The INS (inertial navigation system) mode maintained the plane on lateral course lines between selected flight plan waypoints programmed into the INS computer.When the INS navigation systems were properly programmed with the filed flight plan waypoints, the pilot could turn the autopilot mode selector switch to the INS position and the plane would then automatically track the programmed INS course line, provided the plane was headed in the proper direction and within 7.5 miles (12.1 km) of that course line.If, however, the plane was more than 7.5 miles (12.1 km) from the flight-planned course line when the pilot turned the autopilot mode selector from HEADING to INS, the plane would continue to track the heading selected in HEADING mode as long as the actual position of the plane was more than 7.5 miles (12.1 km) from the programmed INS course line.
Then, when over the Bethel beacon, the flight could start using INS mode to follow the waypoints that make up route Romeo-20 around the coast of the U. 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 7.5 miles (12.1 km) tolerance permitted by the inertial navigation computer.
Whatever the reason, the autopilot remained in the HEADING mode, and the problem was not detected by the crew.
The VOR/LOC mode maintained the plane on a specific course, transmitted from a VOR (VHF omnidirectional range, a type of short-range radio signal transmitted from ground beacons) or Localizer (LOC) beacon selected by the pilot.
The ILS (instrument landing system) mode caused the plane to track both vertical and lateral course beacons, which led to a specific runway selected by the pilot.
The crew received a NOTAM (Notice to Airmen) of this fact, which was not seen as a problem, as the captain could still check his position at the next VORTAC beacon at Bethel, 346 miles (557 km) away. At about 10 minutes after take-off, KAL 007, flying on a heading of 245 degrees, began to deviate to the right (north) of its assigned route to Bethel, and continued to fly on this constant heading for the next five and a half hours.