Flight Crew Commercial
Human-Machine Interface; Confusion; Communication Breakdown
Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance
Human Factors
Descending into PDX via the Moxie STAR we were cleared to cross seven miles south of VANTZ Intersection at 10;000 MSL. Because of how unusual this clearance was (VANTZ at 10 or 12 thousand is the norm) I queried ATC asking to confirm the clearance and ATC responded that was correct. The pilot flying mistakenly set 7;000 into the altitude alerter and I mistakenly confirmed 7;000 as a correct altitude. We descended to 9;500 where we re-considered the clearance; briefly talked and decided to stop the descent. At this time ATC asked our altitude. I explained we had become confused or misunderstood the clearance and were descending to 7;000 MSL; 7 miles south of VANTZ. From there ATC cleared us to descend and maintain 9;000. Flight continued normally.Being initially confused by the clearance to cross seven miles south of VANTZ and then seeing 7;000 in the altitude window may have drawn an incorrect correlation resulting in a positive response in my mind because of the relation between the seven mile south restriction and seeing the '7' recur in the altitude window. This is only educated guess to as why seeing 7 thousand set produced a positive response in my mind. Increased scrutiny of altitude set in the altitude alerter. Even though the pilot not flying may initially concur with the pilot flying's altitude selection it doesn't necessarily mean that is the correct altitude. Do not just agree with what digits are set in the window. Make sure to agree that the proper altitude set.
An unusual crossing restriction from Approach while descending on the MOXIE STAR to PDX led the flight crew of an EMB-120 to descend below their cleared altitude.