Hot Air Balloon Pilot Practice Test

You must have your current pilot certificate in your possession whenever you are acting as the pilot in command or as a required crewmember. This requirement exists so that, if an authorized FAA representative asks, you can immediately show you’re legally authorized to fly and current in the privileges you’re exercising. The certificate should be in your personal possession or readily accessible in the aircraft.

That means it applies to all flights where you’re PIC or you’re part of the required crew, not just flights with passengers or international flights, and it’s not something you ignore. Even if you’re flying solo, you’re the PIC, so you need to carry it.