Why are airplane windows rounded?

Answer: Distributes stress around edges

Distributes stress around edges ✓ — Correct! Early planes had square windows, but several crashed from metal fatigue cracks at the sharp window corners. At altitude, the cabin is pressurized (8-11 psi higher than outside). Square corners create stress concentration points where cracks start. Rounded windows distribute stress evenly around the perimeter, eliminating stress concentration. This simple design change prevented catastrophic failures.

Cheaper to manufacture — Wrong. Rounded windows are actually more expensive to manufacture than square ones (harder to cut and seal). They're used because they're safer—square corners create stress concentration points where cracks initiate under pressure differential. At altitude, cabin pressure is 8-11 psi higher than outside. Rounded shapes distribute this stress evenly, preventing dangerous metal fatigue cracks. Safety justifies the extra cost.

Prevents rain from entering — Wrong. Rain prevention isn't a concern (most rain occurs at low altitude where planes fly through it). Rounded windows are critical for safety: cabin pressurization creates 8-11 psi pressure difference at altitude. Square windows concentrate stress at corners, causing metal fatigue cracks (several early jets crashed from this). Rounded windows distribute stress evenly, eliminating crack initiation points. It's about pressure vessel integrity, not weather.