Is it common that advisor has no idea about what is going on and totally helpless? Before going to graduate school, I had been under the impression that professors were necessarily extremely intelligent.  I had also assumed that professors would tend to be smarter than assistant professors or postdocs.

Now after more than 1 year in my research group, I know that I was very wrong. 
My supervisor has no clue to assess student work. They even need help to understand basic calculations, and they cannot help me with questions about papers.

My advisor often fails to understand basic content in my own reports after repeated explanations, becoming angry with me over it.

Questions:


Is this a common situation for PhD students to find themselves in?
What can I do to improve the situation?
No, it is not common. 

However, everyone in academia probably will meet someone like your supervisor along the years. Incompetence and unprofessionalism can be found in every corner of society and academia is not an exception.

If you are able, change to another supervisor as soon as possible.