Date: 201201
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: Gulfstream G200 (IAI 1126 Galaxy)
Operator: Fractional
Operating under: Part 91
Flight plan: IFR
Mission: Passenger
Flight phase: Takeoff / Launch
ATC / advisory: Tower ZZZ
Aircraft component: Cabin Crew Seat
Component problem: Malfunctioning
Reporter organization: Fractional
Reporter function: Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Confusion; Human-Machine Interface; Time Pressure; Troubleshooting; Communication Breakdown
Anomaly: Aircraft Equipment Problem Critical; 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 Crew
When detected: Other Post flight
Result: General None Reported / Taken
Contributing factors: Aircraft; Chart Or Publication; Human Factors
Primary problem: Chart Or Publication

Narrative: When we arrived at the FBO I requested our paperwork from the front desk and ordered fuel. Shortly thereafter the First Officer; our Flight Attendant and I headed for the aircraft. We were fueled and had finished our pre-flight checks as we awaited our passengers. Once the passengers arrived and were settled; we started the engines and began our taxi. ATIS was reporting takeoff from Runway 01L; however; we were instructed to taxi to 19R. This is an extremely short taxi from our location. The First Officer informed our Flight Attendant of the change and asked her if she would be ready for takeoff soon. She then proceeded to attempt to pull out the jump seat. Then; for quite some time; she pulled on the jump seat in an apparent attempt to extend it for use which was very loud; and I'm sure; disturbing to our passengers as it was for me. The First Officer then advised her that she would probably just have to sit in the cabin; to which there was no reply. There was absolutely no mention of the fact that she didn't have a seat. As we held short of Runway 19R I instructed the First Officer to tell the Tower that we needed more time before we departed. The Flight Attendant began to pull on the jump seat a second time and I had asked the First Officer to ask her to stop. She then [unbeknownst to us] proceeded to stand in the galley. The First Officer asked her if she was ready; and she replied; 'I'm ready.' We were then cleared for takeoff and I informed the First Officer that there was no way I was going to takeoff with her standing in the galley. I then said to her that we were taking off right now and she proceeded to the cabin of the aircraft. Prior to power initiation; I said; 'Everybody seated?' the First Officer checked the back and did not see anything out of the ordinary.The rest of the flight was very normal. At no time did the Flight Attendant alert us to the fact that there were 9 people on the aircraft. She did not inform us of anything; nor do I recall her saying a word to us during the entire flight. During descent into our destination; at the time when she would normally pull out the jump seat; she again said nothing about 9 people being on board or her having no seat on the aircraft. After landing; I was in my seat finishing paperwork and the First Officer and the Flight Attendant were speaking about bags; cabs; etc... It was at that time that she informed us that she did not have a seat. That is when I realized that there were nine passengers on the aircraft. This was after all of the passengers had departed in their vehicles. I then preceded to double check my trip sheet; and it showed 8 people. We then realized that the Flight Attendant did not have a proper seat in the rear of the aircraft.I would have never departed an aircraft with knowledge of the fact that every person was not secured in a proper seat. I also would not have willingly departed an aircraft with knowledge of an extra passenger on a Part 91K flight.




Synopsis: When the Flight Attendant was unable to extend her jump seat for occupancy and an unreported passenger had taken the last passenger seat the flight crew of a Part 91K G-IV proceeded to takeoff with the Flight Attendant still standing.