Date: 201909
Local time of day: 0601-1200
Location: ORD.Airport, IL
Aircraft: EMB ERJ 170/175 ER/LR
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Takeoff / Launch
Airspace: Class B ORD
ATC / advisory: Tower ORD
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Human factors: Communication Breakdown; Time Pressure
Anomaly: Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Ground Incursion Runway
Detector: Person Flight Attendant; Person Flight Crew
When detected: In-flight
Result: General None Reported / Taken
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: Alerted after departure that we may have had an issue.  At Gate XX; [we were] pushed back tail north and got a CAS message for which couldn't clear a maintenance issue.  With assistance from Maintenance Control and a complete shutdown and startup on the ramp the issue was resolved; but all the necessary data had to be re-entered. The checklists were rerun and we were given taxi instructions to Runway 22L.Tower gave line up and wait instructions for Runway 22L and a note for wake turbulence due to a heavy aircraft.Captain called for Before Takeoff Checklist which was completed. I remember hearing radio chatter while the Captain asked if the flight attendants had called ready. The Captain called the flight attendants via call button and had reverified as other radio traffic was commencing. Flight attendants reconfirmed they were ready as we were rolling into position as Captain and I were scanning all the panels; Captain started takeoff roll and said 'TOGA.' I questioned Captain and said 'Hey did he clear us to go?' The Captain looked up at the light panel and said 'He must have. The lights are on; didn't he?'; or something similar to that. My eyes went wide and looked at the IAS and it was increasing fast. At that point he convinced me that I may have missed the call and I believed it to be safer to focus on the task of a safe takeoff roll and deal with this at a safe altitude.I didn't question it again and take off roll continued; Tower did not question the takeoff roll.After hand off to the second controller about 5 minutes into the climb Center alerted us to the possible deviation.The flight continued uneventfully after that.While in flight when I had time to think about what had happened; it bothered me that I could not fully recall whether or not I had heard an instruction from Tower to take off or not. I had to think about it for a while and then it dawned on me that had I heard the call; I would've answered it and confirmed the instructions to take off. I did query the Captain to confirm whether the instructions were given; but I was persuaded that instructions were given; he insisted because the lights were turned on; which to him was a mental trigger that served him as a confirmation.At that instant I had thought of quickly contacting Tower to confirm the take off clearance while we were rolling but I was worried it would be more unsafe to abort takeoff at that point than it would be to continue and felt compelled to back up the Captain believing that I was in error.I believe some of the contributing factors were multiple communications were going on at the same time which lead to confusion. Relying on the lights being in the on position as a confirmation of a take off clearance; the Tower Controller having us line up and wait on the runway during the period of wake turbulence dissipation instead of having us hold short may have helped decrease the possibility of an inadvertent take off; and of course since I was responsible for operating the radios on this leg; I should've been insistent to confirm with Tower if I had any doubt about the takeoff clearance.




Synopsis: E-175 First Officer reported a communication breakdown between flight crew and ATC resulting in takeoff without clearance.