I was just curious, in the past, smoking on planes was permitted.
What is the law surrounding smoking in your own private plane?
The short answer to "Can I smoke in my own private aircraft?" is "Yes, usually".
As Promised, I looked up the relevant FAR (23.853 - Passenger & Crew Compartment Interiors).
The actual rule is long and verbose, but pretty common sense. The important regulatory bits for most personal aircraft are just two points though. You must:
For "commuter category" aircraft there are even more requirements. Those requirements don't apply to most "personal" planes, so even though it's probably a good idea to meet some of those requirements if you're going to allow smoking in your personal aircraft they're not legally required.
All that said, while it's likely legal (unless you've removed your ashtrays or your plane didn't come with them) common sense generally dictates you don't want to smoke in your plane - It's bad for the vacuum-driven instruments' filters, will require even more attention to cleaning the inside of your windows, and a loose cigarette in turbulence or a bunch of ashes blown about by an open air vent can be a pretty substantial distraction.