Chapanis was notably active in improving aviation safety around the time of World War II, although his career covered a wide range of domains and applications.
One of his major contributions was shape coding in the aircraft cockpit. After a series of runway crashes of the Boeing B-17, Chapanis found that certain cockpit controls were confused with each other, due partly to their proximity and similarity of shape. Particularly, the controls for flaps and landing gear were confused, the consequences of which could be severe. Chapanis proposed attaching a wheel to the end of the landing gear control and a triangle to the end of the flaps control, to enable them to be easily distinguished by touch alone. Thereafter for that aircraft there were no further instances of the landing gear being mistakenly raised while the aircraft was still on the ground. This particular shape-coding of cockpit controls is still used today.