes according to type of aircraft.