Date: 202108
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: B757-200
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Final Approach
Airspace: Class B ZZZ
ATC / advisory: Tower ZZZ
Aircraft component: Autopilot
Component problem: Improperly Operated
Reporter organization: Air Carrier
Reporter function: Pilot Flying; First Officer
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
Human factors: Confusion; Human-Machine Interface; Situational Awareness
Second reporter function: Pilot Flying; Captain
Anomaly: Aircraft Equipment Problem Less Severe; 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: Automation Air Traffic Control; Person Flight Crew; Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Took Evasive Action; Flight Crew Overcame Equipment Problem; Flight Crew Became Reoriented
Contributing factors: Aircraft; Procedure; Human Factors
Primary problem: Aircraft

Narrative: Assigned ZZZZZ Visual to Runway XX at ZZZ. Briefed use of VNAV in accordance with Approach Briefing Guide. Prior to ZZZ1 VOR; instructed to descend from 4;000 to 3;000 ft. MSL and FLCH was utilized. Upon crossing ZZZ1; pilot not flying; while also saturated with LCA (Line Check Airman) duties; set 1;500 ft. for ZZZZZ1 and then touchdown zone elevation while slowing and configuring aircraft. We did not notice FLCH was still engaged until ATC provided an altitude alert at 900 ft. MSL. Pilot not flying/LCA took command of aircraft and performed a momentary level-off before giving aircraft back to original pilot flying for a stabilized approach and landing.


Second narrative: Pilot not flying is an LCA (Line Check Airman) conducting an Initial Operating Experience flight for pilot flying; who is a transition First Officer from the other aircraft builder.Assigned to fly the ZZZZZ Visual Approach to Runway XX at ZZZ. Installed the RNAV V XX procedure in FMC and briefed RNAV/visual approach procedures. ATC cleared us direct ZZZ1 and cross ZZZ1 at 3;000 ft. LNAV was engaged to fly direct ZZZ1 and FLCH was engaged to descend to 3;000 ft. Crossing ZZZ1; pilot not flying entered 1;500 ft. in MCP altitude window for ZZZZZ1 crossing altitude. While descending on profile; aircraft was slowing to 180 kt. and configuring for landing. PM was tracking the configuration changes and verifying on course with LNAV; and then selected TDZE after passing ZZZZZ2 intersection. Pilot flying was having difficulty finding the runway which was out his right window behind [of] his shoulder line. Pilot not flying looked outside to acquire runway to direct pilot flying to look in the right direction. At that moment; ZZZ Tower Controller called 'Altitude Alert' for our flight. Pilot not flying looked inside and noticed altitude was 900 ft. crossing ZZZZZ1; below the 1;500-ft. altitude that was programmed in the FMC. Pilot not flying then also noticed that VNAV was not engaged but the aircraft was descending in FLCH; so was therefore not on the VNAV descent path. Pilot not flying took control of the aircraft; leveled the descent; and continued to follow the LNAV course to line up with Runway XX. Once aligned and in proper position to land; pilot not flying gave control of aircraft back to pilot flying; who conducted a stable visual approach to a safe landing.


Synopsis: B757 flight crew reported an incorrect selection on the auto-flight system control panel resulted in a descent below a crossing altitude during final approach and an altitude alert from ATC.