Date: 201601
Local time of day: 1201-1800
Location: MDW.Airport, IL
Flight conditions: VMC
Light: Daylight
Aircraft: B737 Next Generation Undifferentiated
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Taxi
ATC / advisory: Tower MDW
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Communication Breakdown; Situational Awareness
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Conflict Ground Conflict; Less Severe; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Ground Incursion Runway
Detector: Person Air Traffic Control; Person Flight Crew
When detected: Taxi
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: We were cleared to taxi to Runway 31C at MDW via P; G. While on Taxiway G; there was a Company aircraft holding short of 31C on taxi line E1. With E2 and E3 taxi lanes open; I taxied behind the E1 aircraft and held short of 31C on taxi line E3. Almost clear of the E1 holding aircraft's tail; MDW Ground Control called and said I should not have passed behind the aircraft. After stopping at the hold short; MDW Tower cleared a smaller business aircraft to change from 31C and cleared him to land on 31R. Even though there was no aircraft inbound to 31R when I taxied around/behind the aircraft at the 31C hold short; I think I crossed the hold short line for 31R with my left gear. I still think Taxiway G ends and dumps into the huge apron of E1; E2; and E3 hold lines for 31C. I have done this many times; and never been called by Ground Control. I have often been holding short for 31C on E3 and/or E2 and had aircraft be cleared to land 'over' me on 31R. This is why I did not consider my movement around the holding aircraft to be an encroachment of another runway.


Second narrative: We pushed and deiced off of [the] gate and were cleared to taxi to 31C via P; G; cross 31R. We understood the instructions and taxied uneventfully most of the way. Approaching 31C; a Company aircraft was waiting for departure on the E1 line. My Captain began to taxi around him and I saw we were going to encroach upon 31R. I alerted the Captain and asked if we were going to enter 31R to which he replied (paraphrase) no; he was only going to go around Company. I checked final; which was clear; while I told him directly that we were about to cross the hold short line; and then a second later let him know; unnecessarily; that we were on a runway without clearance. I told him I was very uncomfortable; to which he replied that he'd been doing that for 15 years and it wasn't a problem. A moment later Tower called us and said (paraphrase) that in future we should not taxi around other aircraft without permission. I acknowledged the transmission and Tower came back clearing an aircraft on final to 31C to sidestep and land on 31R. We lined up on E3; which has the shortest distance between runways; but as we were not in a 737-800; I don't believe that posed any additional threat with landing traffic on 31R.


Synopsis: B737NG flight crew reported a runway incursion at 31R in MDW.