A B737-300 FMS dropped LNAV and executed a wide turn off the SLC DELTA arrival after being given JAMMN.
Synopsis
A B737-300 FMS dropped LNAV and executed a wide turn off the SLC DELTA arrival after being given JAMMN.
Narrative
As we were arriving on the DELTA Arrival into SLC we were given direct JAMMN cross at 17;000. As we were leveling off we were given; 'Expect Runway 17.' The PF (First Officer) put Runway 17 into the FMC and executed it very quickly. I expected a right turn to on course on the arrival as we approached the fix. However; as the aircraft was at 280 KTS it rolled into 30 degrees of bank and turned well past the heading for the arrival to about 045. I was asking; 'Where are we going?' and verifying the FMC and telling the pilot flying we need to turn back to the left. He stated the LNAV had kicked off and the aircraft was turning back to the south. I did not know if it was trying to hold or return back to JAMMN. I again called for a turn to the north and to return to the arrival. ATC called and asked where we were going and to turn to 310 degrees. We did that and were already turning back through 010 degrees. We had started the turn and I was clearing our flight path and concerned about being too rough in the turn. We had already begun a correction. I feel that the FMC was changed and executed too quickly for the phase of flight (leveling off and turning at a fix); Verify; verify; verify.
Second reporter narrative
It was the second leg of a 5-leg turn. We had an unscheduled plane change at LAX prior to departure and were running slightly late. Communication; Crew coordination and rapport with the Captain were all excellent. He was a Check Airman who was very standardized IAW the FOM and easy to fly with. I was the pilot flying and the flight up to the incident was uneventful. Approximately 2-3 NM from JAMMN we were told by Approach to expect Runway 17. Both the altitude and airspeed restrictions at JAMMN were met and my situational awareness was high. So I called up the arrival page on the CDU and changed the previously selected Runway 16R to 17. I do not recall selecting or accidentally inducing any other changes to the arrival such as STARS; transition; or runways other then changing from 16R to 17. Once selected; I accepted the change via the CDU. The pilot monitoring also did not recall seeing me make any other changes other then the runway. After accepting the change I immediately called up the LEGS page to crosscheck the points for the new runway. I went first to the LEGS page covering the 17 approach and at first glance noted the points were correct. At about this time we were directly over JAMMN and the aircraft began a 30 degree bank turn to the right. I was still looking at the Runway 17 waypoints on the LEGS page but my situational awareness told me something was wrong because I knew the plane should just continue straight ahead to DRAPR. I then went to LEGS page 1 and noted we had just passed JAMMN; there was a discontinuity on the first line and DRAPR was the next point and LNAV was deselected. It seemed like the airplane was either attempting to enter holding at JAMMN or was in a turn to go back to JAMMN which was behind us.We reached a heading of approximately 040 degrees from our original 360 degrees heading before selecting HDG on the control panel and initiating a left turn back to a 310 degrees heading while we sorted out what had occurred. We were VMC the whole time and clearing our flight path. Nothing else was said by Approach for the next 2-3 minutes but then they called and asked us to call when we landed about a possible Pilot Deviation. The rest of the arrival and landing were uneventful. I am unclear on what actually occurred. I know everything was correct prior to the runway change. Something in the process of selecting a new runway entered a Route Discontinuity at our present position and deselected LNAV. Why the plane started a right turn is also unclear. With LNAV deselected the aircraft should have continued straight. We had to manually select HDG to turn the plane back to the left.I'm not sure if I made an error in the process or it was an aircraft malfunction. I tend to believe the former; but don't know what I did to induce the problem. Late runway changes via the CDU on LNAV arrivals are a frequent and normal occurrence at places like LAX; but I've never encountered a problem like today. It does highlight the need to always make sure the pilot monitoring is monitoring any CDU changes. Also to cross check LNAV and the LEGS page immediately after making any changes; especially if close to any waypoints.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.