Narrative:

Approaching clarr intersection on clarr 1 arrival into las vegas, leveling 13000 ft, slowing to 250 KTS (assigned), controller told us to descend via the skebr (RNAV) 2 arrival. We informed him we were previously assigned the clarr 1 (as per our flight plan). He then asked if we would mind switching to the skebr 2. We said ok. First officer (the PF) dialed in 7000 ft into the altitude selector window as per company policy, set to lowest of route fix altitudes (kimme at 190 KTS at 7000 ft). When the new arrival was programmed into the FMC, the first fix on the skebr STAR, I (PNF) turned manually to dag VOR, selected 031 degree course to assure proper course, instructed first officer to turn 10 degrees right to get back on course. Controller then instructed us to fly heading 340 degrees. He, at the same time, said 'looks like the computer didn't accept the routing change.' we were preoccupied by the STAR change and difficulty of getting the FMC to go to clarr, even though it was selected as the next 'to' fix. We both noticed altitude was now approaching 11000 ft and responded by leveling at 11050 ft just as controller instructed us to climb and maintain 12000 ft. We said the FMC was not allowing us to select the next fix 'kepec,' so he said try 'ipumy' which the computer accepted. We climbed to 12000 ft and continued the skebr approach. The ATC supervisor advised there was no loss of separation. Once on the ground, I spoke to ATC supervisor. I told her I thought the last min change, approximately 6 mi from clarr, inputting a new arrival confused the computer. Then the company policy to put in the 'lowest' instead of 'next' altitude after passing the fix, both together, resulted in the controller taking us off the previously assigned route, but left the computer without direction vertically. It probably defaulted to 'cruise descent' mode and would have continued to 7000 ft had we not stopped it at 11050 ft. Because it was VMC, full moon, the safety of the flight was not in doubt. Also, an aircraft to our right which we could see departing las was restr below us to either 9500 ft or 10500 ft. Controller advised there was no loss of separation. Because the controller vectored us off course I'm reasonably sure there was no altitude deviation. Our thanks to the controller and his handling of the situation. In retrospect, I would have (and will in the future) refused close in FMC/STAR modifications -- especially on 'hardball' FMC driven HSI. I will, by copy of this report, ask company to look into our procedure of selecting the lowest RNAV STAR altitude instead of passing the fix then selecting lower altitude. I certainly learned from the experience. Supplemental information from acn 598939: we resumed the arrival with radar vectors. I also had been flying for 6 days straight, with 1 day off. I do feel that fatigue had some role here.

Google
 

Original NASA ASRS Text

Title: AN ARRIVING B737 FLT CREW SUFFERS A HDG TRACK DEV AND AN ALT EXCURSION AFTER ATTEMPTING TO PROGRAM AN ARR CHANGE IN THEIR FMS AT THE REQUEST OF APCH CTLR AT L30, NV.

Narrative: APCHING CLARR INTXN ON CLARR 1 ARR INTO LAS VEGAS, LEVELING 13000 FT, SLOWING TO 250 KTS (ASSIGNED), CTLR TOLD US TO DSND VIA THE SKEBR (RNAV) 2 ARR. WE INFORMED HIM WE WERE PREVIOUSLY ASSIGNED THE CLARR 1 (AS PER OUR FLT PLAN). HE THEN ASKED IF WE WOULD MIND SWITCHING TO THE SKEBR 2. WE SAID OK. FO (THE PF) DIALED IN 7000 FT INTO THE ALT SELECTOR WINDOW AS PER COMPANY POLICY, SET TO LOWEST OF RTE FIX ALTS (KIMME AT 190 KTS AT 7000 FT). WHEN THE NEW ARR WAS PROGRAMMED INTO THE FMC, THE FIRST FIX ON THE SKEBR STAR, I (PNF) TURNED MANUALLY TO DAG VOR, SELECTED 031 DEG COURSE TO ASSURE PROPER COURSE, INSTRUCTED FO TO TURN 10 DEGS R TO GET BACK ON COURSE. CTLR THEN INSTRUCTED US TO FLY HDG 340 DEGS. HE, AT THE SAME TIME, SAID 'LOOKS LIKE THE COMPUTER DIDN'T ACCEPT THE ROUTING CHANGE.' WE WERE PREOCCUPIED BY THE STAR CHANGE AND DIFFICULTY OF GETTING THE FMC TO GO TO CLARR, EVEN THOUGH IT WAS SELECTED AS THE NEXT 'TO' FIX. WE BOTH NOTICED ALT WAS NOW APCHING 11000 FT AND RESPONDED BY LEVELING AT 11050 FT JUST AS CTLR INSTRUCTED US TO CLB AND MAINTAIN 12000 FT. WE SAID THE FMC WAS NOT ALLOWING US TO SELECT THE NEXT FIX 'KEPEC,' SO HE SAID TRY 'IPUMY' WHICH THE COMPUTER ACCEPTED. WE CLBED TO 12000 FT AND CONTINUED THE SKEBR APCH. THE ATC SUPVR ADVISED THERE WAS NO LOSS OF SEPARATION. ONCE ON THE GND, I SPOKE TO ATC SUPVR. I TOLD HER I THOUGHT THE LAST MIN CHANGE, APPROX 6 MI FROM CLARR, INPUTTING A NEW ARR CONFUSED THE COMPUTER. THEN THE COMPANY POLICY TO PUT IN THE 'LOWEST' INSTEAD OF 'NEXT' ALT AFTER PASSING THE FIX, BOTH TOGETHER, RESULTED IN THE CTLR TAKING US OFF THE PREVIOUSLY ASSIGNED RTE, BUT LEFT THE COMPUTER WITHOUT DIRECTION VERTLY. IT PROBABLY DEFAULTED TO 'CRUISE DSCNT' MODE AND WOULD HAVE CONTINUED TO 7000 FT HAD WE NOT STOPPED IT AT 11050 FT. BECAUSE IT WAS VMC, FULL MOON, THE SAFETY OF THE FLT WAS NOT IN DOUBT. ALSO, AN ACFT TO OUR R WHICH WE COULD SEE DEPARTING LAS WAS RESTR BELOW US TO EITHER 9500 FT OR 10500 FT. CTLR ADVISED THERE WAS NO LOSS OF SEPARATION. BECAUSE THE CTLR VECTORED US OFF COURSE I'M REASONABLY SURE THERE WAS NO ALTDEV. OUR THANKS TO THE CTLR AND HIS HANDLING OF THE SIT. IN RETROSPECT, I WOULD HAVE (AND WILL IN THE FUTURE) REFUSED CLOSE IN FMC/STAR MODIFICATIONS -- ESPECIALLY ON 'HARDBALL' FMC DRIVEN HSI. I WILL, BY COPY OF THIS RPT, ASK COMPANY TO LOOK INTO OUR PROC OF SELECTING THE LOWEST RNAV STAR ALT INSTEAD OF PASSING THE FIX THEN SELECTING LOWER ALT. I CERTAINLY LEARNED FROM THE EXPERIENCE. SUPPLEMENTAL INFO FROM ACN 598939: WE RESUMED THE ARR WITH RADAR VECTORS. I ALSO HAD BEEN FLYING FOR 6 DAYS STRAIGHT, WITH 1 DAY OFF. I DO FEEL THAT FATIGUE HAD SOME ROLE HERE.

Data retrieved from NASA's ASRS site as of July 2007 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.