Mental Model - Mode Errors/Interface

Other mode errors are due to poor interface design. To reduce the "clutter" on display and control panels and economize on space requirements, designers may resort to using the same displays and controls for several "modes" of operation. Typically, a push-button or switch is used to change modes. An Air Inter, Airbus A320, crash on approach to the Strasbourg Airport in France was attributed to a mode error. The flight crew intended to set a 3.3 degree flight path angle of descent. Instead, they commanded a 3,300 foot per minute descent. Both the mode for descent in degrees and the mode for descent in vertical speed were set using the same knob. Mode selection was accomplished by a push-button. The same display was also used for both modes. The only difference was that degrees were displayed as "3.3" and feet per minute as "33".

Subsequently, the display was changed so that degrees were displayed in decimal form (3.3) and vertical speed was displayed as a four-digit number (3,300).

Air Intra, Strasbourg France, Crash Attributed to Interface Design Error