Air carrier flight crew reported receiving a low altitude alert from ATC due to descending below crossing fix altitude on approach.
Synopsis
Air carrier flight crew reported receiving a low altitude alert from ATC due to descending below crossing fix altitude on approach.
Narrative
We inadvertently descended below the intermediate altitude on the ILS 30L approach into SJC. We were initially given the RAZRR5 star which leads to KLIDE on the ILS 30L approach and this is what we briefed. As we were flying to SJC we were given direct KLIDE which is on the ILS 30L approach. After that we were given direct to BORED then direct KLIDE then direct SJC. We were told to cross BORED at 8000 then descend to 7000. As we were approaching BORED we received a frequency change and NorCal told us to cross BORED at 6000 and cleared for ILS 30L into SJC. We did notice there was a waypoint SWIGS between BORED and KLIDE but because it was a straight line between BORED and KLIDE we did not put this point in. Unfortunately we didnt realize the altitude of at or above 5200 at SWIGS. Passing BORED we set 2700 in for the final approach fix and started a descent to KLIDE at 4000. Passing 5000 the controller called us and told us to climb immediately to 5200. He said he got a low altitude alert. I turned the Autopilot off and climb back to 5200. The lowest altitude I saw was 4800. The rest of the approach was uneventful. After we were on the ground we were told to contact NorCal. In hindsight I realized we should have reloaded the approach from BORED. I was thinking we were going direct to BORED then direct to KLIDE to start the approach. We were rushed with the changes and since it was a straight line between BORED and KLIDE we thought we were good. We didnt recognize the altitude associated with SWIGS. I also feel we should have clarified with the controller that we were starting the approach at BORED instead of KLIDE. We were cleared direct BORED direct KLIDE and the next clearance we received was cross BORED at 6000 cleared for the ILS 30L approach. I know we missed the altitude but I also feel the clearance could have been better. We were never told cleared to BORED for the ILS. If we had known earlier that we would fly the approach from BORED we would have had time to program this in the FMC and verify the altitudes before the approach.
Second reporter narrative
We inadvertently descended below an at or above altitude on the ILS 30L into SJC. An incorrect FMS setup of the approach left a critical at or above altitude out of the legs page as we set up the approach. Here are the specifics: We were filed on the RAZZR 5; STUBL transition into SJC. We anticipated flying the RAZZR to KLIDE on the approach into 30L and briefed this prior to T/D. Once we crossed over the Sierras we were immediately vectored off the arrival. As we approached the terminal area; NORCAL gave us direct to KLIDE. After a frequency change; the next controller gave us direct to BORED; direct KLIDE; direct SJC. He also told us to descend to cross BORED at 8000 then descend to 7000. A few miles from BORED we got another frequency change. The next controller told us to cross BORED at 6000; cleared the ILS 30L. We were rushed at this point. We did notice the intermediate point of SWIGS was not in the nav sequence; but since it was on the same course between BORED and KLIDE; we didnt reload the approach. Unfortunately we failed to notice the at or above altitude of 5200 prior to SWIGS. We stayed in VNAV and set the FAP altitude of 2700 in the Mode Control Panel (MCP). After crossing BORED at 6000; we started a descent to cross KLIDE at or above 4000. Passing 5000; NORCAL called us and told us to immediately climb to 5200. I saw 4800 as the lowest altitude we got to after the Captain disengaged the Autopilot and manually climbed back to 5200. We then realized there was an at or above 5200 at SWIGS. Because our initial clearance before our approach did not include SWIGS; we needed to reload the approach with the BORED transition for SWIGS to be in the nav sequence. The rest of the approach was uneventful and after landing SJC Ground gave us a phone number to call NORCAL. In hindsight; we should have queried NORCAL about our clearance prior to reaching BORED. Since we were initially cleared direct to BORED then KLIDE; we should have asked if we were commencing the approach from BORED and not KLIDE. After getting clarification from NORCAL; we would have had more time to reload the approach and transition so that the routing and altitudes were correct in the FMS. We definitely screwed this up; but an earlier clearance for the approach prior to reaching BORED would have been helpful. It would have given us more time to get the FMS correct and verify the altitudes on the approach plate matched the box prior to reaching BORED.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.