When is smoking banned on airplanes?

Smoking is always prohibited on airplanes. An airplane is filled with pressurized air, so there is no way to clear smoke from a cabin. Also, disposing of a cigarette in a trash can could start a fire that produces toxic smoke and disables control systems, thus causing the plane to crash. The TSA or your local airport security agency will confiscate any cigarettes you attempt to take aboard.

According to FAA rules smoking is not allowed in the aircraft while the aircraft is being fueled or taking off or landing. Any other time it is permitted. Commercial airlines however, since they operate in many different jurisdictions, some of which prohibit smoking in all public places, have all gone to prohibiting smoking on the aircraft entirely although there is a rarely-used exception for the pilot.