AFAIK, airline crew need to be listed on the crew manifest for the flight and arrive at immigration/customs in their uniform. Then their airline ID (which will, without fail, have a picture on it) is checked against the manifest immigration/customs received for the flight and they're allowed through without having to show further identification.

The keys are to be listed on the manifest and to be in uniform. Otherwise, they will still need their passports or other acceptable travel documents.