Watch first the clip of the crash of a single engine plane.
YouTube - Cameron Park plane crash
This is the link to the NTSB preliminary report on the crash. According to NTSB, the pilot was commercial rated.
LAX07FA258
And then watch this demonstration of what happened, on a simulator.
YouTube - Re: Plane Crash Caught on Tape(killing 2) 08-30-2007
There are numerous things that went wrong and all could have been avoided by the pilot in command. I am fast learning that flying is very save, but only because it is made safe by following strict protocols. If we treat flying like we drive a car it is very unsafe.