B767-300 and 300 ER
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 133; Flight Crew Total 12700; Flight Crew Type 701
Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued New Clearance; Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Aircraft; Environment - Non Weather Related
We were given a 6 NM right of track offset; which we performed. The aircraft was right where it was supposed to be. We subsequently were changing our speed; when [ATC] asked if we were maintaining the offset. The captain and I were both surprised and a quick check showed that we were indeed slowly straying off the offset. I recall seeing about a 1 NM deviation at the time. The Captain corrected back to the offset; and we notified ATC that we were on the offset. We were really puzzled by what happened in this event. The only two things I can think of [are]:1.The aircraft was in a turn while the FMC was still trying to compute the correction to the adjusted airspeed; therefore; the turn was not applied correctly.2. The possibility of GPS jamming. There were increased threats being made from North Korea to South Korea. We were aware of numerous accounts of GPS jamming according to the NOTAMS. The aircraft continued to operate well after the event. I do not believe we were more than a mile off the offset. I could be wrong; but that is what I remember seeing. [ATC] caught the deviation early; and it was a bit difficult initially to see that we were deviating since we were in a turn already. I believe we were both very alert and attentive to what the aircraft was doing; even though at the time we were engaged in changing and checking the airspeed of the aircraft via the FMC.
We were given an offset of 6 NM right [(6R)] by [Air Traffic] Control and we executed it. The aircraft was tracking 6R in LNAV as programmed. As we approached DONVO; the aircraft properly made the turn but instead of maintaining 6R in LNAV; It began to slowly navigate towards the next active waypoint; SANKO. We did not catch it. The controller asked us if we were maintaining 6R and we responded 'yes'. We looked at the map and crosstrack error and noticed we were; in fact; very slowly tracking towards SANKO (not the usual aggressive turn when you delete a large offset; such as 6R). The MAP display still showed the 6R magenta line. I immediately went to heading select and attempted to reestablish the 6R offset. The controller then gave us direct ANRAT and all seemed well for the remainder of the flight; including subsequent offsets. My First Officer and I were very puzzled as I have never seen this in my years on the B-767. We; of course; then began a discussion as to what may have happened. We concluded one of two things. We had just increased the Mach from .77 to .80 around DONVO and possibly (never have seen this before); the FMC was unable to calculate the rate of turn and was trying to compensate for the turn radius. Secondly; possible GPS jamming out of Seoul (28 minutes into flight and it was in the NOTAMS). We had no EICAS messages or other warnings. I did not write it up because I did not believe it to be an airplane discrepancy. It worked as advertised the rest of this flight and the subsequent flight. In the future; I would monitor the crosscheck and MAP display more closely; especially in areas of heightened awareness.
B767 flight crew experienced a track deviation in Chinese airspace approach DONVO; possibly due to GPS jamming. ATC detects the error while it is still minor and the anomaly does not reoccur during the remainder of the flight.