Date: 201902
Local time of day: 0001-0600
Location: ORD.Airport, IL
Flight conditions: IMC
Weather elements / visibility: 1.5
Aircraft: Widebody Transport
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Final Approach
Airspace: Class B ORD
ATC / advisory: TRACON C90
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness; Communication Breakdown
Second reporter function: First Officer; Pilot Flying
Anomaly: Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Clearance
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: On approach 28C approximately 20 NM final at 5;000 MSL; we were cleared to maintain 5;000 MSL until established clear for the ILS 28C. FO (First Officer) requested 4;000 feet; Captain set 4;000 feet while talking on radio and identifying the ILS. When I checked the step down altitudes; I realized wrong altitude was set but descend was already initiated. The correct step down was to stay at 5;000 feet until RZUKO; then step down to 4;000 MSL to HAZIL; we arrived RZUKO at 4;000 MSL. All SOPs were followed; however an error was made. We stayed at 4;000 MSL until glideslope capture; followed by normal approach and landing. There were no traffic conflicts throughout the approach.


Second narrative: On ILS approach to 28C at O'Hare we descended too early to 4;000 feet. Our clearance was to maintain 5;000 until established on the approach; cleared approach to 28C. Conditions were IMC; 700 OVC; 1.5 miles visibility. The clearance was given on dogleg to final approximately 30 NM from ORD. As flying pilot with the autopilot on; I selected LNAV to intercept final approach instead of LOC. The reason for LNAV was to limit the airplanes tendency to bank back and forth as it searches and tries to maintain the localizer course with LOC selected at this distance (30 NM) from the runway. On the FMC; the fix HAZIL was extended with fixes RZUKO and MEMAW entered into the fix pages. They were also visible on the navigation display. The aircraft successfully intercepted the course as LNAV engaged. Our altitude was 5;000 feet throughout these moments. My error: Looking at the Primary Flight Display; I saw what I thought was a glideslope indicator showing us above glideslope. I asked for confirmation from the other pilots that we were indeed cleared for the approach and received it. So I mistakenly believed I was somehow high on approach. Since the ceiling and vis was relatively low for the arrival; and with upcoming stabilized criteria in mind; I wanted to correct a high on approach situation quickly. I saw HAZIL as the active waypoint on the ND and called for 4;000 feet to be set in the MCP (Mode Control Panel.) It was set and I engaged FLCH. The aircraft descended to 4000'. However; due to my situational unawareness; we were not complying with the stated clearance. We were outside RZUKO and not established on the glideslope and not at 5;000 feet as instructed by ATC. We crossed RZUKO at 4;000 feet; not 5;000 feet. My impression that we were above glideslope was wrong and led to this error. I realized the error shortly before crossing RZUKO; too late to correct. We intercepted the glideslope thereafter and landed normally. There was no communication to or from ATC regarding our error at any time during flight.


Synopsis: Air Carrier flight crew reported descending prior to crossing restriction on approach.