Originally Posted by
USFlyerUS
It's an FAA issue. Airlines are required enforce the rules they set per the FAA. It's in effect incorporated into their air operator's certificate. Therefore, the "former" here is what should matter -- the rules are the rules and need to be followed.
I am not aware of any FAA requirement. Only FAA guidance to airlines.
The only thing I found close . . . quickly mind you . . . is as follows.
https://www.faa.gov/documentLibrary/.../AC121-29B.pdf