What Causes Sudden Fainting?

My dad has had an episod of passing out all of the sudden 4 time in the last 2 years we have been asking him to get himself checked out but he seems to be in a denial phase! that there is nothing wrong with him he also has high blood pressure for the last 10 years or so. His most resent episod was during his flight from New York to Delhi just 2 days ago. My question is what do u think is causing this???? plz answer a very worried daughter!