Date: 201509
Local time of day: 1201-1800
Location: SLC.Airport, UT
Flight conditions: VMC
Light: Daylight
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class B SLC
ATC / advisory: TRACON S56
Aircraft component: FMS/FMC
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; First Officer
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Training / Qualification; Confusion
Second reporter function: Pilot Flying; Captain
Anomaly: Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Aircraft; Human Factors; Procedure
Primary problem: Human Factors

Narrative: While over SLAGG waypoint; on the WAATS 3 Arrival; landing south at SLC we had 16R selected and were on LNAV/VNAV PATH.  Approach told us to switch to 16L.  I told the Captain who was PF I would put in the new runway.  This caused a disconnect with the autopilot and put us in CWS Steering mode.  I asked for vectors since we were no longer on the STAR and heading east when we should have been turning north on the STAR.  We were given vectors and then realized by putting in a new runway that caused the route discontinuity.  Luckily this happened during VMC conditions. Wait to put in new runway while on a STAR.


Second narrative: Upon check-in with ATC on WAATS 3 RNAV; we were given the arrival to 16R but told that 16L may become available. We were ready for the change and briefed both possibilities. WX was clear VFR so either approach would be acceptable. Inside of DUGGY we were given a change to 16L. This should have been no problem but when the FO selected and executed; for some reason the LNAV disconnected and continued on last heading. I recognized it fairly quickly and went to manual control and turned downwind prior to the airport to a heading of 340. We immediately asked for vectors due to the lack of guidance on the LNAV. I did not watch closely as the FO selected; other than to acknowledge the runway change; but he is a fairly seasoned FO and don't believe that it was a selection error. I believe this was more of a glitch in the actual program for this arrival. During the reselection it should have placed a discontinuity at the end of the WAATS Arrival and simply changed the runway. We had not selected any direct entry or closed the discontinuity from the previous selection. While it did not lead to a gross error mostly due to the fact that it was VFR; had we been IFR this would have been a serious problem; and our recognition of it would have taken much longer.To be clear I believe this to be a programming error associated with the LNAV and this particular approach. During the reselection; as the Pilot Flying; I should have been looking much closer prior to selection. However; we are still unclear as to why the LNAV disconnected before the end of the WAATS Arrival despite the reselection.


Synopsis: B737 flight crew experienced a route discontinuity when the runway is changed from 16R to 16L during the WAATS3 RNAV to SLC. The aircraft goes to heading mode and vectors are requested from ATC.