Abstract: An intrusion detector for passenger aircraft has a portable housing including on board power means, at least one sensor operable to detect activity along a detection beam alignable to the aircraft and an indicator such as a strobe light providing a flashing-light alarm visible through the aircraft windows. The detector is self-sufficient, being carryable onto the aircraft by security personnel, and there activated without exposure to possible tampering. The sensor and/or indicator are armed after a delay allowing the security personnel to depart the detection zone, and activation of the indicator is delayed following detected activity, allowing security personnel to disarm the device by entry of a code, without indicating alarm conditions.