In three-crew cockpit can a person qualified as pilot work in the position of flight engineer? Does a flight enginner have different qualifications that are not covered by an ATPL, thus rendering a pilot unqualified for the position?
The pilot cannot just sit down in the flight engineer station. First there is a different FAA flight engineer certificate that the pilot needs to obtain. Secondly, the pilot would need to go through the airline's training program for that station.
Granted, a pilot who has flown that airplane before should quickly get those done.