Date: 201706
Local time of day: 1201-1800
Location: ORD.Airport, IL
Flight conditions: VMC
Light: Daylight
Aircraft: Medium Transport; Low Wing; 2 Turbojet Eng
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Taxi
ATC / advisory: Ground ORD
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Communication Breakdown; Confusion; Distraction; Workload; Situational Awareness
Anomaly: ATC Issue All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Ground Incursion Taxiway
Detector: Person Air Traffic Control
When detected: Taxi
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew FLC complied w / Automation / Advisory
Contributing factors: Human Factors; Procedure; Airport
Primary problem: Human Factors

Narrative: We were in the Foxtrot alleyway on the east line when we contacted metering we were number two. In the time it took us to check in; the aircraft in front of us had vacated the Foxtrot alleyway east line and we became number one on the east line. When we received our taxi clearance we were given to taxi to runway 28R/N5 by following the company aircraft in front of us. The problem was that there was not an aircraft in front of us; however; there was an aircraft number one in the Foxtrot alleyway on the west line that was our company airplane with no one behind him. We assumed that is what he meant and followed him out of the alleyway onto the active taxiway where he entered A then proceeded to A13. We immediately became confused when the aircraft in front of us made a left turn and started moving east bound on taxiway B. Both the captain and I being familiar with ORD know that a normal taxi to 22L is via B and D and had a split second decision to make. In fear of following the wrong airplane to the wrong runway and entering taxiways without a clearance the captain and I made the split second decision not to follow the airplane we originally thought was to be correct. Instead of making the left turn and heading east on B we made a right turn on B to head west followed by a left turn on N2 and a left turn on N to enter the line. We were then told to switch to tower. After switching to tower the tower called us and said that we needed to talk to ground. We switched back to ground and knew that we made a mistake once we switched and saw the airplane we were originally thought was correct to follow turn right on A15 and make a left turn onto N to skip a part of the line we had mistakenly put ourselves in.This event was caused by a couple of factors. To kick things off we received a poor ATC clearance to follow the aircraft in front of us when there was not anyone in front of us and didn't receive a specific clearance to taxi to the runway like taxi to runway 28R/N5 via A13;B; A15; N which in hindsight what we were supposed to do. The second contributing factor is that the captain and I made some assumptions as to what was meant by the clearance and we attempted to fill in the gaps on our own instead of asking for clarification. In addition to this error; we proceeded onto an active taxiway deeming what we thought was correct which obviously went wrong. The issue we faced was a high amount of confusion when we watched the aircraft in front of us which originally was off our left side turn left on taxiway B. We again had the opportunity to stop the aircraft and seek clarification; however we kept moving along a route we thought was more correct. An additional factor that should be noted was that ground control was very busy and it was difficult to attempt to contact ground.In an attempt to try to avoid this from happening again; the crew should seek clarification from ATC before moving the aircraft when a vague clearance is received.




Synopsis: An air carrier First Officer reported being cleared to Runway 28R following company traffic on the Foxtrot Alleyway. The crew followed the incorrect Company aircraft and entered an active taxiway.