What document must a commercial pilot have with them when acting as pilot in command?

A commercial pilot must have a valid pilot certificate with them when acting as pilot in command because this document serves as proof of their authorization to operate an aircraft for compensation or hire. The pilot certificate confirms that the pilot has met the necessary training requirements, passed the required tests, and possesses the skills and knowledge to operate the aircraft safely. It is a legal requirement for pilots to carry their pilot certificate while conducting flight operations, ensuring compliance with aviation regulations and maintaining safety in the airspace.

While other documents like a flight plan, medical certificate, or logbook may be important for various reasons, they are not mandatory to be in possession of during the actual act of piloting. The pilot certificate is specifically required as proof of qualification to ensure that pilots operating aircraft are officially recognized and certified by aviation authorities.