Abstract: An aircraft monitoring system comprising a computer system and a receiver for receiving tracking data from a tracking device associated with an aircraft, the monitoring system having a passive tracking mode in which the computer system is configured to receive tracking data and an active tracking mode in which the computer system is configured to determine a condition indicative of an aircraft incident based on the absence of received tracking data. The monitoring system is automatically triggered into the active tracking mode when a signal is received from the aircraft indicating the aircraft has taken-off.