This happened last night. I didn't see any other threads about it. I was playing Franklin and driving a bike at Blaine Co's airport, when all the sudden a Mammutus (the old dodo) landed. I knocked the guy out and took the plane, flew around the map. I didn't want to risk screwing my game up so I did not save it. I drove back over to the airport and it spawns along with the stunt plane from time to time. They always land and get out.
So the plane IS in singleplayer...why can't we buy it? It's obviously coded and able to fly? Has anyone else seen this?