Date: 201901
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Light: Dusk
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class B ZZZ
ATC / advisory: TRACON ZZZ
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Human factors: Communication Breakdown; Human-Machine Interface
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Detector: Person Air Traffic Control; Person Flight Crew
When detected: Pre-flight
Result: Flight Crew Became Reoriented; Flight Crew Regained Aircraft Control; Flight Crew Returned To Clearance
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: Just outside the FAF with an altitude intercept of 2000 feet; and a FAF crossing of at or above 1700 feet; the Tower cleared us to land at the same time the VNAV indicator rapidly descended and my FO (First Officer) said 'There is your five-mile point' which I should have been at 1700 feet instead of the 2000 feet I was at. I thought I was fairly high and over responded by going Vertical Speed and blindly entering two scrolls down; which unbeknownst to me resulted in a 4100 fpm descent rate putting us below the crossing altitude of 1700 feet or above. We ended up at just over 1400 feet as I quickly tried to recover manually. Then we received the low altitude alert from the Tower and continued to recover back to the glideslope; landing without further incident. I should have entered 1700 feet in the preselect altitude when cleared to land and then the automation would have worked fine. I also should have visually verified my V/S input; I would never purposefully select 4100 feet; especially that low. My FO also had just entered a five-mile circle without telling me ahead of time; I thought we were at the five-mile point and therefore too high; when he said there is your five-mile point; along with the rapidly descending slope indicator. What my FO was meaning; and just wanting to help; is that he had just entered a five-mile fix circle; not that we were at the five-mile point.


Second narrative: RNAV GPS Runway XX ZZZ; autopilot on. FMC cruised at 2000 feet with speed window open and 180 knots ATC assigned speed. We were level flight; just outside [the FAF]; approximately seven-mile final. Approach Controller said 'Company XXXX; maintain 180 knots until five miles out; call the Tower at [FAF.]' At that point I went heads down to put a five-mile ring on the fix page and switched to the Tower. Without my knowledge the Captain switched from VNAV to Vertical Speed 4100 fpm down to regain what he presumed as a 300 feet high vertical deviation with zero set in the altitude window. We weren't high at all. At that time I the PM; FO (Pilot Monitoring; First Officer); was assessing why we had this situation that was controlled seconds earlier. At this point I stated altitude to the Captain and I could hear the Low Altitude Alert going off in the Controller Tower over the Air Traffic Controller's communications with other aircraft. ATC questioned our altitude and I responded 'CORRECTING.' In seconds (I'd estimate five to seven seconds) we had gone from level at 2000 feet to 1400 feet and 300 feet below the 1700 feet crossing altitude at [the FAF.]Captain perceived situational awareness that he was high. We were not high. Next; we never set -4100 fpm down while at 2000 feet in the terminal area. Lastly verbalize to the PM; FO; in this case; that you are leaving VNAV.


Synopsis: B737 flight crew reported a communications breakdown between Captain and First Officer regarding approach clearance resulted in an altitude deviation and low altitude alert.