Narrative:

While en route from lax to las, ZLA cleared us to descend via the skebr RNAV 1 arrival landing runway 25L into las. My first officer, the PF, set the lowest altitude constraint on the arrival, 7000 ft, into the altitude window. This is the common practice at our airline. Just outside clarr intersection, we were xferred over to las approach. We reported on with las approach control 'descending through 12300 ft and cleared to descend via the skebr arrival.' the approach controller again cleared us to descend via the skebr arrival, except cross povac at or above 9000 ft. At this point, we had already crossed clarr at 12000 ft and the aircraft had already begun a descent to cross ipumy at or above 11000 ft. My first officer entered the new crossing restr, povac at or above 9000 ft. My first officer then activated the approach phase on our FMC. In the approach phase, the computer would want to slow the aircraft toward final approach speed. The pilot normally selects a manual speed before activating the approach phase, then when established on fonak, the pilot selects managed speed and the aircraft slows to the approach speed. This is exactly what my first officer had done. This would have been the normal procedure during most arrs had we not had further speed restrs on our RNAV procedure. However, the skebr arrival had 2 additional speed restrs. My first officer noted that he had activated the approach mode prematurely, but he could manually control the speed for the remainder of the procedure. I took this to mean he would rather be back in the descent phase. I should have let him manually control the speed. Instead, as we were leveled at 11000 ft for ipumy, I entered 11000 ft as our new cruise altitude to reactivate the descent mode. Doing that, however, had a negative effect as this FMC deletes any constraints ahead at or above the new cruise altitude (ie, ipumy at or above 10000 ft). I must have received a message that a constraint ahead was being removed, but I disregarded its meaning. The aircraft began to descend to the next restr, povac at or above 9000 ft. Having never flown this arrival before, I must have assumed we had already passed the 11000 ft restr, but I ws also concerned at how low we were at that point on the arrival. The WX was clear with unrestr visibility. The controller then inquired about our altitude and pointed out our error. At this point we were still above the MVA. Approach issued a new clearance direct bld VOR and we were given a new altitude. I called the TRACON on the ground. The TRACON supervisor said he had talked to the controller and would not pursue any enforcement action. He said we were not the first, just be more careful in the future. During this event, I made a number of errors: 1) not spending more time reviewing an unfamiliar type of arrival, 2) making changes to the FMC when it was not my leg, 3) skipping over messages without taking the time to comprehend their meaning, and 4) not verifying constraints ahead after making a change. I think in the future, when making changes to altitude constraints while actively flying a managed descent arrival, I will put the next constraint in the altitude window, manually flying the subsequent altitude constraints, until all the changes are made and verified in the FMC, and then -- and only then -- will I continue to fly the arrival as a managed descent.

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Original NASA ASRS Text

Title: A319 CREW DSNDED BELOW THE MINIMUM ALT LISTED ON THE SKEBR 1 STAR INTO LAS.

Narrative: WHILE ENRTE FROM LAX TO LAS, ZLA CLRED US TO DSND VIA THE SKEBR RNAV 1 ARR LNDG RWY 25L INTO LAS. MY FO, THE PF, SET THE LOWEST ALT CONSTRAINT ON THE ARR, 7000 FT, INTO THE ALT WINDOW. THIS IS THE COMMON PRACTICE AT OUR AIRLINE. JUST OUTSIDE CLARR INTXN, WE WERE XFERRED OVER TO LAS APCH. WE RPTED ON WITH LAS APCH CTL 'DSNDING THROUGH 12300 FT AND CLRED TO DSND VIA THE SKEBR ARR.' THE APCH CTLR AGAIN CLRED US TO DSND VIA THE SKEBR ARR, EXCEPT CROSS POVAC AT OR ABOVE 9000 FT. AT THIS POINT, WE HAD ALREADY CROSSED CLARR AT 12000 FT AND THE ACFT HAD ALREADY BEGUN A DSCNT TO CROSS IPUMY AT OR ABOVE 11000 FT. MY FO ENTERED THE NEW XING RESTR, POVAC AT OR ABOVE 9000 FT. MY FO THEN ACTIVATED THE APCH PHASE ON OUR FMC. IN THE APCH PHASE, THE COMPUTER WOULD WANT TO SLOW THE ACFT TOWARD FINAL APCH SPD. THE PLT NORMALLY SELECTS A MANUAL SPD BEFORE ACTIVATING THE APCH PHASE, THEN WHEN ESTABLISHED ON FONAK, THE PLT SELECTS MANAGED SPD AND THE ACFT SLOWS TO THE APCH SPD. THIS IS EXACTLY WHAT MY FO HAD DONE. THIS WOULD HAVE BEEN THE NORMAL PROC DURING MOST ARRS HAD WE NOT HAD FURTHER SPD RESTRS ON OUR RNAV PROC. HOWEVER, THE SKEBR ARR HAD 2 ADDITIONAL SPD RESTRS. MY FO NOTED THAT HE HAD ACTIVATED THE APCH MODE PREMATURELY, BUT HE COULD MANUALLY CTL THE SPD FOR THE REMAINDER OF THE PROC. I TOOK THIS TO MEAN HE WOULD RATHER BE BACK IN THE DSCNT PHASE. I SHOULD HAVE LET HIM MANUALLY CTL THE SPD. INSTEAD, AS WE WERE LEVELED AT 11000 FT FOR IPUMY, I ENTERED 11000 FT AS OUR NEW CRUISE ALT TO REACTIVATE THE DSCNT MODE. DOING THAT, HOWEVER, HAD A NEGATIVE EFFECT AS THIS FMC DELETES ANY CONSTRAINTS AHEAD AT OR ABOVE THE NEW CRUISE ALT (IE, IPUMY AT OR ABOVE 10000 FT). I MUST HAVE RECEIVED A MESSAGE THAT A CONSTRAINT AHEAD WAS BEING REMOVED, BUT I DISREGARDED ITS MEANING. THE ACFT BEGAN TO DSND TO THE NEXT RESTR, POVAC AT OR ABOVE 9000 FT. HAVING NEVER FLOWN THIS ARR BEFORE, I MUST HAVE ASSUMED WE HAD ALREADY PASSED THE 11000 FT RESTR, BUT I WS ALSO CONCERNED AT HOW LOW WE WERE AT THAT POINT ON THE ARR. THE WX WAS CLR WITH UNRESTR VISIBILITY. THE CTLR THEN INQUIRED ABOUT OUR ALT AND POINTED OUT OUR ERROR. AT THIS POINT WE WERE STILL ABOVE THE MVA. APCH ISSUED A NEW CLRNC DIRECT BLD VOR AND WE WERE GIVEN A NEW ALT. I CALLED THE TRACON ON THE GND. THE TRACON SUPVR SAID HE HAD TALKED TO THE CTLR AND WOULD NOT PURSUE ANY ENFORCEMENT ACTION. HE SAID WE WERE NOT THE FIRST, JUST BE MORE CAREFUL IN THE FUTURE. DURING THIS EVENT, I MADE A NUMBER OF ERRORS: 1) NOT SPENDING MORE TIME REVIEWING AN UNFAMILIAR TYPE OF ARR, 2) MAKING CHANGES TO THE FMC WHEN IT WAS NOT MY LEG, 3) SKIPPING OVER MESSAGES WITHOUT TAKING THE TIME TO COMPREHEND THEIR MEANING, AND 4) NOT VERIFYING CONSTRAINTS AHEAD AFTER MAKING A CHANGE. I THINK IN THE FUTURE, WHEN MAKING CHANGES TO ALT CONSTRAINTS WHILE ACTIVELY FLYING A MANAGED DSCNT ARR, I WILL PUT THE NEXT CONSTRAINT IN THE ALT WINDOW, MANUALLY FLYING THE SUBSEQUENT ALT CONSTRAINTS, UNTIL ALL THE CHANGES ARE MADE AND VERIFIED IN THE FMC, AND THEN -- AND ONLY THEN -- WILL I CONTINUE TO FLY THE ARR AS A MANAGED DSCNT.

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.