For each airport there is a parameter TAXITIME . A flight without a slot if supposed to take off at EOBT+TAXITIME +-15 MIN . If it does not make it then a FLS message is sent. TAXITIME is set by ATC and does not concern the operators. An AO at a busy airport will file its plan with a EOBT which indicates that the plane is ready . CFMU computes the actual takeoff time taking in account TAXITIME. The delay message is only concerned with EOBT , not with what happens after startup.