B737-800 flight crew reported a track deviation occurred on the BOSSS2 RNAV arrival into den when th...

Narrative:

We were expecting to land runway 35R at den. We had already set up and briefed the ILS 35R CAT III. While descending through FL180 we were told to expect ILS 35L. We then set up and briefed the ILS 35L CAT III. We were on the BOSSS2 RNAV arrival which transitions into the approach. As we passed bosss we received an FMC alert saying the approach and frequency does not match. The airplane started turning right and descending for ILS 35R instead of staying at 12;000 feet like it should have for 35L. We immediately selected altitude hold and heading select and turned back on course. We told denver approach what had happened and asked if we needed to climb back up to 12;000 feet. He said it was no problem and told us to descend and maintain 10;000 feet.while re-briefing the approach we did not catch that the FMS still had the previous approach loaded. A contributing factor was that we are still trying to get familiar with our new checklists and flows. This runway change happened last minute while we were performing our descent checklist and before landing checklist. Denver field elevation is 5;434 feet; so our before landing checklist that begins at 10;000 afl came quickly after passing FL180.when briefing an approach; always make sure the FMS is loaded with the correct approach.

Original NASA ASRS Text

Title: B737-800 flight crew reported a track deviation occurred on the BOSSS2 RNAV arrival into DEN when the FMC was improperly programmed following a late runway change.

Narrative: We were expecting to land runway 35R at DEN. We had already set up and briefed the ILS 35R CAT III. While descending through FL180 we were told to expect ILS 35L. We then set up and briefed the ILS 35L CAT III. We were on the BOSSS2 RNAV arrival which transitions into the approach. As we passed BOSSS we received an FMC alert saying the approach and frequency does not match. The airplane started turning right and descending for ILS 35R instead of staying at 12;000 feet like it should have for 35L. We immediately selected altitude hold and heading select and turned back on course. We told Denver Approach what had happened and asked if we needed to climb back up to 12;000 feet. He said it was no problem and told us to descend and maintain 10;000 feet.While re-briefing the approach we did not catch that the FMS still had the previous approach loaded. A contributing factor was that we are still trying to get familiar with our new checklists and flows. This runway change happened last minute while we were performing our descent checklist and before landing checklist. Denver field elevation is 5;434 feet; so our before landing checklist that begins at 10;000 AFL came quickly after passing FL180.When briefing an approach; always make sure the FMS is loaded with the correct approach.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1631442
Time
Date	201903
Local Time Of Day	0601-1200
Place
Locale Reference	D01.TRACON
State Reference	CO
Aircraft 1
Make Model Name	B737-800
Operating Under FAR Part	Part 121
Flight Phase	Descent
Flight Plan	IFR
Component
Aircraft Component	FMS/FMC
Person 1
Function	Pilot Not Flying First Officer
Qualification	Flight Crew Air Transport Pilot (ATP)
Person 2
Function	Captain Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	ATC Issue All Types Deviation - Procedural Clearance Deviation - Procedural Published Material / Policy Deviation - Track / Heading All Types

B737-800 flight crew reported a track deviation occurred on the BOSSS2 RNAV arrival into den when the FMC was improperly programmed following a late runway change.