Date: 201206
Local time of day: 1801-2400
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Light: Daylight
Aircraft: B737 Undifferentiated or Other Model
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: First Officer; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Flight Engineer; Flight Crew Multiengine
Human factors: Other / Unknown
Anomaly: Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Became Reoriented
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: Approximately 30-35 minutes from landing; the Captain stated he needed to go to the lavatory to 'relieve himself'. He and the Lead Flight Attendant went through the standard security procedure of entering/exiting the cockpit. When the Captain was ready to re-enter the cockpit (with the flight attendant occupying the cockpit with me); he 'called' the cockpit (after approximately 4-5 minutes of his initial exit); to re-enter the cockpit. The Lead Flight Attendant then asked me if I was going back to use the lavatory as well. I responded in the affirmative; to which she asked; 'You want to just let the Captain enter the cockpit while you exit the cockpit? (i.e.; 'swapping out'; implies that she would remain in the cockpit; while the Captain and I 'swapped in/out of the cockpit'). This is not the normal operating procedure for security and safety reasons (i.e.; leaving both cockpit seats unoccupied). Normally; in this situation; the flight attendant would open the cockpit door; allowing the cockpit crewmember (in this case; the Captain) back inside the cockpit; while the flight attendant exits; thereby; having both pilots back in their seats. Then; the other cockpit crewmember (myself; in this case) would begin the entire process again for entering/exiting the cockpit).What actually occurred; was that I got out of my seat to let the Captain back into the cockpit. I exited the cockpit while the Captain simultaneously entered back into the cockpit; while the Lead Flight Attendant remained in the cockpit during this entire process. The longest period of time in which neither cockpit seat was occupied was approximately 5 seconds; as the Captain and myself 'swapped out' of our seats quickly; especially; when the Captain realized I was the one opening the cockpit door to allow him to re-enter the cockpit; instead of allowing the flight attendant to open the cockpit door to allow him to re-enter.I should have remained in my seat while the flight attendant let the Captain re-enter the cockpit. Both the flight attendant directly involved in this situation; and; myself; is very aware and knowledgeable of the standard operating procedure for entering/exiting the cockpit during flight. Even though the flight attendant made the recommendation that the captain and I merely 'swap' in/out of the cockpit; ultimately; the responsibility lies with me to make sure we all follow the policy/procedure for entering/exiting the cockpit. Fortunately; there was no emergency that arose with the aircraft (i.e.; rapid depressurization; engine failure; etc...); or; ATC; while both the Captain and I were not occupying our seats. As I look back on this situation; I still find it hard to believe I allowed myself to alter the standard operating procedure. I realized the error I made as soon as I opened the cockpit door; and; the Captain stood there to re-enter; and; acted very surprised to see me instead of the flight attendant. In no way; did I intentionally; or; willfully; desire to violate the standard operating procedure. This situation was honestly and sincerely; a result of me having a momentary lapse in judgment. In the future; when I am faced with this situation with one; or both of us; as Captain and First Officer; going back to use the lavatory; I vow to follow; exactly; the procedure of exiting/re-entering the cockpit. If I had taken just 2 seconds to think about the potential consequences of what I was doing (getting out of my seat in the cockpit; thereby; not having either the Captain or myself in a cockpit seat); I'm sure I would have not altered the exiting/re-enter procedure.




Synopsis: B737 First Officer reports leaving his seat at FL240 to open the cockpit door for the Captain returning from his bathroom break.