Cosmo,
This is my very first post, so I'm sticking to a subject with which I am familiar!
<font face="Verdana, Arial, Helvetica" size="2">Objective of the Investigation
The sole objective of the investigation of an accident or incident shall be the prevention of accidents and incidents. It is not the purpose of this activity to apportion blame or liability.</font>
If you're looking for comparison between air and road as part of your research, then there is little difference (although in a road traffic sense the immediate aim is not prevention: this comes with recommendation to the appropriate bodies afterwards).
What is paramount is an objective search for the truth (as Matelot pointed out), and absolute impartiality; although not many believe that!
If you want some accident investigation sites - and some do cover air accidents - you could start initially with the Institute of Traffic Accident Investigators (ITAI)
www.itai.org. They have links all over.
Be pleased to help further if I can.