Date: 201903
Local time of day: 1801-2400
Location: BWI.Airport, MD
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class E ZDC
ATC / advisory: TRACON PCT
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Communication Breakdown; Situational Awareness; Troubleshooting; Workload
Anomaly: ATC Issue All Types; Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance
Detector: Person Flight Crew; Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: We were descending via the ANTHM3 landing Runway 10 at BWI. The FMC was programmed correctly; autopilot 2 was engaged and the aircraft was managing the STAR as published. Event began approximately 16;000 feet outside of EAGLL in a managed descent. The ANTHM3 has multiple altitude and airspeed restrictions; the final altitude is 4;000 feet at STRPS. The altitude restriction at EAGLL is: -13;000/+12;000; then ROKTT is: 11;000; distance between EAGLL and ROKTT is 23.7NM. We were with Potomac Approach on frequency. Approximately 16;000 feet descending via the ANTHM3; outside of EAGLL; Potomac Approach revised our clearance; 'descend to 11;000 for traffic; then continue descend via ANTHM3.' PM (Pilot Monitoring) read back the clearance verbatim. PF (Pilot Flying) decided the managed descent would be sufficient; however PM understood the clearance to descend immediately to 11;000; which would level the aircraft just prior or at EAGLL. After a brief discussion to ensure we both understood the revised clearance; PF set 11;000 in the altitude selector and pulled for OPEN DES. The aircraft was nearing 11;000 at or shortly after EAGLL. Satisfied that the instructions were understood; PM shifted attention to another task. We were in a higher than normal workload and at some point PF reset the altitude selector to 4;000 and inadvertently pushed the selector to manage the descent. Both pilots distracted by an ATC call did not realize the aircraft began to descend to 4;000. With 4;000 set in the altitude window; there was no altitude warning horn. Potomac Approach inquired our altitude as we slowly descended through 10;500. The PF stopped the descent at 10;300 and began to climb back to 11;000. Potomac Approach revised our clearance again to maintain 10;000 until ROKTT; then descend via remainder of ANTHM3. We were handed off to another Potomac Approach Controller and PM advised the new altitude. Potomac Approach reaffirmed the revised clearance; we continued the STAR and approach to landing as normal and uneventful. Both pilots are highly experienced in both total time and time in type. Both pilots had flown together; have mutual respect for each other; used SOP and CRM. The issues that led to the event are: the timing of the revision led to confusion; both pilots understood the revised clearance differently; but assumed they understood it the same; another similar sounding call sign on ATC led to slight distraction; PM multi-tasking in a higher workload environment. Lessons learned are: PM should have paused other tasks at altitude changes to confirm and back up PF. PF confirm altitude changes with PM to ensure both pilots are in agreement of descent mode/initiation.




Synopsis: Air carrier pilot monitoring reported confusion attributed to not understanding ATC clearance; and ATC altitude changes from the procedure while on the ANTHM3 landing Runway 10 at BWI.