Air carrier flight crew reported receiving a low altitude alert from ATC on descent into BIL.
Synopsis
Air carrier flight crew reported receiving a low altitude alert from ATC on descent into BIL.
Narrative
Flew below a required altitude on YLSTN 3 RNAV STAR into BIL. When departing ZZZ; our release fuel was 15713 and the fuel we actually received was 16160. That in combination with a late addition of 4 passengers put us over our maximum landing weight. FMS initially showed our landing weight as 75;300 but then reduced to 75;150. As we approached BIL; we were cleared by ATC to descend via the YLSTN 3 STAR and the First Officer (FO) set 8000 as the bottom altitude for the STAR. Just prior to beginning descent on the YLSTN 3 STAR; we started the APU to begin burning extra fuel to lower our weight. The FO extended 1/2 speed brake to increase drag and we slowed the aircraft to 230 KIAS in order to lower the landing gear to also help with drag and fuel burn. When the FO was beginning the descent; we did not have the Descend Now prompt on the FMS; so he chose to use vertical speed and began descending at about 1000 fpm. After he set the descent up; we looked at the fuel situation again to discuss what we could do to burn enough to be below maximum landing weight and lost focus on the mandatory 15;000-foot altitude at the YLSTN waypoint. We still had not reached the YLSTN waypoint on the STAR when ATC called and gave us a low altitude alert and said we needed to be at minimum of 13;500 in that sector. At that point we were at 12;700 feet and hadn't yet crossed the YLSTN waypoint; so ATC reminded us that we were required to maintain 15;000 until that point. We raised the landing gear after acknowledging the call and the FO disengaged the autopilot and autothrottles and we climbed back up to 15;700. As we were climbing I noticed the speed brakes deployed and I stowed them. After we were back that the required altitude; the remainder of the arrival and approach into BIL were uneventful. ATC at both Salt Lake Center and Billings Approach asked us if we had any malfunctions and if we required assistance and I replied that we did not.When we came up with our plan to lower the gear to help us burn for fuel; we should have set aside the fuel problem temporarily; rather than continuing to fixate on it. We would have allowed ourselves to focus on flying the STAR correctly and then periodically revisit the fuel status. If we were still unable to get the weight below the maximum landing weight; we could have either asked ATC for holding or for extended vectors to help us to burn enough fuel to get to a proper landing weight. There were two better options that I should have proposed to the FO when he selected vertical speed for the descent. Since the YLSTN waypoint is listed as 15;000 or above; he could have made it a hard altitude and done a vertical direct to that point and then continued down the path of the STAR. The second option would have been to do a vertical direct to the BEARE waypoint; which is the bottom altitude on the STAR and what we had been cleared to fly. Either course of action would have protected the altitude at YLSTN and we would not have descended below that required altitude at that point.
Second reporter narrative
[Narrative contained no additional information.]
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.