Abstract: An apparatus monitors doors on an aircraft. A system monitor has a circuit with an interrogation switch, a receiver, a plurality of red lights and at least one green light source. A plurality of door sensor units each has a housing with a protruding limit switch having a door-open position and a door-closed position. Each limit switch is in a sensor circuit with a battery and a transmitter. The sensor circuit transmits with a unique transmission if the limit switch is in the door-open position. If the system monitor receives a door-open position signal from one of the door sensor units when the interrogation switch is activated, a corresponding one of the red lights is illuminated, and if the system monitor receives all door-closed position signals, the green light is illuminated.