Passenger seats - as approved by the airline operator, are the only seats that can be occupied by non-crewmembers. Passengers are not to occupy a F/A jump seat, i.e. to increase the "passenger capacity" of the cabin.
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Obviously, nothing wrong about a passenger occupying such seat for a few minutes, i.e. while waiting for a toilet nearby to be free... But an "extra passenger" cannot be accommodated on such seat for the flight.
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I have flown airplanes (some 707 or 727s) which had one regular passenger seat in the overwing exit row marked "crew only"... these seats were deducted from the number of usable passenger capacity.
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My airline permitted flight crewmembers, cabin crews and technical staff to occupy our jump seats, but in no case were "extra passengers" to be using such seats for takeoff or landing. A retired pilot now, I no longer have the privilege to occupy such seats should the flight be full, even though I know perfectly well how to open a door of an airplane...
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Happy contrails