Date: 201511
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Light: Daylight
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Parked; Takeoff / Launch; Cruise
Airspace: Class A ZZZ; Class B ZZZ
ATC / advisory: Center ZZZ; Ramp ZZZ
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Communication Breakdown; Distraction; Time Pressure
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Deviation / Discrepancy - Procedural Weight And Balance; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Other / Unknown
Detector: Person Flight Attendant
When detected: In-flight
Result: Flight Crew Became Reoriented; Flight Crew FLC complied w / Automation / Advisory; Flight Crew Took Evasive Action
Contributing factors: Company Policy; Human Factors; Procedure
Primary problem: Company Policy

Narrative: Departed gate with full aircraft. Told all jumpseaters accommodated. Video inop so manual safety demo required. Flight Attendants (FA) allowed FA trainees to assist demo. Short taxi to Runway XY. Prepared and chimed FAs and was cleared for takeoff. FA chimed us as we started our takeoff roll. I elected to continue rather than abort. After cleanup we contacted #1 and learned that all seats and cabin jumpseats were full and 1 FA did not have a seat. I contacted dispatch and ops for guidance. Under agreement with dispatch and the duty chief pilot we continued to filed destination with a qualified non-working FA.


Second narrative: During takeoff roll heard a chime from cabin. We had been cleared for takeoff and Captain chose to not break sterile in this critical time so continued takeoff. After cleanup called cabin and we were informed there had been some training flight attendants on board. There were not enough jumpseats for one of them. They were seated sharing a jumpseat but there was not enough seatbelts for all of them. I flew the airplane while the Captain consulted with dispatch and IOC. I believe they consulted with a chief pilot on duty also. They mentioned something about in their books there is a situation where someone with no seat can sit on the floor. Since it doesn't sound safe to have a person sitting on the floor for landing. A qualified line FA that was non reving sat in the cockpit jumpseat so everyone could have a seat. We were also told it would be best to continue the flight as the seating situation would be the same whether landing returning to land or continuing. We agreed that would be best so continued the flight. Possibly a headcount of who has a jumpseat and/or passenger seat assigned during preflights. At this point we are not sure how the boarding passes were mis-counted but it's suspected an agent possibly tore off part of boarding pass but didn't scan it. FAs being seated with a little more time before entering the runway. Or receiving a call over the intercom from the cabin stating the cabin is ready for takeoff.


Synopsis: A B737-800 departed with two seats too few for passengers; crew; two trainee flight attendants and a Flight Attendant jump seater. Gate agents failed to account for the trainees and jump seater having given their seats to passengers.