Air carrier flight crew reported descending below a required altitude on approach and not recognizing it until they were advised by Air Traffic Control.
Synopsis
Air carrier flight crew reported descending below a required altitude on approach and not recognizing it until they were advised by Air Traffic Control.
Narrative
This was my first leg as Pilot Flying since date and second overall for the day. I had vacation and two weeks of leave in between. My flight time over 90 days is low because of leave; COVID and vacation. I flew a bit in the Aircraft Y for currency. I was clearly rusty; a bit tired as I had just gotten back from a very rough weekend. I was emotionally fatigued and not 100% ready to fly; but felt well enough to fly the four-leg west coast shuffle. I did have about a 45 minute nap in crew room before flight. I had commuted in from Reno at about XX25L.We were being vectored around for the RNAV; no RNP due to traffic. I thought I heard a clearance for the approach when he cleared us direct to OKAIN. So I armed 2000 ft. for FAF altitude and LNAV/VNAV. The aircraft started descending from 4000 ft. to 3600 ft. (OKAIN crossing altitude). Then ATC said we were cleared to OKAIN at or above 3800 ft.; cleared approach. We both looked at each other and knew something was definitely misunderstood. Captain queried that our plates show 3600 ft. for OKAIN. ATC said his MVA was 3800 ft. The aircraft had already neared 3600 ft. altitude and we began to climb; but the aircraft was already on top of OKAIN so we elected to let LNAV VNAV continue. We continued the approach.The problem was definitely expectation bias and not fully comprehending the initial ATC instruction; which we think might have been cleared the lateral portion of the approach. I don't remember the verbiage used by ATC in the radio call. Like a robot; I utilized LNAV VNAV thinking we were good to go; and once on final with LNAV VNAV path; we would zero out and go. Clearly there was some rust and expectation bias that surprised us when the final clearance came. I blame myself for not verifying what I heard.
Second reporter narrative
While on a vector into the San Diego terminal area; we were on a vectored downwind and we on era descent. (I don't recall the assigned altitude; at this time. Either 4; or 5 thousand feet.) The First Officer was the Pilot Flying at the time; and the autopilot was engaged. We were given a turn to fly direct to the OKAIN point on the Runway 27 approach; with an assignment to intercept the inbound course on the approach. I was heads down verifying the FMC; and entering the next frequency (Control Tower); when the First Officer; who later said he thought we were cleared for the approach; had entered 2000 ft. in the MCP; and activated the VNAV mode; resulting in a descent. I didn't see him do that; as I was heads down. Moments later; ATC instructed us to cross OKAIN at 3800 ft. and that we cleared for the approach. The published crossing altitude over OKAIN is '3600 ft.'. I looked at our altitude and noticed we were at 3660 ft. I called ATC to verify his altitude assignment over OKAIN; ATC replied with 3800 ft. We were less than a mile from OKAIN and I asked the First Officer why he had descended. He stated that he thought we were previously cleared for the approach.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.