What are the legal requirements to be a "crewmember"? 14 CFR 1.1 includes the following definitions related to crewmembers:


  Crewmember means a person assigned to perform duty in an aircraft during flight time.
  
  Flightcrew member means a pilot, flight engineer, or flight navigator assigned to duty in an aircraft during flight time.


Some of the regulations (although not all instances) also refer to a required crewmember.

Anecdotaly I have heard of people being assigned as a crewmember on the spot so that they could fly in an aircraft that is limited to "crewmembers only" (i.e. some world war two era aircraft, etc.) by saying something like:     


  "If the normal gear extension fails during this flight, then you will be responsible to pull this emergency handle over here.  Okay, got it?"


I'm not sure whether or not that would technically qualify someone as a crewmember, so are there any legal interpretations from the FAA or any guidance on what actually does qualify someone as a crewmember?
I'll take a stab at this one, though I can't guarantee the correctness of my interpretation (the regs don't seem exactly clear on the subject - at least not to me, so there's a lot more "inference" here than I'd normally be comfortable with).

In most cases under Part 91 the only requirement to be a "crewmember" is that the pilot in command says you're a crewmember -- i.e. the PIC determines that a person is qualified to perform a specific duty and designates them to perform it.
In the absence of anything more specific this is the only requirement I can turn up (and it's an inferred requirement - I didn't find anything specifically stating this, but 91.533 regarding flight attendants strongly implies that it's the correct interpretation. There's also a counsel opinion from 2008 - the Schehr letter which talks a little about who is a "crewmember" for purposes of riding in jumpseats that are placarded "For Crew Use Only". That interpretation is based on Part 135, but could be logically extended to Part 91 (and possibly some 121) operations too).



There are frequently more specific requirements though, particularly for positions that are important to flight operations. For example if you buy yourself a nice shiny Lockheed Electra to duplicate Amelia Earhart's trip under Part 91 rules you still cannot simply designate someone off the street to be the Flight Navigator: the FAA has specific requirements for Flight Navigator certification, and someone acting as the flight navigator would need to meet those requirements.

There are also more stringent requirements that are imposed for Part 121 and 135 operators. For example they can't simply designate a person as a flight attendant based on the PIC's determination that they are qualified: There are requirements for filling that crew position which must be met (FAR 121.421 &amp; FAR 135.295 respectively, and possibly other regulations).