Milde, p53 According to the ICAO, the autopilot was not operating in the INS mode either because the crew did not switch the autopilot to the INS mode (shortly after Cairn Mountain), or they did select the INS mode, but the computer did not transition from INERTIAL NAVIGATION ARMED to INS mode because the aircraft had already deviated off track by more than the 7 miles tolerance permitted by the inertial navigation computer.