Date: 202110
Local time of day: 0001-0600
Location: ORD.Tower, IL
Flight conditions: IMC
Weather elements / visibility: Thunderstorm; Rain
Aircraft: Medium Large Transport
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Final Approach
Airspace: Class B ORD
ATC / advisory: Tower ORD
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Human factors: Communication Breakdown; Situational Awareness; Workload; Fatigue
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Clearance; Flight Crew Became Reoriented
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: Arriving [early in the morning] in Chicago after an all night flight. Weather in ZZZ was very poor as was weather in Chicago; with lots of turbulence en-route. Descending via Bradford Seven arrival and then vectors to ILS 10C; in heavy rain; thunderstorms; moderate turbulence; lightning and busy ATC communications. On vectors for ILS 10C and in level change descending to 4;000 feet; given clearance to intercept localizer. Continuing to descend to 4;000 feet; given clearance for ILS 10C. MCP altitude was changed without request; verbalization or verification to the marker crossing altitude. While descending; slowing; configuring; and intercepting the localizer in moderate turbulence and heavy rain the MCP altitude change went unnoticed by Pilot Flying (PF). We descended 300 feet below the last crossing restriction on the localizer prior to intercepting the glide slope. After that we intercepted the glide slope and flew a normal approach and landing. Debriefed at the gate about the importance of verbalizing; verifying; and monitoring all changes made to MCP.


Second narrative: Day 2 of a 3 day pairing with an all night flight to ORD with [an early morning] arrival. Weather was IMC on departure and approximately 1 hour of flight was spent in light to moderate turbulence. On arrival into ORD there were gusty winds; rain; broken clouds at 900 feet AGL and visibility of about 3 miles. Vectored onto ILS 10C approach [and] told to intercept the localizer and descend to 4;000 feet. Later we were given approach clearance. Once given the approach clearance the Pilot Monitoring (PM) changed the MCP altitude to the FAF altitude without request from the Pilot Flying (PF) and without verbalizing the change. The change in the MCP window occurred prior to the aircraft capturing the 4;000 feet level off and the FMAs changed to V/S and continued to descend below 4;000 feet. The PF noticed the altitude deviation and intervened by using ALT HOLD. The aircraft ended up deviating 280 feet below the 4;000 foot altitude restriction and captured the glide slope shortly after the PF used ALT HOLD. The rest of the approach and landing were uneventful. PF and PM debriefed what led to the event and what could have been done better after safely parking the aircraft at the gate.


Synopsis: Air carrier flight crew reported descending below charted altitude on approach to ORD in instrument conditions when the First Officer improperly set the MCP altitude.