Air carrier flight crew reported their aircraft descended below the glidepath while on approach.
Synopsis
Air carrier flight crew reported their aircraft descended below the glidepath while on approach.
Narrative
We were on the ZZZZZ1 X for Runway XX to ZZZ; 'descending via'; and then given direct ZZZZZ1. Full automation was engaged; autopilot; autothrottles; LNAV/VNAV. No approach assignment was given at this time. The next ATC instruction was 'direct ZZZZZ; cross ZZZZZ at 4000 ft.; cleared for the RNAV RNP Z Runway XX.' With the approach previously loaded; ZZZZZ was moved under ZZZZZ; from the STAR. A hard 4000 ft.; at ZZZZZ was verified in the FMC; 1500 ft. was set in the altitude window and verified for the crossing restriction at the FAF ZZZZZ2. Full automation was engaged and windows closed. About six miles from ZZZZZ the aircraft began to slow from 240 knots to 210 knots; which seemed early for the speed restriction at ZZZZZ3. My attention was outside; looking for the field; and an aircraft ahead of us. We noticed the aircraft was at 2800 ft.; but we were still four miles from ZZZZZ. The First Officer (OF) immediately began a climb back to 4000 ft. Approach then said they showed us low on the arrival; and would we like the visual to Runway XX; or would we be able to return to 4000 ft. by ZZZZZ. We crossed ZZZZZ at 4000 ft.; re-verified all inputs and automation; and continued with the approach. Following this; the approach; landing; and taxi-in were uneventful; and no request to contact on ATC on landline.
Second reporter narrative
On Date; at approximately (time); we were cleared off the ZZZZZ1 X RNAV Arrival at ZZZ direct to ZZZZZ; with a descent clearance to 4000 ft. After just a few minutes; we were cleared for the RNAV (RNP) Z starting at ZZZZZ and cleared to cross ZZZZZ at or above 4000 ft. I had 4000 ft. on the altitude select window from the initial clearance and once cleared for the approach; I verified and verbalized LNAV VNAV/PATH; set in 1500 ft. for the FAP altitude; and changed the RNP value to 0.3. Around 2700 ft.; I realized my altitude deviation; saw no PATH; VNAV was not selected and the Speed Window was open displaying 240 kts. I announced my discovery and immediately started a climb back up to 4000 ft. ATC called to inquire; just a moment after I started the climb. In my 'mind's eye'; I was on the PATH and the aircraft was slowing for the preprogrammed speed at ZZZZZ: 229 kts. I do recall being outside; searching for the airport; when I returned back inside to see my altitude. I don't recall how long I was 'outside'. I did not announce that I was 'outside' to prompt my Captain to share my mental model. I have no explanation other than complacency and expectation bias that must have convinced me that I was on PATH. Did I make the callouts without confirming the Primary Flight Display and Mode Control Panel while setting in the 1500 and resetting the RNP value? It is hard to accept but I must have. Did I make an entry on the FMS; which caused me to come out of VNAV? I cannot be certain. But; I am certain that there was a poor division of attention at the needed time; which allowed this deviation to occur.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.