EMB ERJ 170/175 ER/LR
Flight Crew Air Transport Pilot (ATP)
Communication Breakdown
Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Flight Crew Became Reoriented; Flight Crew Regained Aircraft Control
Human Factors
I was pilot monitoring for a leg with a normal takeoff. We were assigned a heading while climbing out on vectors to eventually intercept the departure.As pilot monitoring I was watching the turn and noticed we were approaching a point at which I'd expect to begin rolling out and noticed the pilot flying had no indication of beginning a roll out so I waited an extra few seconds to give them the benefit of the doubt and as we were approaching our assigned heading I spoke up and said 'hey you're going to stop this turn right?' and there was no immediate response from the pilot flying so I put my hands on the controls and was preparing to take control if the pilot flying did not immediately respond. The pilot flying did immediately respond when he felt my input on the controls as he was hand flying and sort of snapped out of it and immediately corrected and apologized profusely.I'd estimate we ended up going through our assigned heading by 10-20 degrees during the recovery with a relatively quick but non aggressive return to our assigned heading. I noticed traffic in the general direction of the continued turn about 500 feet above us on TCAS; but I don't believe we were close and it certainly didn't result in even a TA.The pilot flying First Officer mentioned he had zoned out in the turn and had I not stepped in they probably would have continued turning. I noticed at the time our assigned heading was perpendicular to our FMS arrow for the departure we were not on yet. I think the pilot flying was possibly fixating on the arrow while hand flying and simply forgot to roll on out the heading bug.ATC didn't mention it and gave us routing shortly after.This event occurred on our 3rd leg out of 5 for the day and we discussed the event and our intent to ASAP it in a debrief. All other events for the day were normal.I spoke up when we were approaching the point of no return in terms of exceeding our heading clearance and I probably could've done it a bit sooner or put my hands on the controls sooner for a physical prompt; but I didn't have reason to suspect I'd have to. I've never had someone who was staring at the PFD making a turn just zone out. It was kind of a weird event and a first for myself. It's something I'll keep an increased awareness for in the future and I'm sure the pilot flying will as well.
The Captain of an Embraer EMB-175 reported adding control input to the aircraft because the First Officer did not rollout on heading.