In general aviation, it's almost ubiquitous that pilots yell "clear" or "clear prop" before engaging the starter. Are there any regulatory requirements to take this action either in the form of 14 CFR or maybe in the Airman Certification Standards?
Like @mongo, I am not aware of a regulation and don’t recall seeing anything about it in the AIM. However, the Airplane Flying Handbook (FAA-H-8083-3B) says this about engine start:
Prior to engine start, the pilot must ensure that the ramp area surrounding the airplane is clear of persons, equipment, and other hazards from coming into contact with the airplane or the propeller. Also, an awareness of what is behind the airplane prior to engine start is standard practice. A propeller or other engine thrust can produce substantial velocities, result in damage to property, and injure those on the ground. The hazard of debris being blown into persons or property must be mitigated by the pilot. At all times before engine start, the anti-collision lights should be turned on. For night operations, the position (navigation) lights should also be on. Finally, just prior to starter engagement, the pilot should always call “CLEAR” out of the side window and wait for a response from anyone who may be nearby before engaging the starter.
To my knowledge it is not a regulatory requirement. Assuring that there are no people or objects near the propellers is a regulatory matter.
Good practice is to call out, and then clear the area visually. Leave enough time for the line boy to get out from under the plane who you didn't notice clearing the chocks. (grin)
There’s no regulation for yelling that out, save, I suppose that one could cite a flight crewmember under §91.13 guidelines for careless and reckless operation, if they attempted to start an aircraft engine with personnel nearby without giving proper warning to alert bystanders of the hazard. It is operating the aircraft in a careless or reckless manner that would endanger the life or property of another.
Many aircrews do not yell that out during commercial operations, but, in general, these are occurring on sterile ramps with trained personnel nearby. You are, in these situations, still responsible to be cognizant of the whereabouts of ground personnel and refrain from starting up or moving if they are near the aircraft.
That being said, if you are acting as PIC, you are directly responsible for, and the final authority on, the operation of that aircraft (§91.3). Should someone be injured or killed by a propstrike from an aircraft you are acting as PIC of during startup or taxi, and it’s determined that the cause of the accident is a result of your gross negligence, you could potentially be charged with criminal vehicular homicide and definitely face the prospect of a civil tort for wrongful death.