In version 7.5 when flying over the skyline out of view the plane would still crash into the ground.
On both version 8.1 and 8.2 the plane can be flow at a negative altitude. Yet I've only had this happen when the plane is over the horizon.
I noticed when I answered a phonecall and came back to find my plane still in the air, yet at -626M altitude.
Not really a big problem when flying normally however.