Date: 202104
Local time of day: 1801-2400
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Light: Night
Aircraft: EMB ERJ 170/175 ER/LR
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; First Officer
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Communication Breakdown; Situational Awareness
Anomaly: Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Security; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Other / Unknown
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Became Reoriented; Flight Crew Regained Aircraft Control
Contributing factors: Company Policy; Procedure; Human Factors
Primary problem: Human Factors

Narrative: In cruise I told the CA (Captain) I needed to use the lavatory. We called back to the FA's (Flight Attendants) who said they were finishing up service and would come up when they could. When they called up; they gave the code word as normal. I did a positive exchange of controls to the Captain since it was my PF (Pilot Flying) leg. He took controls as normal. Door unlocked and the crew swap went normal. Upon finishing in the lavatory the B FA called up to begin the next crew exchange. She informed me that the CA wanted to use the lavatory too. I was waiting in the galley expecting the A FA to come out so we could do the normal process of me sitting down; getting controls back; getting any flight updates; and then having the FA's call back up so the CA could use the lav. Instead; the flight deck door opened and out walked the CA. No one was at the controls; the A FA was in the process of moving from my seat to the CA seat. I immediately said 'she's supposed to come out first; no one is at the controls'. The CA responded 'oh; right' or something along those lines. The B FA and myself expressed our concerns over the incorrect lavatory exchange procedure. He then proceeded to open the lavatory door. Not go back into the flight deck. I rushed into the flight deck; sat down; got myself situated and locked the door since there was no pilot. I got the aviate; navigate done and now wanted to communicate. I asked the A FA what happened and she said 'I don't really know; he told me to switch seats with him so we squeezed past each other'. I told her that's not normal and that I needed to talk to the CA. The B FA called up; said the code word; and the CA came back in. I talked to him about what happened and he said something like 'oh right; that did seem strange'. I talked to him about the correct procedure which he; I assume; did not know. I told him we had no one at the controls and how dangerous that is. Upon getting to the hotel the FA's asked me if I was going to fill out a report. I called the Pilot Union Professional Standards to get their opinion on the situation. I used this as a teaching moment; however; the whole event should never have happened.




Synopsis: First Officer reported that during a lavatory break; both pilots were out of the cockpit at the same time and the flight attendant was swapping seats in the cockpit; leaving the aircraft uncontrolled.