I f you often luxuriate yourself basking in the glow of the offerings of the premium class cabin, you might not have noticed those black triangles above the windows because they are located in the economy class cabin of many airplanes.
…So Why Are Black Triangles Above the Windows?
There are typically two sets of triangles found above two windows on each side of the airplane inside of the economy class cabins of airplanes manufactured by Airbus: the ones closer to the front designate the location of the window from which the leading edges of the wings of the aircraft and its slats are best viewed; while the ones several rows back designate the location of the window from which the trailing edges of the wings of the aircraft and its flaps are best viewed. The positions of the slats and flaps — as well as their datum marks and numbers — can be checked from inside of the airplane through these windows by members of the flight crew in the unlikely event during which a problem may occur.
The topic was raised during a recent flight by a passenger seated aboard an airplane; and the explanation I received was from multiple sources. I personally have not seen those black triangles in airplanes manufactured by The Boeing Company.
My understanding is that flight attendants typically check through these windows discreetly if there is a problem or issue under the instructions of the captain of the aircraft and report the findings of what they see back to the pilots — supposedly so as not to cause concern amongst passengers who might become nervous if they notice a pilot conducting an inspection of a wing during a flight.