Narrative:

While enroute to las; I attempted to retrieve the ATIS via ACARS approximately 250 NM out. ACARS no comm message was displayed and I was forced to wait until I could receive ATIS via VHF radio. Due to the mountainous terrain northwest of las; I was unable to receive the ATIS until we were on an early descent; due to a bumpy ride at cruise altitude. At approximately FL250 I was able to receive ATIS on VHF while we were on a descent to FL210. While I was off getting the ATIS; the first officer (first officer); who was pilot flying; was inputting the landing runway info in the CDU. As soon as I was back from being off frequency; the first officer started the delayed arrival briefing. Earlier in the flight he had reviewed the sunst 3 fixes on the arrival before we began our initial descent; and all was in check with the sunst arrival. We now were to review the remainder and the approach and landing. We both then noticed that the aircraft was proceeding directly to the sunst fix on the arrival; bypassing bty; mycal; fuzzy; tragr; and ipumy. This deviation put us north of the arrival course. I immediately contacted ATC and requested direct to fuzzy in order to get us back on the arrival. As far as I could tell we were about four to five [miles] north of mycal when I requested direct to fuzzy. We were given 'direct to fuzzy; cross fuzzy at 16;000 feet 250 knots'; [and] the controller said that she showed us north off course. I told her that we were aware we had a navigational error and were correcting it at that time. A few minutes later she gave us a number to call when we landed in las. The remainder of the arrival and landing were uneventful.upon landing I called ATC and the gentlemen I talked to asked if I had flown that arrival often lately. He then told me that there have been several of the same deviations lately on the same arrival. The next day I found out that [another flight to las on a different day] had the same issue and also filed [a report]. Neither the first officer nor I could figure out why; when simply adding the landing runway to the FMC; several of the fixes on the arrival were deleted and the aircraft was proceeding directly to sunst from approximately the bty VOR.

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Original NASA ASRS Text

Title: A B737 Captain reported a track deviation occurred when the FMS dropped some waypoints on SUNST arrival into LAS for unknown reasons.

Narrative: While enroute to LAS; I attempted to retrieve the ATIS via ACARS approximately 250 NM out. ACARS NO COMM message was displayed and I was forced to wait until I could receive ATIS via VHF radio. Due to the mountainous terrain northwest of LAS; I was unable to receive the ATIS until we were on an early descent; due to a bumpy ride at cruise altitude. At approximately FL250 I was able to receive ATIS on VHF while we were on a descent to FL210. While I was off getting the ATIS; the First Officer (FO); who was Pilot Flying; was inputting the landing runway info in the CDU. As soon as I was back from being off frequency; the FO started the delayed arrival briefing. Earlier in the flight he had reviewed the SUNST 3 fixes on the arrival before we began our initial descent; and all was in check with the SUNST arrival. We now were to review the remainder and the approach and landing. We both then noticed that the aircraft was proceeding directly to the SUNST fix on the arrival; bypassing BTY; MYCAL; FUZZY; TRAGR; and IPUMY. This deviation put us north of the arrival course. I immediately contacted ATC and requested direct to FUZZY in order to get us back on the arrival. As far as I could tell we were about four to five [miles] north of MYCAL when I requested direct to FUZZY. We were given 'direct to FUZZY; cross FUZZY at 16;000 feet 250 knots'; [and] the Controller said that she showed us north off course. I told her that we were aware we had a navigational error and were correcting it at that time. A few minutes later she gave us a number to call when we landed in LAS. The remainder of the arrival and landing were uneventful.Upon landing I called ATC and the gentlemen I talked to asked if I had flown that arrival often lately. He then told me that there have been several of the same deviations lately on the same arrival. The next day I found out that [another flight to LAS on a different day] had the same issue and also filed [a report]. Neither the FO nor I could figure out why; when simply adding the landing runway to the FMC; several of the fixes on the arrival were deleted and the aircraft was proceeding directly to SUNST from approximately the BTY VOR.

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.