Unless you want to face harsh consequences, you should never smoke on a plane no matter how bad your craving is.  Smoking on an aircraft could result in steep fines, being forced to land the plane early at a different location, or in some cases, getting arrested and thrown in jail. It is also not a smart idea to try and sneak a cigarette into the bathrooms. Along with their dozens of no-smoking signs, there are also smoke detectors so they will be able to tell you were smoking. It's smart just to wait until the plane lands to smoke.